23. The Digestive System

Anatomy, Motility, Secretion, Digestion, and Absorption

Learning Objectives

23.1 What the Digestive System Is For

Every cell in your body runs on molecules it cannot make and did not manufacture. Those molecules arrive as food: bound up in polymers far too large to cross a membrane, mixed with material that is useless, and occasionally accompanied by organisms that would like to eat you. The digestive system solves all three problems with one strategy — a tube.

The alimentary canal (the gastrointestinal tract, or gut) is a continuous muscular tube running from mouth to anus, roughly 7.5 metres long in a living adult and closer to 9 metres in a cadaver, where the loss of muscle tone lets it relax and lengthen. Along its length sit the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive organs — teeth, tongue, salivary glands, liver, gallbladder, and pancreas — are not part of the tube. They sit outside it and deliver secretions into it through ducts, or, in the case of the teeth and tongue, act on its contents mechanically.

The six essential activities

Notice that four of the six are mechanical or transport functions and only one is chemistry. Students arrive expecting digestion to be a story about enzymes. It is mostly a story about muscle, timing, and surface area, with enzymes as the final short step. A hamburger spends about four hours in the stomach being physically dismantled and perhaps twenty minutes' worth of that time being chemically digested there.

The lumen is outside the body

Here is the single most useful conceptual move in this chapter, and it takes some getting used to.

Imagine a doughnut. The hole is surrounded by doughnut on every side, but nothing in the hole has ever been inside the doughnut. You are a doughnut — a very long, very narrow one. The lumen of your alimentary canal is continuous with the outside world at both ends, and material in it has not yet entered your body. It enters your body only at the instant it crosses the epithelium of the mucosa. Until then it is outside, being processed in a controlled external chamber that you happen to carry around with you.

This is not a word game. It has three consequences that run through the whole chapter:

• Contents can be harsh. The stomach can maintain a pH of 1.5, and the colon can host a hundred trillion microorganisms, because neither is in contact with your tissues. Both are in contact with an epithelium whose entire job is to be a wall. When that wall fails — an ulcer, a perforation, an inflamed appendix — you get a catastrophe out of proportion to the size of the hole, because you have connected the outside world to the inside of the abdomen.
• Absorption is a decision, not a leak. Nothing enters passively just because it is in the lumen. It enters because a specific transporter, on a specific cell, in a specific segment, moved it. This is why you absorb iron poorly and glucose almost perfectly, and why a particular drug must be given intravenously.
• Immunity is regional. The gut epithelium is patrolled by more immune tissue than any other site in the body — the mucosa-associated lymphoid tissue of Chapter 20 — precisely because it is the largest and most heavily populated interface between you and the environment. Its default posture is tolerance, not attack, which is the opposite of the posture everywhere else.

   THE ALIMENTARY CANAL AND THE ACCESSORY ORGANS
   ══════════════════════════════════════════════════════════════════════

   ORAL CAVITY ──────────► [ teeth · tongue ]        ACCESSORY (in brackets)
        │  ingestion, mastication,                    deliver secretions
        │  salivary amylase, bolus                    into the tube, and
        │                                             are NOT part of it
        │  ◄──── [ PAROTID · SUBMANDIBULAR ·
        │          SUBLINGUAL glands ]  1.0-1.5 L/day
        ▼
   PHARYNX ─── deglutition, pharyngeal phase (involuntary, CN V,IX,X,XI,XII)
        │
        ▼
   ESOPHAGUS  ~25 cm · pure conduit · NO digestion · NO absorption
        │      upper esophageal sphincter (skeletal)
        │      LOWER esophageal sphincter (smooth) + diaphragmatic crura
        ▼
   ╔═══════════════════════════════════════════╗
   ║  STOMACH   cardia·fundus·body·pylorus     ║   HCl to pH 1.5-2.0
   ║  50 mL empty → 1.5 L comfortable → 4 L    ║   pepsin, intrinsic factor
   ║  churns 3 waves/min · empties in 2-4 h    ║   ~2 L gastric juice/day
   ╚═══════════════════════════════════════════╝
        │  pyloric sphincter — lets through ~3 mL at a time, < 2 mm
        ▼
   ┌───────────────────────────────────────────────────────────────────┐
   │ SMALL INTESTINE  ~3 m living / 6-7 m cadaver                      │
   │   DUODENUM  25 cm  ─◄── [ LIVER 1.4 kg ] ──► bile 600-1200 mL/day │
   │      │                        │                                   │
   │      │                   [ GALLBLADDER ] stores + concentrates    │
   │      │                        │            5-20x, 30-50 mL        │
   │      │             ◄────── [ PANCREAS ] ──► 1.2-1.5 L/day         │
   │      │                     enzymes + HCO3-                        │
   │   JEJUNUM  ~2.5 m   ── MOST ABSORPTION HAPPENS HERE               │
   │   ILEUM    ~3.6 m   ── bile salts, vitamin B12 (terminal ileum)   │
   └───────────────────────────────────────────────────────────────────┘
        │  ileocecal sphincter
        ▼
   ┌───────────────────────────────────────────────────────────────────┐
   │ LARGE INTESTINE  ~1.5 m   cecum → ascending → transverse →        │
   │   descending → sigmoid → rectum → anal canal    [ APPENDIX ]      │
   │   water + electrolyte recovery (~0.9 L/day) · microbiome ·        │
   │   vitamin K and B synthesis · 12-36 h transit                     │
   └───────────────────────────────────────────────────────────────────┘
        │  internal anal sphincter (smooth, involuntary)
        │  external anal sphincter (skeletal, voluntary)
        ▼
   DEFECATION    ~150 g/day · ~100-200 mL water

23.2 One Wall, Modified Six Ways

From the mid-esophagus to the anal canal, the wall of the alimentary canal is built on a single four-layer plan. Learn the plan once, then learn the modifications, and you will never again have to memorize the histology of an individual organ — you will derive it.

The template

1 · Mucosa (innermost — faces the lumen). Three sublayers:

• Epithelium. Stratified squamous where abrasion is the threat (mouth, pharynx, esophagus, anal canal); simple columnar everywhere secretion and absorption are the job (stomach through rectum). Read the epithelium and you know whether that segment's problem is friction or exchange.
• Lamina propria. Loose areolar connective tissue carrying the capillaries that nourish the epithelium and, in the intestine, receive nearly everything absorbed. It also contains the scattered lymphoid follicles of the mucosa-associated lymphoid tissue (MALT), which aggregate into Peyer's patches in the ileum — placed exactly where the bacterial load begins to climb.
• Muscularis mucosae. A thin sheet of smooth muscle that twitches the mucosa itself, throwing it into local folds and ridges. It does not move food along; it wiggles the absorptive surface so that fresh chyme keeps contacting fresh villi. It is the difference between a static filter and an active one.

2 · Submucosa. Dense irregular (moderately dense areolar) connective tissue containing the larger blood vessels, lymphatics, lymphoid follicles, and the submucosal (Meissner) nerve plexus. Its elastic fibres let the tube distend after a meal and recoil afterwards. This is also the layer surgeons care about: it holds the sutures, because it is the strongest layer.

3 · Muscularis externa. Two sheets of smooth muscle at right angles to each other:

• an inner circular layer, whose contraction narrows the lumen;
• an outer longitudinal layer, whose contraction shortens the segment.

Between them lies the myenteric (Auerbach) plexus. Alternate the two layers in a travelling wave and you have peristalsis; contract the circular layer rhythmically at intervals without a travelling wave and you have segmentation. The circular layer thickens at strategic points to form sphincters.

4 · Serosa or adventitia. Where the organ hangs free within the peritoneal cavity, the outermost layer is a serosa — the visceral peritoneum, a sheet of areolar connective tissue covered by a mesothelium that secretes slippery serous fluid (Chapter 1). Where the organ is retroperitoneal or lies outside the peritoneal cavity altogether — the esophagus, the descending and ascending colon, the rectum — it is instead an adventitia, fibrous connective tissue that binds it to surrounding structures. The distinction is clinical: an adventitia tethers, a serosa lubricates and lets the organ slide.

  THE UNIVERSAL FOUR-LAYER WALL — AND ITS ORGAN-SPECIFIC EDITS
  ══════════════════════════════════════════════════════════════════════

        LUMEN  (topologically OUTSIDE the body)
   ═══════════════════════════════════════════════════════════
   ┌──────────┐  1 MUCOSA
   │ epithel. │    epithelium ....... barrier + secretion + absorption
   │ ───────  │    lamina propria ... capillaries · lacteals · MALT
   │ lam.prop │    muscularis mucosae  wiggles the surface
   │ ───────  │
   │ m.mucosae│
   ├──────────┤  2 SUBMUCOSA
   │ vessels  │    big vessels · lymphatics · elastic fibres
   │ SUBMUC.  │    SUBMUCOSAL (MEISSNER) PLEXUS ──► secretion + blood flow
   │ PLEXUS   │
   ├──────────┤  3 MUSCULARIS EXTERNA
   │ CIRCULAR │    inner circular ... narrows lumen · forms sphincters
   │ ~~~~~~~~ │    MYENTERIC (AUERBACH) PLEXUS ──► motility
   │ MYENT.PL │    outer longitudinal  shortens segment
   │ ~~~~~~~~ │
   │ LONGIT.  │
   ├──────────┤  4 SEROSA (visceral peritoneum) — free organs
   │ serosa   │    or ADVENTITIA (fibrous) — esophagus, retroperitoneal
   └──────────┘

   ─── HOW EACH ORGAN EDITS THE TEMPLATE ────────────────────────────────

   ESOPHAGUS    epithelium → STRATIFIED SQUAMOUS (abrasion)
                muscularis → skeletal (upper 1/3) → mixed → smooth
                outer layer → ADVENTITIA (no serosa, it is not in a cavity)
                WHY: a chute. Speed and toughness; no absorption at all.

   STOMACH      + a THIRD muscle layer, INNERMOST OBLIQUE
                epithelium → simple columnar, ALL of it secretory
                mucosa thrown into RUGAE that flatten as it fills
                WHY: it must CHURN in three planes, not just squeeze.

   SMALL INT.   mucosa + submucosa thrown into PLICAE CIRCULARES
                mucosa raised into VILLI; each cell bears MICROVILLI
                lamina propria packed with capillaries + a LACTEAL
                WHY: absorption. Surface area is the entire design brief.

   COLON        NO villi (nothing left to absorb but water and ions)
                goblet cells everywhere — mucus for a drying, solid load
                longitudinal layer reduced to THREE BANDS: TENIAE COLI
                WHY: storage and dehydration, not absorption of nutrients.

   RECTUM/ANUS  epithelium → back to STRATIFIED SQUAMOUS at the anal canal
                circular layer thickened → INTERNAL ANAL SPHINCTER (smooth)
                surrounded by EXTERNAL ANAL SPHINCTER (SKELETAL, voluntary)
                WHY: abrasion again, and conscious control of the exit.

23.3 Control: The Second Brain, the Long Loops, and Six Hormones

The gut is the only organ system that could, in principle, run without the brain. Isolate a segment of intestine, keep it alive in a bath, put food in it, and it will digest and propel that food using its own nervous system. This is not true of the heart (which needs no nerves but has no reflexes either) and is emphatically not true of skeletal muscle.

The enteric nervous system

The enteric nervous system (ENS) is a complete nervous system embedded in the gut wall, containing on the order of 500 million neurons — more than the spinal cord, roughly as many as a cat's entire brain. It contains sensory neurons, interneurons, and motor neurons; it uses more than thirty neurotransmitters, most of which also appear in the CNS; and it can generate reflexes end to end without consulting anyone. Hence the popular label, "the second brain." The label is fair as long as it is understood as an executive system, not a thinking one: it runs digestion, it does not deliberate.

It has two plexuses, and their positions predict their jobs exactly.

Short reflexes and long reflexes

  CONTROL OF DIGESTIVE ACTIVITY — TWO LOOP LENGTHS
  ══════════════════════════════════════════════════════════════════════

           ╔══════════════════════════════════════════╗
           ║   CNS — brainstem (medulla), hypothalamus ║
           ║   sight · smell · thought · emotion ·     ║
           ║   distension reported from far away       ║
           ╚══════════════════════════════════════════╝
              ▲                                    │
    afferent  │  vagal + spinal                    │  efferent
              │                                    ▼
      ┌───────────────┐                  PARASYMPATHETIC          SYMPATHETIC
      │  LONG REFLEX  │                  vagus (CN X) to the      T5-L2 via
      │  (vagovagal,  │                  left colic flexure;      celiac, sup.
      │   enterogastric)                 pelvic splanchnic        + inf. mes.
      └───────────────┘                  (S2-S4) below it         ganglia
              │                                    │                   │
              │                          ▲ STIMULATES:         ▼ INHIBITS:
              │                            motility              motility
              │                            secretion             secretion
              │                            sphincter relax.      + vasoconstricts
              ▼                                    │                   │
   ══════════════════════════════════════════════════════════════════════
     GUT WALL                                      ▼                   ▼
   ┌────────────────────────────────────────────────────────────────────┐
   │  MUCOSA   chemoreceptors (pH, osmolality, nutrients)               │
   │           mechanoreceptors (stretch)  ──┐                          │
   │  ───────────────────────────────────────┼────────────────────────  │
   │  SUBMUCOSAL PLEXUS  ◄───────────────────┤   SHORT REFLEX:          │
   │       │ secretion, blood flow           │   entirely within the    │
   │  ─────┼─────────────────────────────────┤   gut wall. No CNS.      │
   │  MYENTERIC PLEXUS   ◄───────────────────┘   Milliseconds to        │
   │       │ motility                            seconds.               │
   │  ─────▼───────────────────────────────────────────────────────     │
   │  SMOOTH MUSCLE + GLANDS  =  the effectors                          │
   └────────────────────────────────────────────────────────────────────┘
              │
              ▼   and a THIRD control channel, slowest of all:
   ┌────────────────────────────────────────────────────────────────────┐
   │  ENTEROENDOCRINE CELLS in the mucosa release hormones into BLOOD   │
   │  gastrin · secretin · CCK · GIP · motilin · ghrelin · somatostatin │
   │  Minutes. Acts on distant gut, on accessory organs, and on brain.  │
   └────────────────────────────────────────────────────────────────────┘

The division of labour is elegant. Short reflexes handle everything local and immediate: a bolus arrives, the segment above it contracts and the segment below it relaxes. Long reflexes exist for coordination between distant regions and for input the gut cannot sense itself. The stomach cannot see a sandwich; the brain can, and the cephalic phase of gastric secretion is a long reflex from eye to stomach. Conversely, the duodenum can detect that acid is arriving faster than it can neutralize it, and the enterogastric reflex carries that complaint backwards to slow the stomach — a long reflex used as a brake.

Autonomic modulation follows the general rule from Chapter 14, with the digestive tract as its clearest illustration. Parasympathetic activity — "rest and digest" — increases motility and secretion. Sympathetic activity inhibits both and constricts the splanchnic vessels, which is why you lose your appetite when frightened, why a marathon runner's gut shuts down (see the Exercise sidebar in §23.6), and why severe pain or shock can stop peristalsis entirely.

The gastrointestinal hormones

The gut is, by mass, the largest endocrine organ in the body. Its enteroendocrine cells sit singly in the mucosal epithelium with a chemosensitive apical surface in the lumen and a secretory basal surface facing capillaries: a taste bud wired to the bloodstream.

Read the table for its logic rather than as a list. Three of the seven are triggered by what has arrived in the duodenum and every one of those three tells the upstream organs to slow down and the downstream organs to get ready. Secretin says "acid is here — send bicarbonate, and stomach, ease off." CCK says "fat and protein are here — send enzymes, squeeze the gallbladder, and stomach, ease off." GIP says "sugar is here — pancreas, start insulin now, before the glucose arrives." The duodenum is the control room of the entire tract, which is one reason it is only twenty-five centimetres long: it is not there to absorb, it is there to decide.

23.4 Mouth, Pharynx, and Esophagus — and Why Lying Down Undoes It All

The oral cavity

The mouth is a mechanical processor with a chemical starter kit. Its walls are lined by stratified squamous epithelium — keratinized on the gums, hard palate, and dorsal tongue, where friction is greatest, and non-keratinized elsewhere.

The tongue is a mass of interlacing skeletal muscle in three planes. Intrinsic muscles, which begin and end within the tongue, change its shape; extrinsic muscles, which anchor to bone, change its position. Together they perform three jobs: they mix food with saliva and compress it into a bolus; they hold that bolus against the palate and push it back for swallowing; and they position it for speech. Its surface bears papillae — filiform (mechanical, no taste buds), fungiform, vallate, and foliate (all with taste buds; Chapter 15).

The teeth do the actual work of mastication. A human grows two sets: 20 deciduous teeth, erupting between about six and twenty-four months, and 32 permanent teeth. The permanent dental formula, per quadrant, is 2 incisors, 1 canine, 2 premolars, 3 molars — written 2-1-2-3 — and each tooth's shape is a statement about its job. Incisors are chisels for cutting; canines are cones for tearing; premolars and molars have broad cusped surfaces for grinding and crushing.

A tooth is a crown (above the gum, covered in enamel), a neck, and one or more roots (in a bony socket, covered in cementum and anchored by the periodontal ligament — the fibrous joint called a gomphosis). Inside, dentin surrounds a pulp cavity carrying vessels and nerves. Enamel is the hardest substance in the body, about 96–97% calcium phosphate as hydroxyapatite crystals, and it is produced by cells that die when the tooth erupts. Enamel therefore cannot be repaired. This one histological fact explains the entire existence of dentistry.

Saliva and the salivary glands

Three pairs of glands produce 1.0–1.5 litres of saliva per day.

Saliva is 97–99.5% water, slightly hypotonic, with a pH of 6.75–7.00. The rest matters:

• Salivary amylase, which begins starch digestion in the mouth and continues working inside the bolus for perhaps 20–30 minutes in the stomach until acid finally penetrates and denatures it.
• Lingual lipase, secreted by glands on the tongue, which begins fat digestion — a minor contributor in adults, a significant one in infants.
• Mucin, a glycoprotein that, hydrated, becomes the lubricant that lets a dry cracker become a swallowable bolus.
• Lysozyme, IgA, defensins, and lactoferrin — a genuine antimicrobial package. The common observation that animals lick their wounds is not superstition; saliva also contains epidermal growth factor.
• Bicarbonate and phosphate buffers, which neutralize the acids bacteria produce from sugar. This is the direct link between salivary flow and dental caries.

Salivary secretion is almost purely parasympathetic and, unusually, is increased by both divisions of the autonomic system in different ways: parasympathetic stimulation (CN VII to submandibular and sublingual, CN IX to parotid) produces copious watery saliva, while sympathetic stimulation produces a small volume of thick, mucus-rich saliva. That is why fear makes your mouth feel dry — flow drops, viscosity rises — even though secretion has not stopped.

Deglutition

Swallowing is one of the most complex reflexes in the body: about 22 muscle groups in a fixed sequence, coordinated by the deglutition centre in the medulla and lower pons, using cranial nerves V, VII, IX, X, XI, and XII.

Phase 1 · Buccal (voluntary). The tongue tip presses against the hard palate and the tongue contracts from front to back, forcing the bolus into the oropharynx. This is the last voluntary act. Everything after it is committed.

Phase 2 · Pharyngeal–esophageal (involuntary). Once the bolus contacts receptors in the oropharynx, the following happens in under a second:

1. The soft palate and uvula elevate, sealing the nasopharynx. (Fail here and you get liquid out of your nose.)
2. The larynx elevates and moves anteriorly; the epiglottis folds back over the laryngeal inlet; the vocal folds adduct. Three separate mechanisms guard one opening — a level of redundancy that tells you how expensive a mistake would be.
3. Breathing stops — deglutition apnea — for 0.5–1.5 seconds, enforced by the medullary centre that also runs respiration.
4. The upper esophageal sphincter (the cricopharyngeus, skeletal muscle) relaxes.
5. A peristaltic wave carries the bolus down. Liquids reach the stomach in about 1 second, largely by gravity; solids take 4–8 seconds and require peristalsis. In the absence of gravity — lying flat, or upside down — peristalsis alone still succeeds, which is a good demonstration that the esophagus is an active pump and not a drainpipe.

The esophagus and its two sphincters

The esophagus is a 25 cm muscular tube running from the cricoid cartilage at C6, through the mediastinum posterior to the trachea, and through the esophageal hiatus of the diaphragm at the level of T10 to reach the stomach at about T11. Its muscularis externa is skeletal in the upper third, mixed in the middle, and smooth in the lower third — a smooth handover from voluntary to involuntary control. It has an adventitia, not a serosa.

The upper esophageal sphincter is straightforward: a ring of skeletal muscle, tonically contracted, that opens on command during swallowing and keeps air out of the esophagus during breathing.

The lower esophageal sphincter (LES) is not so simple, and it is the answer to Case File question 3.

The antireflux barrier has four components

1 · Intrinsic LES tone. The distal 2–4 cm of esophageal smooth muscle maintains a resting pressure of about 10–30 mm Hg above intragastric pressure. It relaxes for swallowing, and it relaxes at other times too — these transient lower esophageal sphincter relaxations (TLESRs) are a normal vagovagal reflex triggered by distension of the gastric fundus, and they exist to let you belch. They are also the single commonest mechanism of reflux, in health and in disease.

2 · The diaphragmatic crura. The esophagus does not pass through a simple hole. It passes through a sling formed mainly by the right crus of the diaphragm, whose fibres wrap around it and act as an external sphincter. Every time you inhale, cough, strain, or lift, the diaphragm contracts and this sling pinches the esophagus shut — precisely at the moments when abdominal pressure is highest. The internal and external sphincters are anatomically superimposed and their pressures summate.

3 · The intra-abdominal segment. Two to four centimetres of esophagus lie below the diaphragm, inside the abdominal cavity. This is a flutter-valve arrangement of some sophistication: any rise in intra-abdominal pressure presses on the outside of this segment at the same time as it presses on the stomach. The squeeze on the tube rises exactly in step with the pressure trying to push contents up it, so the barrier is self-adjusting. The phrenoesophageal ligament anchors this segment below the diaphragm.

4 · The angle of His. The esophagus does not enter the stomach end-on. It enters at an acute angle — normally under 60° — between the left side of the esophagus and the fundus. The fundus is a dome that balloons up above the entry point. When the fundus fills, it presses the flap of tissue at the angle against the esophageal opening: a flap valve that closes harder the fuller the stomach gets. In a hiatal hernia, part of the stomach slides up through the hiatus into the chest, and this angle is straightened — flattened toward 90° or more. The flap valve is destroyed, the crural sling no longer coincides with the LES, and the intra-abdominal segment is lost. All three anatomical components fail at once, which is why hiatal hernia and reflux travel together.

  THE ANTIREFLUX BARRIER — AND WHAT LYING DOWN AFTER A LARGE MEAL DOES
  ══════════════════════════════════════════════════════════════════════

  UPRIGHT, STOMACH MODERATELY FULL            LYING FLAT, STOMACH FULL
  ─────────────────────────────────           ─────────────────────────────
        ESOPHAGUS                                    ESOPHAGUS
            ║                                    ═══════════════
            ║                                          ▲  ▲  ▲
   ┌────────╫────────┐  DIAPHRAGM         ┌─────────────╫──────────┐
   │  right crus ►◄  │  crural sling      │  crus ►◄ (relaxed in   │
   └────────╫────────┘  pinches           └──────────  quiet supine│
            ║                                          breathing)  │
      angle │ of His  < 60°                                        │
       ╱────╨────╲                            ╔══════════════════════════╗
      ╱  FUNDUS   ╲  gas bubble sits          ║  ACID POCKET now lies    ║
     │   (gas)     │ HERE, above the          ║  directly ON the         ║
     │  ▲▲▲▲▲▲▲    │ liquid                   ║  cardia. Angle of His    ║
     ├─────────────┤                          ║  flattened toward 90°.   ║
     │ ~~~~~~~~~~~ │ ACID + FOOD              ║  ~~~~~~~~~~~~~~~~~~~~~~  ║
     │ ~~~~~~~~~~~ │ liquid level is          ║  ~~~ acid + food ~~~~~~  ║
     │ ~~~~~~~~~~~ │ BELOW the cardia         ╚══════════════════════════╝
     │ ~~~~~~~~~~~ │ — gravity assists
     └─────────────┘

   FOUR BARRIERS            UPRIGHT + SMALL MEAL      SUPINE + LARGE MEAL
   ──────────────────────   ─────────────────────     ──────────────────────
   1 LES intrinsic tone     10-30 mm Hg, few TLESRs   fundic distension
                                                       TRIGGERS TLESRs by
                                                       vagovagal reflex ▼▼▼
   2 Crural sling           reinforced by every        quiet supine breathing
                            inspiration, cough, lift   = minimal reinforcement
   3 Intra-abdominal seg.   abdominal pressure         same pressure, but now
                            squeezes tube AND stomach  the column of acid sits
                            equally — self-adjusting   ON the junction
   4 Angle of His           acute; fundic gas presses  flattened; liquid, not
                            the flap valve SHUT        gas, contacts the cardia

   AND: gravity, which normally keeps the fluid level below the junction,
        is removed entirely. Emptying is also slowed by a large, fatty meal
        (CCK + enterogastric reflex), so the reservoir stays full for hours.

23.5 The Stomach: A Reservoir That Makes Acid

Gross anatomy

The stomach lies in the left upper quadrant, largely under the diaphragm and behind the ribs and the left lobe of the liver. It has four regions:

• Cardia — the small area surrounding the esophageal opening.
• Fundus — the dome that bulges superolaterally, above the level of the cardia. In an upright person it holds the swallowed gas bubble. It is also the compliance organ: it relaxes to accept volume without raising pressure.
• Body — the large central portion; the main reservoir and the site of the acid-producing glands.
• Pyloric part — the funnel-shaped outlet, comprising the pyloric antrum, the pyloric canal, and the pylorus itself, whose thickened circular muscle is the pyloric sphincter. This is where the gastrin-producing G cells live, and where the grinding is done.

The concave right border is the lesser curvature, from which the lesser omentum runs to the liver; the convex left border is the greater curvature, from which the greater omentum drapes down over the intestines like a fatty apron — a structure with a genuine function, since it migrates to and walls off sites of inflammation, and is sometimes called the abdominal policeman.

When empty, the mucosa and submucosa are thrown into large longitudinal folds, the rugae, which flatten out as the organ fills. They allow the stomach to expand from roughly 50 mL empty to 1.5 L comfortably — and up to about 4 L — without any true stretching of the epithelium, which could not survive it.

The third muscle layer

The stomach is the only part of the alimentary canal with three layers of muscularis externa: the standard circular and longitudinal layers plus an innermost oblique layer, strongest over the body and fundus. A two-layer tube can squeeze and shorten. Add a third, diagonally oriented layer and the organ can also twist and shear. That is what churning is — mechanical breakdown in three planes at once, which is why the stomach reduces a swallowed piece of meat to a slurry in a way no amount of peristalsis alone could.

The gastric glands and their five cell types

The entire gastric mucosa is a secretory epithelium. Its surface is pitted with millions of gastric pits, each leading down into two to seven gastric glands. The cells lining them change with region, but in the fundus and body — the acid-producing zone — there are five types.

Two of those entries deserve emphasis before we go on. Intrinsic factor is the only gastric secretion that is strictly indispensable: without it, vitamin B₁₂ cannot be absorbed in the terminal ileum, and the eventual result is pernicious anemia and irreversible neurological damage. You can live without stomach acid. You cannot live without intrinsic factor, or without an injection to replace what it would have delivered.

And the 3–6 day turnover is not trivia. It is a load-bearing part of the mucosal barrier: the stomach's answer to chemical injury is to throw away the damaged cells faster than the acid can kill them.

How the parietal cell makes acid

The parietal cell is one of the most metabolically extreme cells in the body. It contains more mitochondria per unit volume than almost any other, and when stimulated it undergoes a dramatic morphological change: cytoplasmic tubulovesicles, loaded with proton pumps, fuse with the apical membrane and unfold it into deep secretory canaliculi, multiplying the apical surface several-fold within minutes. It builds the machinery only when told to.

The chemistry is a beautiful piece of accounting. The cell does not conjure hydrogen ions; it splits carbonic acid and disposes of both halves in opposite directions.

1. CO₂ + H₂O → H₂CO₃, catalyzed by carbonic anhydrase inside the cell.
2. H₂CO₃ dissociates into H⁺ and HCO₃⁻.
3. The H⁺/K⁺ ATPase — the proton pump — in the apical membrane pumps H⁺ into the gland lumen in exchange for K⁺, using ATP directly. This pump is the only one in the body that can move H⁺ against a million-fold gradient: intracellular pH 7.2 against luminal pH 1.0.
4. The HCO₃⁻ leaves through the basolateral membrane in exchange for Cl⁻. That bicarbonate enters the blood, which is why venous blood leaving the stomach after a meal is transiently alkaline — the alkaline tide.
5. The imported Cl⁻ exits apically through a chloride channel, following H⁺ electrically. H⁺ and Cl⁻ combine in the lumen as HCl.
6. K⁺ recycles back out through an apical channel so the pump never runs out of substrate.

Three signals stimulate the parietal cell, each through its own receptor:

• Acetylcholine from vagal and enteric neurons, on M₃ receptors;
• Gastrin from antral G cells, on CCK-B receptors;
• Histamine from ECL cells, on H₂ receptors.

These are not additive but synergistic: histamine is the final common amplifier, and both ACh and gastrin work partly by causing ECL cells to release histamine. That single fact is why an H₂ blocker such as famotidine suppresses acid secretion driven by all three pathways, and it is the pharmacological logic behind an entire drug class.

  THE GASTRIC GLAND, AND THE PARIETAL CELL'S ACID PUMP
  ══════════════════════════════════════════════════════════════════════

   LUMEN  pH 1.5-2.0            MUCUS GEL LAYER 200-600 um thick
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   ░░░░░░░ pH 7 AT THE CELL SURFACE — a 6-unit gradient across 0.5 mm ░░
   ┌──┬──┬──┬──┬──┬──┬──┬──┐  SURFACE MUCOUS CELLS
   │  │  │  │  │  │  │  │  │  alkaline bicarbonate-rich mucus
   └┬─┴──┴──┴──┴──┴──┴──┴─┬┘
    │   GASTRIC PIT       │
    │                     │      STEM CELLS at the isthmus →
    ├─── MUCOUS NECK ─────┤      whole surface replaced in 3-6 DAYS
    │    CELLS            │
    │  ╭───────────╮      │
    │  │ PARIETAL  │ ─────┼───► HCl  +  INTRINSIC FACTOR
    │  │   CELL    │      │
    │  ╰───────────╯      │      stimulated by: ACh (M3) · gastrin
    │  ╭───────────╮      │                     (CCK-B) · HISTAMINE (H2)
    │  │  ECL cell │──────┼───► histamine ──► amplifies the other two
    │  ╰───────────╯      │
    │  ╭───────────╮      │
    │  │ CHIEF     │──────┼───► PEPSINOGEN (inactive) + gastric lipase
    │  │  CELL     │      │
    │  ╰───────────╯      │
    │  ╭───────────╮      │      G cell → GASTRIN (antrum)
    │  │ ENTERO-   │──────┼───►  D cell → SOMATOSTATIN (the OFF switch)
    │  │ ENDOCRINE │      │
    │  ╰───────────╯      │
    └─────────────────────┘

   ── INSIDE ONE PARIETAL CELL ──────────────────────────────────────────

     BLOOD side (basolateral)          LUMEN side (apical canaliculus)

        HCO3- ──────►  out             ◄────  K+   (recycles)
          ▲   exchanged for Cl-                ▲
          │                                    │  ATP
        H2CO3                          ╔═══════╧════════╗
          ▲   carbonic anhydrase       ║  H+/K+ ATPase  ║ ──► H+  ──┐
      CO2 + H2O                        ║ "PROTON PUMP"  ║           │
          ▲                            ╚════════════════╝           ▼
     from metabolism                                              H+ Cl-
                                        Cl- ────────────────────►  = HCl
        Cl- ──────►  in                                          pH 1.0-2.0
                                       (moves H+ against a MILLION-fold
                                        gradient: pH 7.2 inside → 1.0 out)

     ALKALINE TIDE: the HCO3- leaving basolaterally makes gastric venous
     blood transiently alkaline after a meal. Nothing is created; the cell
     splits carbonic acid and sends the halves in opposite directions.

Pepsinogen and the zymogen principle

Chief cells secrete pepsinogen, which is inactive. In the acid of the gastric lumen, a 42-amino-acid fragment is cleaved off and the molecule becomes pepsin, an endopeptidase with a pH optimum of 1.8–3.5. Once a little pepsin exists it cleaves more pepsinogen — autocatalysis — so the conversion is explosive once started.

This is the zymogen principle, and it recurs throughout the chapter and throughout the book: any enzyme capable of digesting the cell that makes it is synthesized in an inactive form and activated only after it has left. Pepsinogen in the chief cell is harmless. Pepsinogen in the lumen becomes pepsin in seconds. And note the elegant safety interlock: pepsin requires acid, so it can only become active where acid is, which is only in the lumen. Move pepsin to the duodenum, where secretin has summoned bicarbonate and pH is 7–8, and it is irreversibly inactivated. The enzyme is confined to its intended workspace by chemistry alone.

The three phases of gastric secretion

About 2 litres of gastric juice are produced each day, and its production is anticipated, stimulated, and then shut off in three overlapping phases.

Cephalic (reflex) phase — 20–30% of the response, before food arrives. Sight, smell, taste, and even the thought of food activate the hypothalamus and the vagal nuclei; efferent vagal fibres release ACh and gastrin-releasing peptide in the stomach wall, priming acid and pepsinogen secretion. Its whole purpose is to have the acid waiting. It is abolished by loss of appetite, by depression, and by vagotomy.

Gastric phase — 60–70%, lasting 3–4 hours. Three stimuli, all local:

• Distension activates stretch receptors, driving both short (myenteric) and long (vagovagal) reflexes.
• Peptides and amino acids in the lumen directly stimulate G cells to release gastrin.
• Rising pH. This one is counterintuitive and important. Food, especially protein, is a buffer; when it arrives, luminal pH climbs from 1.5 toward 4 or 5. That removes the inhibition on the G cell, and gastrin is released. Acid secretion is therefore triggered, in part, by the disappearance of acid.

The inhibitory arm of the gastric phase is the mirror image: as acid accumulates and pH falls below about 2, D cells release somatostatin, which shuts off the G cell and directly inhibits the parietal cell. This is a textbook negative feedback loop with pH as the regulated variable, the D cell as the receptor and control centre, and the parietal cell as the effector.

Intestinal phase — a brief stimulatory arm and a dominant inhibitory one. When partially digested protein first enters the duodenum, duodenal G cells release a small amount of intestinal gastrin, briefly encouraging the stomach to continue. That lasts minutes. Then the duodenum begins to protest, and the protest is emphatic. Acid (pH < 2), fat, hypertonic chyme, or distension in the duodenum triggers:

• the enterogastric reflex — short and long neural pathways that inhibit vagal stimulation, excite sympathetic fibres, and inhibit the myenteric plexus; and
• the enterogastrones — secretin, CCK, and GIP — which inhibit gastric secretion and, crucially, gastric emptying.

Gastric motility and emptying

Interstitial cells of Cajal in the greater curvature act as the stomach's pacemaker, generating a basic electrical rhythm of about 3 waves per minute that spreads through the gap-junction-coupled smooth muscle. Whether a given wave produces a contraction depends on neural and hormonal input riding on top of it — but the rhythm is intrinsic and never stops.

Two motility behaviours matter:

Receptive relaxation and accommodation. As you swallow, a vagovagal reflex releases nitric oxide and VIP in the fundus, which relaxes. The stomach accepts a litre with almost no rise in intragastric pressure. (This is also the reflex whose failure after certain operations produces early satiety, and whose over-activity — fundic distension — triggers the TLESRs of §23.4.)

Propulsion, grinding, and retropulsion. Peristaltic waves begin weakly in the fundus and intensify enormously toward the pylorus. When a strong wave arrives, the pyloric sphincter is already closing. Only about 3 mL of the most liquid chyme squirts through; the rest slams into the closed sphincter and is thrown violently backwards into the body of the stomach — retropulsion. This is the grinder. Solid particles are not allowed into the duodenum until they are smaller than about 2 mm.

Emptying takes roughly 2–4 hours for a mixed meal, and the composition determines the rate: liquids leave fastest and in an exponential pattern; carbohydrate meals empty in 2–3 hours; protein slower; fat slowest of all, up to 6 hours, because of the CCK-mediated brake. A large, fatty meal at 08:00 is still substantially present at 10:00 — which matters to Amara, who is asleep and horizontal for all of it.

The mucosal barrier — and Case File questions 1 and 2

Here is the problem the stomach has solved. It secretes a fluid capable of dissolving metal and an enzyme capable of digesting protein, and its own wall is made of protein and sits in direct contact with both. Something must be protecting it, and that something has five components.

1 · The mucus gel layer. Surface mucous cells secrete a thick, insoluble, adherent gel 200–600 µm deep. Its critical property is not thickness but that it is unstirred: it does not mix. Bicarbonate secreted by the same cells is trapped within it and does not wash away, so a stable pH gradient exists across the gel — pH 1.5–2.0 at its luminal face, pH ~7 at the cell surface. Any H⁺ that starts to diffuse inward is neutralized on the way. Pepsin, a large molecule, cannot penetrate the gel at all, and in any case would be inactivated by the neutral pH it would meet if it did.

2 · Tight junctions. The epithelial cells are welded to one another by tight junctions (Chapter 3) that prevent acid from leaking between cells into the lamina propria. The barrier is only as good as its seals.

3 · Rapid epithelial replacement and restitution. The surface epithelium is completely renewed every 3–6 days. Faster still is restitution: when a few surface cells are killed, neighbouring cells flatten and migrate sideways to cover the gap within minutes, before any new cells are made. Small injuries are resurfaced almost as fast as they occur.

4 · Mucosal blood flow. The gastric mucosa receives a generous blood supply whose job is partly metabolic and partly janitorial: it carries away the small quantity of H⁺ that does back-diffuse, delivers the bicarbonate the surface cells need, and supplies the oxygen and nutrients that restitution and cell division require. When mucosal blood flow falls, every one of the other four defences degrades.

5 · Prostaglandins. And here is the keystone. The gastric mucosa constitutively expresses cyclooxygenase-1 (COX-1), which converts arachidonic acid into prostaglandins E₂ and I₂. These prostaglandins:

• stimulate mucus secretion (component 1),
• stimulate bicarbonate secretion (component 1),
• maintain mucosal blood flow by vasodilating submucosal arterioles (component 4),
• promote epithelial restitution and proliferation (component 3), and
• modestly inhibit acid secretion by the parietal cell.

Three of the five defences are prostaglandin-dependent, and the fourth is prostaglandin-assisted.

23.6 The Small Intestine: An Organ Built Entirely Around Surface Area

Almost all chemical digestion and virtually all nutrient absorption occur here, in three regions with a division of labour.

Bile and pancreatic juice arrive together at the hepatopancreatic ampulla, opening at the major duodenal papilla about 7–10 cm beyond the pylorus and guarded by the sphincter of Oddi, which CCK relaxes.

Three tricks, multiplied

A smooth cylinder 3 m long and 2.5 cm across offers about 0.4 m² of surface. That is not remotely enough to absorb a meal in the four hours available. So the small intestine amplifies it three times over, at three different scales.

  THREE AMPLIFICATIONS OF SURFACE AREA  —  0.4 m² BECOMES ~200 m²
  ══════════════════════════════════════════════════════════════════════

  SCALE 1 — centimetres        PLICAE CIRCULARES  (circular folds)
  ┌──────────────────────┐     Permanent folds of MUCOSA + SUBMUCOSA,
  │ /\  /\  /\  /\  /\   │     up to 1 cm tall. NOT flattened by
  │/  \/  \/  \/  \/  \  │     distension (unlike gastric rugae).
  └──────────────────────┘     They also force chyme to SPIRAL,
                               slowing transit and improving mixing.
                                                          ×3
  SCALE 2 — millimetres        VILLI
   ∩ ∩ ∩ ∩ ∩ ∩ ∩ ∩ ∩ ∩         Finger-like projections of MUCOSA only,
   │ │ │ │ │ │ │ │ │ │         ~1 mm tall, 20-40 per mm².
   └─┴─┴─┴─┴─┴─┴─┴─┴─┘         Each has its own capillary bed + lacteal.
                                                          ×10
  SCALE 3 — micrometres        MICROVILLI  ("brush border")
   ||||||||||||||||||||        1 um tall, ~3,000 per cell,
   ||||||||||||||||||||        ~200 million per mm².
   ┌──────────────────┐        Their membrane CARRIES the final
   │  ENTEROCYTE      │        digestive enzymes — the brush border
   └──────────────────┘        enzymes are not secreted, they are
                               part of the wall.
                                                          ×20
  ─────────────────────────────────────────────────────────────────────
       0.4 m²  ×  3  ×  10  ×  20   =   ×600   ≈   200 m²
       a bare tube    folds  villi  microvilli   ≈ a tennis court
  ─────────────────────────────────────────────────────────────────────

The villus and the two routes out

Each villus is a projection of mucosa with a core of lamina propria. Its epithelium is simple columnar: enterocytes (absorptive cells, brush-bordered) plus scattered goblet cells. Inside the core sit the two exit routes, and which one a nutrient takes depends entirely on its size and solubility:

• a dense capillary network, draining into venules → the hepatic portal vein → the liver. Everything water-soluble takes this route, and everything on this route is inspected by the liver before it reaches the rest of the body.
• a single blind-ended lymphatic capillary, the lacteal, draining into lymphatics → the cisterna chyli → the thoracic duct → the left subclavian vein. Lipids take this route, and it bypasses the liver's first pass entirely.

Strands of smooth muscle from the muscularis mucosae run into each villus and contract rhythmically, shortening and waving it, which milks the lacteal empty and stirs the layer of fluid immediately around the microvilli.

Between the villi are the crypts (intestinal glands, of Lieberkühn), which contain:

• stem cells, dividing continuously — the entire epithelium is replaced every 3–6 days, the fastest turnover in the body, which is both a defence against chemical injury and the reason chemotherapy causes mucositis and diarrhea;
• Paneth cells at the base, secreting lysozyme, defensins, and phospholipase A₂ — an antimicrobial garrison stationed beside the stem cells they exist to protect;
• enteroendocrine cells, the source of secretin, CCK, and GIP.

The duodenum adds one unique structure: Brunner's glands in the submucosa, the only submucosal glands in the entire tract, which secrete a viscous alkaline mucus in response to acid arriving from the stomach. They are the first line of duodenal defence, and they are most numerous in the first few centimetres — exactly where the acid lands.

Intestinal juice amounts to 1–2 L/day, is mostly water and mucus, and is alkaline (pH 7.4–7.8). Note what it does not contain: enzymes. The brush border enzymes are membrane-anchored, not secreted, so digestion's final step happens while the substrate is already in contact with the cell that will absorb the product.

Motility

• Segmentation is the dominant pattern after a meal. Rings of circular muscle contract and relax at intervals without a travelling wave, chopping and mixing chyme against the absorptive surface. Its frequency is set by the pacemaker gradient: about 12–16/min in the duodenum falling to 8–9/min in the ileum. Because contractions are more frequent upstream, the net effect of pure mixing is nonetheless a slow drift downstream.
• Peristalsis takes over as absorption completes, moving residue toward the ileocecal valve. Total small intestinal transit is 3–6 hours.
• The migrating motor complex (MMC) operates only in the fasting state. Every 90–120 minutes, driven by motilin, a powerful wave sweeps from stomach to ileum over about 90 minutes, clearing residue, mucus, and bacteria. It is the housekeeper, and eating abolishes it instantly. Its loss — as in some cases of diabetic autonomic neuropathy — permits small intestinal bacterial overgrowth, which is a good demonstration that motility is itself an antimicrobial defence.

23.7 The Accessory Organs: Liver, Gallbladder, Pancreas

The liver

The liver weighs about 1.4 kg, sits in the right upper quadrant under the diaphragm, and has four lobes (right, left, caudate, quadrate) separated superficially by the falciform ligament, in whose free edge runs the round ligament — the fibrous remnant of the fetal umbilical vein.

The lobule. The functional unit is the hepatic lobule: a hexagonal prism about 1–2 mm across with a central vein running down its axis and portal triads at its corners. Plates of hepatocytes, one to two cells thick, radiate outward from the central vein like the spokes of a wheel. Between the plates run sinusoids — leaky capillaries whose endothelium is fenestrated and lacks a basement membrane, so that plasma has direct access to the hepatocyte surface across the space of Disse. Resident Kupffer cells (stellate macrophages) sit in the sinusoidal lumen filtering portal blood, and hepatic stellate (Ito) cells in the space of Disse store vitamin A — and, when chronically activated, produce the collagen of cirrhosis.

Two fluids, opposite directions. This is the fact to hold on to:

• Blood flows inward, from the portal triads at the periphery, through the sinusoids, to the central vein — then to hepatic veins and the inferior vena cava.
• Bile flows outward, from bile canaliculi (tiny channels formed between adjacent hepatocyte membranes, sealed by tight junctions) at the centre, toward bile ductules in the portal triads — then to hepatic ducts and the gut.

Every hepatocyte therefore has a blood face and a bile face, and it moves material from one to the other. That is what a hepatocyte is.

The dual blood supply. The liver receives roughly 1.5 L/min, about 25% of cardiac output, from two sources:

• the hepatic portal vein, ~75% of the flow and ~50% of the oxygen, carrying nutrient-rich, partly deoxygenated blood collected from the entire gut, spleen, and pancreas;
• the hepatic artery proper, ~25% of the flow and ~50% of the oxygen.

The hepatic portal system is the whole reason the liver's functions are what they are. A portal system is one in which blood passes through two capillary beds in series before returning to the heart: gut capillaries → portal vein → hepatic sinusoids → hepatic vein. Everything absorbed by the gut into blood — every nutrient, every toxin, every oral drug — is therefore delivered to the liver first, at high concentration, before the rest of the body sees any of it. This is first-pass metabolism, and it is why some drugs must be given by injection, and why the liver, not the kidney, is the body's chief chemical processing plant.

The liver's roughly 500 jobs, in four groups:

Two clinical inferences fall straight out of that table. A failing liver produces a low albumin (hence edema and ascites) and a prolonged prothrombin time (hence bleeding) — which is why the INR is a liver test, and why hepatic and hematologic failure look so similar from the outside. And because P450 activity is concentrated in the pericentral hepatocytes farthest from the oxygen supply, poisoning by a substance that requires P450 activation (acetaminophen is the classic) produces necrosis in a distinctive centrilobular pattern — the anatomy of the lobule written into the pathology.

  THE HEPATIC LOBULE — BLOOD IN, BILE OUT, IN OPPOSITE DIRECTIONS
  ══════════════════════════════════════════════════════════════════════

        PORTAL TRIAD (at each of the six corners)
        ┌──────────────────────────────┐
        │ • hepatic ARTERY branch  ──► │  O2-rich, 25% flow / 50% O2
        │ • PORTAL VEIN branch     ──► │  nutrient-rich, 75% flow / 50% O2
        │ • BILE DUCT              ◄── │  bile leaving
        └──────────────────────────────┘

                       BLOOD  ─────►  IN     BILE  ◄─────  OUT
              ╱────────────────────────────────────────────╲
        triad ●═══════════════════════════════════════════● triad
              ║  ►►►  hepatocyte plate  ►►►  ◄◄◄ bile ◄◄◄ ║
              ║  ┌───┐┌───┐┌───┐┌───┐┌───┐┌───┐┌───┐      ║
              ║  │ H ││ H ││ H ││ H ││ H ││ H ││ H │      ║
              ║  └───┘└───┘└───┘└───┘└───┘└───┘└───┘      ║
              ║   ~~~~ SINUSOID ~~~~ (Kupffer cells) ~~~  ║
              ║        ▼                    ▼             ║
              ║      ╔═══════════════════════════╗        ║
        triad ●      ║      CENTRAL VEIN         ║        ● triad
              ║      ║   ──► hepatic vein ──►    ║        ║
              ║      ║       INFERIOR VENA CAVA  ║        ║
              ║      ╚═══════════════════════════╝        ║
              ║  ┌───┐┌───┐┌───┐┌───┐┌───┐┌───┐┌───┐      ║
              ║  │ H ││ H ││ H ││ H ││ H ││ H ││ H │      ║
              ║  └───┘└───┘└───┘└───┘└───┘└───┘└───┘      ║
        triad ●═══════════════════════════════════════════● triad
              ╲────────────────────────────────────────────╱

   Between adjacent hepatocytes: BILE CANALICULI — a separate, sealed
   channel network running the OPPOSITE way to blood. Tight junctions
   keep bile out of the sinusoid. Break them and bile enters blood:
   JAUNDICE.

   ZONE 1 (periportal)  best oxygenated — gluconeogenesis, urea cycle
   ZONE 3 (pericentral) least oxygenated — P450 detox; dies FIRST in
                        ischemia and in acetaminophen poisoning

Bile, the gallbladder, and enterohepatic circulation

The liver secretes 600–1,200 mL of bile per day. Its components:

• Bile salts — the working ingredient. Synthesized from cholesterol as cholic and chenodeoxycholic acid, conjugated to glycine or taurine. They are amphipathic: a hydrophobic face and a hydrophilic face on the same molecule, which is precisely what an emulsifier must be. They are not enzymes and digest nothing.
• Bilirubin — a waste product of heme breakdown (Chapter 17), conjugated in the liver and excreted. Bacteria in the colon convert it to urobilinogen and then stercobilin, which makes stool brown. When bile cannot reach the gut, stools turn pale clay-grey.
• Cholesterol, phospholipids (lecithin), electrolytes, and water.

Bile is secreted continuously but needed intermittently, so between meals the sphincter of Oddi is closed and bile backs up into the gallbladder, a 30–50 mL sac that concentrates it 5- to 20-fold by absorbing water and ions. When fat reaches the duodenum, CCK contracts the gallbladder and relaxes the sphincter of Oddi, and concentrated bile is delivered exactly when and where it is needed.

Enterohepatic circulation is the recycling loop that makes this affordable. Synthesizing bile salts from cholesterol is expensive, and the body's entire pool is only about 2–4 g — far less than a single meal requires. So bile salts are reabsorbed: about 95% are recovered by an active Na⁺-coupled transporter in the terminal ileum, returned to the liver in the portal vein, extracted by hepatocytes, and re-secreted. The pool recirculates 2–3 times per meal and 6–8 times a day, and only 0.2–0.5 g/day is lost in stool and replaced by new synthesis.

Two clinical predictions follow immediately. Resect or inflame the terminal ileum (as in Crohn's disease) and bile salts spill into the colon, where they irritate the mucosa and cause secretory diarrhea, while the shrinking pool causes fat malabsorption. And because the only route of cholesterol excretion from the body is as bile salts and biliary cholesterol, interrupting the loop with a bile-acid-binding resin forces the liver to make new bile salts from cholesterol — which lowers plasma cholesterol. A recycling loop, used as a drug target.

The pancreas

The pancreas is both an endocrine gland (the islets, Chapter 16) and, by mass, overwhelmingly an exocrine one. Its acini secrete 1.2–1.5 L/day of pancreatic juice, and it comes in two distinguishable parts from two different cell types under two different hormones:

• Acinar cells → enzymes, stimulated by CCK (and vagal ACh).
• Duct cells → water and bicarbonate, stimulated by secretin. The bicarbonate is exported through a Cl⁻/HCO₃⁻ exchanger working in tandem with the CFTR chloride channel — which is why cystic fibrosis, a CFTR disease, causes pancreatic insufficiency.

The bicarbonate matters as much as the enzymes. Chyme arrives at pH 2; pancreatic juice at pH 7.1–8.2 neutralizes it to about pH 6–7, which stops pepsin dead and creates the alkaline conditions every pancreatic enzyme requires.

The enzymes:

Note the pattern: every protease is a zymogen; nothing else is. Amylase, lipase, and nucleases are secreted active because the pancreas contains little starch, its lipid is in membranes rather than free triglyceride, and its nucleic acids are inside nuclei. It contains enormous quantities of protein, so proteases — and only proteases — must be muzzled.

Activation happens outside the gland. The duodenal brush border carries enteropeptidase (enterokinase), which cleaves trypsinogen to trypsin — and trypsin then activates chymotrypsinogen, proelastase, procarboxypeptidase, procolipase, and more trypsinogen. One enzyme, anchored to the intestinal wall, is the master switch for the entire cascade, and it exists nowhere else. Three further safeguards back it up: the zymogens are packaged in granules separate from the cytosol; the granules contain a trypsin inhibitor that neutralizes any trypsin activated prematurely; and the pancreatic duct is kept flushed at high pressure.

Acute pancreatitis is what happens when those safeguards fail — a gallstone obstructing the ampulla, or the metabolic injury of alcohol — and trypsin is activated within the gland. The cascade then runs in the wrong place, and the organ digests itself and the fat around it. The zymogen principle is not academic; it is the difference between an enzyme and a weapon.

23.8 Chemical Digestion and Absorption, Nutrient by Nutrient

Digestion is hydrolysis: a water molecule is inserted across a bond and a polymer becomes two smaller pieces. Absorption is transport: a monomer crosses two membranes of an enterocyte and enters blood or lymph. Every specific case below is a variation on those two sentences.

  DIGESTION BY NUTRIENT CLASS — ENZYME · SOURCE · SITE · PRODUCT
  ══════════════════════════════════════════════════════════════════════

  CARBOHYDRATE  (starch, sucrose, lactose)
   salivary amylase   salivary glands  MOUTH → stomach   oligosaccharides
   pancreatic amylase pancreas         SMALL INTESTINE   maltose, maltotriose,
                                                         a-limit dextrins
   maltase/sucrase-   BRUSH BORDER     small intestine   GLUCOSE, GALACTOSE,
   isomaltase/lactase (membrane-bound)                   FRUCTOSE
        │
        └──► glucose + galactose: SGLT1 (Na+-coupled, ACTIVE) ─┐
             fructose:            GLUT5 (facilitated)          ├─► GLUT2 ──►
                                                               │   CAPILLARY
  PROTEIN                                                      │   → PORTAL
   pepsin            chief cells       STOMACH (pH 1.8-3.5)    │     VEIN
   trypsin/chymo-    pancreas          small intestine         │
   trypsin/elastase  (as ZYMOGENS)                             │
   carboxypeptidase  pancreas          small intestine         │
   amino- + di-      BRUSH BORDER      small intestine   AMINO ACIDS +
   peptidases                                            DI/TRIPEPTIDES
        │                                                      │
        └──► amino acids: Na+-coupled carriers ────────────────┤
             di/tripeptides: PepT1 (H+-coupled) ───────────────┘

  LIPID  (triglyceride)
   lingual/gastric   tongue, stomach   mouth, stomach    ~10-15% of TAG
    lipase
   BILE SALTS        liver             small intestine   EMULSIFICATION
    (not an enzyme)                                      (surface area ↑↑)
   pancreatic lipase pancreas          small intestine   2-MONOGLYCERIDE
    + colipase                                           + 2 FATTY ACIDS
        │
        └─► MICELLES ferry products across the unstirred water layer
            → enterocyte re-esterifies to TAG in smooth ER
            → packages with apoB-48 = CHYLOMICRON (75-1200 nm)
            → TOO BIG for the capillary basement membrane
            → enters the LACTEAL ──► thoracic duct ──► subclavian vein
            (short/medium-chain FAs, <12 C: straight to the PORTAL VEIN)

  NUCLEIC ACID
   pancreatic RNase, DNase           small intestine     nucleotides
   brush border nucleosidases,       small intestine     pentose + phosphate
   phosphatases                                          + nitrogenous base

  ── WATER: ~9 L/day presented, ~8 L absorbed by small intestine, ~0.9 L
     by colon, 100-200 mL lost in stool. ALL by OSMOSIS following solute.

Carbohydrates

Only three monosaccharides can be absorbed: glucose, galactose, fructose. Everything else must be reduced to them. Amylase attacks only α-1,4 bonds and cannot touch the α-1,6 branch points, which is why it leaves α-limit dextrins for isomaltase to finish. Cellulose, whose bonds are β-1,4, has no human enzyme at all — it passes through as fibre.

Absorption is where the physiology lives. SGLT1 in the apical membrane binds two Na⁺ and one glucose and carries them in together. The energy comes not from ATP directly but from the Na⁺ gradient, which the basolateral Na⁺/K⁺ ATPase maintains — the classic secondary active transport of Chapter 3. This coupling is the physiological basis of oral rehydration therapy: in cholera, the intestine's secretory machinery is jammed open, but SGLT1 still works, so a solution containing both glucose and salt drives Na⁺ absorption, and water follows osmotically. A therapy that has saved tens of millions of lives is a direct application of one cotransporter's stoichiometry.

Proteins

Pepsin accomplishes only 10–15% of protein digestion and is not essential — gastrectomy patients digest protein adequately. The pancreatic proteases do the real work, cutting at specific residues (trypsin after lysine and arginine, chymotrypsin after aromatic residues), and brush border peptidases finish it.

An underappreciated detail: di- and tripeptides are absorbed faster than free amino acids, because PepT1 is a high-capacity transporter with broad specificity, whereas free amino acids compete with one another for a handful of narrow, saturable carriers. Cytosolic peptidases then hydrolyze the absorbed peptides inside the enterocyte, so what enters the portal blood is almost entirely free amino acids either way.

Lipids — and why they go to lymph

Fat has a physical problem no other nutrient has: it is not soluble in the aqueous medium where its enzyme works. Digestion therefore has an obligatory mechanical step before the chemical one.

Emulsification. Bile salts coat large fat droplets, and their amphipathic structure lets them dissolve into the lipid surface with their hydrophilic faces outward. Mutual repulsion prevents coalescence, and churning breaks the droplets progressively smaller. No bonds are broken — but a given mass of fat broken from one 1 mm droplet into a million 10 µm droplets has a hundred-fold greater surface area, and lipase is a surface enzyme. Emulsification does not digest fat; it multiplies the workspace.

Hydrolysis. Pancreatic lipase, anchored by colipase (which pins it to the bile-salt- coated surface), removes fatty acids from positions 1 and 3, leaving 2-monoglyceride.

Micelle transport. The products are still insoluble, and the enterocyte surface is guarded by an unstirred water layer they cannot cross. Bile salts solve this too, aggregating with the products into micelles — 4–7 nm particles with a lipid core and a hydrophilic shell — which diffuse through the water layer and release their cargo at the brush border, where the lipids diffuse straight through the membrane. The bile salts are not absorbed here; they are released to make another trip, and are eventually recovered in the terminal ileum.

Repackaging and the lymphatic exit. Inside the enterocyte, the smooth ER re-esterifies monoglyceride and fatty acids back into triglyceride, and the Golgi wraps that triglyceride, along with cholesterol and phospholipid, in a coat containing apolipoprotein B-48. The result is a chylomicron, 75–1,200 nm across.

That size is the whole answer to the question of route. A blood capillary in the villus is a continuous capillary with a complete basement membrane; its clefts pass molecules of a few nanometres. A chylomicron is up to a thousand nanometres and cannot get through. A lacteal, by contrast, is a lymphatic capillary: it has overlapping, unanchored endothelial cells forming flap valves, and an incomplete basement membrane. Large particles enter easily.

So lipids take the lymphatic route for a purely mechanical reason — but the consequence is physiologically significant. Lymph from the thoracic duct empties into the left subclavian vein, downstream of the liver. Dietary fat is therefore the one class of absorbed nutrient that reaches the heart, the muscles, and the adipose tissue before the liver sees it. It is also why a blood sample taken after a fatty meal looks milky, and why fat-soluble vitamins A, D, E, and K and any fat-soluble toxin follow the same path.

Water, vitamins, and electrolytes

Water. About 9 L are presented to the tract daily — roughly 2 L ingested and 7 L secreted (saliva 1.5 L, gastric 2 L, bile 0.7 L, pancreatic 1.5 L, intestinal 1.5 L). The small intestine reabsorbs about 8 L and the colon about 0.9 L, leaving 100–200 mL in stool. Every drop of it moves by osmosis, following absorbed solute, which is why anything that leaves solute in the lumen causes diarrhea, and why the tract's own secretions are the larger part of what is at stake in severe diarrhea.

Vitamins. Fat-soluble A, D, E, and K ride in micelles and require bile — so any cause of fat malabsorption produces fat-soluble vitamin deficiency. Water-soluble vitamins are absorbed by diffusion or specific carriers in the jejunum, with one famous exception: vitamin B₁₂ is too large and too easily destroyed to be absorbed alone, so it is bound to intrinsic factor from the parietal cell and the complex is taken up by a specific receptor in the terminal ileum. Two organs, at opposite ends of the small intestine, are both required.

Electrolytes. Na⁺ is absorbed all along the tract by several mechanisms and drives water absorption with it. Iron and calcium are absorbed in the duodenum and are unusual in being regulated according to need rather than availability: iron uptake is controlled by hepcidin acting on ferroportin, and calcium uptake by calcitriol (vitamin D), which induces the transport proteins (Chapters 6 and 16). Most nutrients are absorbed as fast as they arrive; these two are metered, because the body has no efficient way to excrete an excess of either.

23.9 The Large Intestine

The colon absorbs no significant nutrient. Its jobs are to recover water and electrolytes, to house the microbiome, and to store residue until elimination is convenient. Its anatomy says so.

Regions: cecum (with the vermiform appendix) → ascending colon → right colic (hepatic) flexure → transverse colon → left colic (splenic) flexure → descending colon → sigmoid colon → rectum → anal canal. The ascending and descending portions are retroperitoneal; the transverse and sigmoid hang on mesenteries, which is why those are the segments that twist (volvulus).

Three unique features, all consequences of one modification:

• Teniae coli — the longitudinal muscle layer is not a continuous sheet but three narrow bands. Their sustained tone gathers the wall into pockets;
• Haustra — those pockets, which permit segmental storage and independent contraction;
• Epiploic (omental) appendages — small fat-filled peritoneal tags of no known function, which occasionally torse and produce sudden abdominal pain that mimics appendicitis or diverticulitis.

Histologically the mucosa has no villi — there is nothing left to absorb that requires surface area — but it is dominated by goblet cells, because a drying, increasingly solid load must be lubricated and the epithelium must be protected from the bacterial mass.

The microbiome

The colon contains on the order of 10¹³–10¹⁴ microorganisms, roughly comparable in number to the body's own cells and weighing about 200 g. This is not a passenger population; it is a metabolic organ.

• It ferments the carbohydrate humans cannot digest — fibre, resistant starch, some oligosaccharides — into short-chain fatty acids: acetate, propionate, and butyrate. Butyrate is the preferred fuel of the colonocyte, supplying 60–70% of its energy. The colonic epithelium is fed by its own bacteria rather than by the bloodstream, which is an arrangement found nowhere else.
• It synthesizes vitamin K and several B vitamins (biotin, riboflavin, folate).
• It metabolizes bilirubin to stercobilin and deconjugates bile acids.
• It provides colonization resistance — an occupied niche is hard to invade — which is why antibiotic treatment predisposes to Clostridioides difficile colitis.
• It educates the immune system, tuning the gut's default posture of tolerance.

Motility and defecation

Haustral contractions occur every 30 minutes or so: a haustrum fills, its wall stretches, and it contracts, squeezing contents into the next. Slow, local, and good for water extraction. Mass movements are different — 3 or 4 times a day, typically after a meal, long stretches of colon contract at once and drive contents rapidly toward the rectum. The trigger is the gastrocolic reflex, initiated by stomach distension. Total colonic transit is 12–36 hours.

The defecation reflex is a spinal reflex with a voluntary override:

1. Mass movement fills the rectum, which is normally empty.
2. Stretch receptors signal via pelvic afferents to the sacral cord (S2–S4).
3. Parasympathetic efferents contract the sigmoid colon and rectum and relax the internal anal sphincter — which is smooth muscle and entirely involuntary.
4. Simultaneously the message ascends to the cortex, producing awareness and, if the moment is wrong, voluntary contraction of the external anal sphincter — skeletal muscle, supplied by the pudendal nerve (S2–S4).
5. If deferred, the rectum relaxes to accommodate, the urge fades, and the reflex is retriggered at the next mass movement. If permitted, the Valsalva manoeuvre raises intra-abdominal pressure, the levator ani straightens the anorectal angle, and defecation occurs.

Two involuntary components and one voluntary one, arranged so that the automatic system does everything except choose the moment. Damage above the sacral cord leaves the reflex intact but removes the choice; damage to the sacral cord or pudendal nerve destroys both.

23.10 Advanced Topic · Pathology as Physiology Run Backwards

Every disease below is one mechanism from this chapter, removed or reversed.

GERD. The antireflux barrier of §23.4 fails — most often through frequent transient LES relaxations, often with a hiatal hernia removing the crural sling, the intra-abdominal segment, and the angle of His at once. Esophageal squamous epithelium is not built for acid; it lacks the mucus gel, the bicarbonate secretion, and the prostaglandin-maintained blood flow that protect the stomach. Chronic exposure produces esophagitis, then metaplasia (Barrett's), then in a small minority adenocarcinoma. Treatment is either mechanical (weight loss, smaller meals, not lying down for three hours after eating, elevating the head of the bed) or chemical (suppressing acid with a PPI so that what refluxes is harmless).

Peptic ulcer and Helicobacter pylori. An ulcer is a breach through the muscularis mucosae, and it occurs when aggression exceeds defence. There are essentially two causes. H. pylori is a spiral bacterium that survives in the mucus gel by secreting urease, which splits urea into ammonia and neutralizes acid in its immediate microenvironment. Where it colonizes determines what disease results: antral-predominant infection destroys D cells, removing the somatostatin brake, so gastrin and acid rise and a duodenal ulcer follows; corpus-predominant infection destroys parietal cells, causing atrophy, hypochlorhydria, and a higher risk of gastric ulcer and gastric cancer. The second cause is NSAID use, by the mechanism worked through in §23.5. In both cases the drug that heals the ulcer is the one that removes the acid, because acid is the common final requirement.

Gallstones. Bile holds cholesterol in solution only because bile salts and lecithin solubilize it. Shift the ratio — more cholesterol, or fewer bile salts — and cholesterol crystallizes. Stasis in the gallbladder (as in prolonged fasting or rapid weight loss) provides the time. A stone impacted in the cystic duct produces biliary colic, the pain peaking 1–2 hours after a fatty meal because that is when CCK contracts the gallbladder against an obstruction. A stone impacted at the ampulla obstructs the pancreatic duct as well, and causes gallstone pancreatitis.

Lactose intolerance. Not a disease but the ancestral human condition: lactase expression normally falls after weaning, and roughly two-thirds of the world's adults are lactase non-persistent. Undigested lactose is osmotically active (drawing water in) and is fermented by colonic bacteria to gas and short-chain fatty acids. Bloating, cramps, flatus, osmotic diarrhea — all from one absent brush border enzyme.

Celiac disease. Gliadin peptides from wheat are deamidated by tissue transglutaminase and presented on HLA-DQ2 or DQ8 molecules, triggering a T-cell response against the small intestinal mucosa. The result is villous atrophy with crypt hyperplasia: the ×10 amplification of Figure 23.6 is destroyed while the crypts frantically try to replace it. Surface area falls by around 90%, and every consequence follows from that single number.

Inflammatory bowel disease. Crohn's disease produces transmural, patchy "skip" inflammation anywhere from mouth to anus, favouring the terminal ileum — hence bile salt malabsorption and B₁₂ deficiency — and, because it is full-thickness, it causes strictures and fistulas. Ulcerative colitis is mucosal only and continuous, beginning at the rectum and extending proximally — hence bloody diarrhea and urgency, but no fistulas. The depth of inflammation, a purely histological fact, predicts the entire clinical picture.

Diverticulitis. Diverticula are herniations of mucosa and submucosa through gaps in the muscular wall where the vasa recta penetrate. By Laplace's law, wall tension is proportional to pressure times radius, so for a given tension the narrowest segment generates the highest pressure: the sigmoid colon, which is exactly where diverticula predominate. Low fibre intake means small, hard stools requiring higher pressures to move. Anatomy plus physics plus diet, in that order.

The four mechanisms of diarrhea. Any diarrhea is one or more of:

Fasting is the single most informative bedside test in that table, and the reason is pure physiology: an osmotic diarrhea has no solute to work with if nothing is eaten, whereas a secretory diarrhea is generating its own solute.

Chapter Summary

§23.1 The digestive system is an alimentary canal — a continuous tube from mouth to anus — plus accessory organs (teeth, tongue, salivary glands, liver, gallbladder, pancreas) that empty into it through ducts. Six activities: ingestion, propulsion, mechanical breakdown, chemical digestion, absorption, defecation. The lumen is topologically outside the body, which is why its contents can be harsh, why absorption is an active decision rather than a leak, and why the gut carries more immune tissue than any other site.

§23.2 From mid-esophagus to anus the wall is mucosa (epithelium, lamina propria, muscularis mucosae), submucosa, muscularis externa (inner circular, outer longitudinal), and serosa or adventitia. Each organ edits the template to suit its job: stratified squamous and skeletal muscle in the esophagus for abrasion and speed; a third oblique layer and rugae in the stomach for churning and storage; plicae, villi, and microvilli in the small intestine for surface area; teniae, haustra, and goblet cells in the colon for storage and lubrication.

§23.3 Three control channels. The enteric nervous system (~500 million neurons) runs short reflexes entirely within the wall — the submucosal plexus governs secretion and blood flow, the myenteric plexus governs motility. Long reflexes route through the CNS, with parasympathetic stimulation and sympathetic inhibition. GI hormones — gastrin, secretin, CCK, GIP, motilin, ghrelin, somatostatin — act over minutes; the three triggered by duodenal contents all say the same thing to the stomach: slow down.

§23.4 Saliva (1.0–1.5 L/day from parotid, submandibular, sublingual) lubricates, buffers, begins starch digestion, and defends. Deglutition has a voluntary buccal phase and an involuntary pharyngeal–esophageal phase with three separate airway guards. The antireflux barrier has four parts — LES tone, the diaphragmatic crura, the intra-abdominal segment, and the angle of His — and lying down after a large meal compromises all four simultaneously while removing gravity.

§23.5 The stomach adds an oblique muscle layer and rugae. Gastric glands hold five cell types; parietal cells make HCl using the H⁺/K⁺ ATPase against a million-fold gradient, plus intrinsic factor; chief cells make pepsinogen, activated by acid. Secretion runs in cephalic, gastric, and intestinal phases, each with an inhibitory arm. The mucosal barrier — mucus gel, tight junctions, rapid restitution, mucosal blood flow, and prostaglandins — is what keeps the stomach from digesting itself, and COX inhibition removes three of its five components at once.

§23.6 The small intestine multiplies surface area ~600-fold — plicae circulares (×3), villi (×10), microvilli (×20) — from 0.4 m² to ~200 m². Each villus carries a capillary bed and a lacteal. Crypts hold stem cells and Paneth cells; the duodenum uniquely has submucosal Brunner's glands. Segmentation mixes after a meal; the migrating motor complex cleans between meals.

§23.7 The liver lobule carries blood inward from portal triads to the central vein and bile outward in canaliculi. Its dual supply is 75% portal, 25% arterial, and the hepatic portal system delivers everything absorbed to the liver first. Bile salts emulsify fat and are recycled by enterohepatic circulation (95% recovered in the terminal ileum). The pancreas secretes bicarbonate from duct cells under secretin and enzymes from acinar cells under CCK; every protease is a zymogen, activated only by brush border enteropeptidase.

§23.8 Carbohydrate → amylase → brush border disaccharidases → SGLT1 and GLUT5 → GLUT2 → portal blood. Protein → pepsin and pancreatic proteases → brush border peptidases → amino acid carriers and PepT1 → portal blood. Lipid → emulsification → lipase → micelles → re-esterification → chylomicrons → lacteal, because chylomicrons are too large for a blood capillary. ~9 L of water is handled daily and moves entirely by osmosis.

§23.9 The colon recovers water and electrolytes, stores residue, and houses a microbiome that ferments fibre to short-chain fatty acids (butyrate feeds the colonocyte), synthesizes vitamin K, and resists colonization. Haustral churning extracts water; mass movements propel. Defecation is a sacral reflex with an involuntary internal and a voluntary external sphincter.

§23.10 Each major GI disease is one mechanism from this chapter removed: the antireflux barrier (GERD), the mucosal barrier (peptic ulcer), bile solubility (gallstones), a brush border enzyme (lactose intolerance), villous surface area (celiac), the depth of inflammation (Crohn's vs colitis), Laplace's law (diverticulitis), and the four routes by which water is lost (diarrhea).

Case File 23 · Resolution

Question 1 — Why the stomach specifically?

Because the stomach is the only organ in the body where three of aspirin's properties overlap.

Prostaglandin dependence. Gastric mucosal integrity depends continuously on locally made COX-1 prostaglandins. PGE₂ and PGI₂ drive mucus secretion, drive bicarbonate secretion, maintain mucosal blood flow, and promote restitution — three of the five components of the mucosal barrier (§23.5), plus part of a fourth. Aspirin irreversibly acetylates COX-1, so all of them fall together. Nowhere else in the body is a surface protected in that specific, prostaglandin-dependent way; the nasal mucosa, the gums, and the bladder urothelium are not.

Ion trapping. Aspirin is a weak acid, pKa ~3.5. In gastric juice at pH 2 it is largely non-ionized and lipid-soluble, so it diffuses into surface cells, where at pH 7.2 it instantly ionizes and can no longer leave. It accumulates and uncouples oxidative phosphorylation. There is no other compartment in the body with a two-hundred-thousand-fold pH step across a single cell membrane, so this mechanism is available nowhere else.

Blocked repair. Both antiplatelet drugs act here. Aspirin's irreversible platelet COX-1 acetylation abolishes thromboxane A₂ for the platelet's 7–10 day lifespan; clopidogrel blocks the P2Y₁₂ ADP receptor. Neither creates a lesion. What they do is prevent the small vessel at the base of an erosion from sealing, and prevent the platelet-delivered growth factors that drive mucosal angiogenesis from arriving. Aspirin makes the hole; both drugs keep it open.

Her laboratory picture confirms the site and the chronicity. Hemoglobin 10.8 → 9.4 g/dL with a falling MCV (84 → 79 fL) and a ferritin of 14 ng/mL is iron deficiency from chronic blood loss, not acute hemorrhage. And the BUN of 26 mg/dL with an essentially unchanged creatinine is a classic upper gastrointestinal finding: blood in the small intestine is digested like any other protein, absorbed as amino acids into the portal vein, and deaminated by the liver to urea. A rising BUN with a stable creatinine localizes the bleeding to above the ileocecal valve — physiology used as a diagnostic instrument.

Question 2 — Why is the stomach not normally digested?

Five overlapping defences (§23.5), of which the first is the most elegant. The mucus gel layer is 200–600 µm thick and, critically, unstirred: it does not mix. Bicarbonate secreted by the surface cells is trapped inside it, so a stable pH gradient exists across half a millimetre — pH 1.5–2.0 at the luminal face, approximately pH 7 at the cell surface. Acid diffusing inward is neutralized before it arrives, and pepsin, a large molecule, cannot penetrate the gel at all, and would be inactivated by neutral pH if it did.

The other four: tight junctions stop acid leaking between cells; the epithelium is completely replaced every 3–6 days and small defects are resurfaced by restitution within minutes; generous mucosal blood flow removes back-diffused H⁺ and supplies bicarbonate and oxygen; and prostaglandins drive the mucus, the bicarbonate, and the blood flow. Add to that the fact that pepsin is secreted as an inactive zymogen that requires acid to activate — so it can only become dangerous in the one place the cell is protected from it.

What aspirin took away was the fifth component, and with it much of the first, third, and fourth. It did not add acid. It removed the defence, and the acid that was always there did the rest.

Question 3 — Why does a large meal before sleep produce reflux?

Four anatomical structures normally guard the gastroesophageal junction, and lying down with a full stomach compromises every one of them at once (§23.4, Figure 23.4).

1. LES tone. Distension of the gastric fundus is the physiological trigger for transient LES relaxations — a vagovagal reflex that exists to permit belching. A large meal maximally distends the fundus and therefore maximally increases TLESR frequency. The sphincter is not overpowered; it is opened, by a normal reflex responding to an abnormal stimulus.
2. The diaphragmatic crura. The right crus forms an external sphincter around the esophagus that tightens with every inspiration, cough, and strain. During quiet supine sleep, that reinforcement is at its weakest of the whole day.
3. The intra-abdominal segment. Its flutter-valve arrangement is self-adjusting to rises in abdominal pressure, but it depends on the column of gastric contents sitting below the junction. Supine, the contents lie against it.
4. The angle of His. The acute cardioesophageal angle works as a flap valve because the fundic gas bubble presses it shut. Lying flat, the gas rises to what is now the highest point — often away from the cardia — and liquid gastric contents, not gas, sit against the junction. Recumbency effectively straightens the valve.

Then add the two aggravating factors specific to Amara. Gravity is removed: upright, the fluid level sits below the junction and reflux must work against a column of liquid; supine, it does not. And her 08:00 meal is her largest of the day and relatively fatty, so CCK and the enterogastric reflex slow gastric emptying to as much as six hours — meaning the reservoir is still full for most of the time she is asleep.

Her night-shift eating pattern is therefore not incidental to her symptoms; it is the mechanism. The intervention that follows is anatomical rather than pharmacological: move the large meal earlier, keep three hours upright after eating, reduce meal size and fat content, and elevate the head of the bed so gravity is partially restored. Her acid secretion is normal. Her posture is the problem.

Systems Integration Case File · Entry 23

Review

Level 1 · Recall

20.1 The layer of the alimentary canal wall that contains the myenteric plexus is the:

a) mucosa    b) submucosa    c) muscularis externa    d) serosa

20.2 Which cell of the gastric gland secretes intrinsic factor?

a) chief cell    b) parietal cell    c) G cell    d) mucous neck cell

20.3 Secretin is released in response to:

a) fat in the duodenum    b) acid in the duodenum    c) distension of the stomach    d) fasting

20.4 The three structural amplifications of small intestinal surface area, in order from largest to smallest scale, are:

a) villi, plicae circulares, microvilli    b) plicae circulares, villi, microvilli    c) microvilli, villi, plicae circulares    d) rugae, villi, microvilli

20.5 Within a hepatic lobule, bile flows:

a) from the central vein toward the portal triads    b) from the portal triads toward the central vein    c) in the same direction as blood    d) into the sinusoids

20.6 Chylomicrons enter lacteals rather than blood capillaries because:

a) they are lipid-soluble    b) they are too large to cross the capillary basement membrane    c) lacteals actively transport them    d) they must avoid the liver

20.7 Which of the following is not part of the gastric mucosal barrier?

a) an unstirred alkaline mucus gel    b) prostaglandin-maintained mucosal blood flow    c) rapid epithelial restitution    d) secretion of pepsin in its active form

20.8 The teniae coli are:

a) three bands of longitudinal smooth muscle in the colon    b) fat-filled peritoneal tags    c) pouches formed in the colonic wall    d) folds of the small intestinal mucosa

Level 2 · Comprehension

20.9 Explain why the pancreas secretes proteases as zymogens but secretes amylase and lipase in active form.

20.10 The gastric phase of secretion is stimulated in part by rising pH. Explain the apparent paradox and identify the feedback loop.

20.11 Why does resection of the terminal ileum cause both diarrhea and, eventually, neurological disease?

20.12 Both the stomach and the colon contain large populations of the body's own defences against their contents, but they defend against different threats. Contrast them.

Level 3 · Clinical Application

20.13 A 58-year-old man has a total gastrectomy for cancer. Predict four long-term nutritional consequences and give the mechanism for each.

20.14 A patient with severe hypotension from sepsis develops abdominal pain out of proportion to examination findings, then bloody diarrhea. Explain using §23.6 and §23.7.

20.15 A 34-year-old woman has episodic right upper quadrant pain beginning 60–90 minutes after meals, worse after fried food, radiating to the right scapula. Explain the timing, the food specificity, and the location of the referred pain.

Level 4 · Integration and Synthesis

20.16 Construct the full causal chain from Amara's night-shift schedule to her endoscopy findings, naming every system involved. Then identify the two points in the chain where an intervention would be most effective, and justify your choices.

20.17 A drug is proposed that blocks CCK receptors, intended to promote weight loss by an unrelated mechanism. Predict its gastrointestinal side effects in as much mechanistic detail as you can, and say whether you would expect it to succeed at its stated purpose.

20.18 Argue for or against: "The gut microbiome should be classified as a human organ." Use at least four specific functions and at least one argument against.

Concept Map to Complete

Copy this onto blank paper and fill in every bracket from memory before checking.

                        THE GASTRIC MUCOSAL BARRIER
                                    │
      ┌──────────┬──────────┬───────┴───────┬──────────────┐
  [ _______ ]  [ tight  ]  [ rapid       ]  [ mucosal   ]  [ __________ ]
   gel layer     junctions   [ _________ ]    [ _______ ]    (COX-1 made)
   pH _____                  every 3-6 d                          │
   at surface                                                     │
   pH _____                                            stimulates:
   in lumen                                            1 [ _______ ]
        │                                              2 [ _______ ]
        │                                              3 [ _______ ]
        │                                              4 [ _______ ]
        ▼
   ACID is made by the [ __________ ] CELL
        │
        ├── pump: [ ____________ ] ATPase, exchanging H+ for [ __ ]
        ├── stimulated by 3 signals: [ ____ ] · [ ______ ] · [ _________ ]
        │      of which [ _________ ] is the final common amplifier
        └── by-product leaving basolaterally: [ ______ ] = the ______ tide

   ASPIRIN blocks [ ______ ] → removes boxes [ ___ ] and [ ___ ]
   CLOPIDOGREL blocks [ __________ ] on platelets → prevents [ _______ ]
   PPI blocks [ ____________ ] → removes the [ ______ ] all three need

Lab / Self-Exploration

1. Time your own gastric emptying, indirectly. Note the clock time of a light, low-fat meal and of the first return of hunger. Repeat on another day with a meal of similar calories but high fat content. The difference — often two hours or more — is CCK and the enterogastric reflex, measured with a kitchen clock.
2. Demonstrate emulsification. Shake oil and water in a jar: two layers within seconds. Add a few drops of dish soap (an amphipathic detergent, structurally analogous to a bile salt) and shake again. The stable cloudy suspension is emulsification. Note that nothing has been digested — only the surface area has changed.
3. Find the referred pain map on yourself. Locate your epigastrium, your periumbilical region, and your hypogastrium. Name one foregut, one midgut, and one hindgut organ for each, and write out which spinal segments carry their visceral afferents.
4. Test salivary amylase. Chew a plain unsalted cracker slowly for 60 seconds without swallowing. The sweetness that develops is maltose, produced by salivary amylase from starch, in real time, in your mouth.
5. Palpate the anatomy. Find your right costal margin at the midclavicular line: the liver edge lies just beneath it and descends on deep inspiration. Now find McBurney's point again from Chapter 1 and state which embryological division of the gut the appendix belongs to and why its pain begins somewhere else.
6. Audit a label. Take any sports drink and calculate its carbohydrate concentration as a percentage (grams per 100 mL). Compare with the 4–8% window in §23.6, and check whether it uses a single sugar or a glucose–fructose combination. Predict its maximum useful delivery rate in grams per hour.

Key Terms

Next: Chapter 24 · Nutrition and Metabolism — where everything absorbed in this chapter is burned, stored, or built into something, and where Amara's metabolic syndrome is finally assembled in full.