Chapter 2
Stomach, Intestines
and Nutrient Absorption

The Stomach and Gastric Digestion

Learning Goal: Name the four regions of the stomach, describe its unique three-layer muscle wall, and explain how it converts a meal into chyme.

1The Four Regions of the Stomach

The stomach is a J-shaped muscular sac sitting just below the diaphragm, with a resting capacity of roughly 50 mL that can stretch to hold 1–1.5 litres after a large meal. Food enters through the cardia, the narrow region just past the lower oesophageal sphincter. Above and to the left of the cardia bulges the fundus, a dome-shaped pocket that serves mainly as a gas-collection chamber — it is where swallowed air and the carbon dioxide released by stomach acid neutralising food tend to collect, which is why a large meal is often followed by burping as fundic gas is vented back up the oesophagus. The body is the largest region, forming the main reservoir where most digestion takes place. The stomach narrows into the antrum, a thicker-walled, more muscular funnel that grinds and propels chyme toward the exit, and finally the pylorus, a short, heavily muscled segment ending in the pyloric sphincter, a ring of muscle that controls exactly how much chyme is allowed to leave at a time.

2An Extra Muscle Layer for an Extra Job

Recall from Lesson 1.3 that most of the gut wall has two muscle layers — circular and longitudinal. The stomach is the one organ in the entire tract with a third: an inner oblique layer. This third layer is what allows the stomach to do something no other digestive organ does — twist and knead its contents rather than simply push them along in one direction, the same way a washing machine drum tumbles clothes rather than merely conveying them. The combined action of all three muscle layers produces powerful, rhythmic contractions that begin gently in the body of the stomach and intensify dramatically as they approach the antrum, mechanically pulverising food alongside the chemical breakdown described in Lesson 2.2.

3Receptive Relaxation: Making Room Without Raising Pressure

A remarkable feature of the stomach is that it can accept a large meal without a large rise in internal pressure. As food arrives, the smooth muscle of the fundus and body relaxes — a reflex called receptive relaxation, triggered by the vagus nerve even before the first bite is swallowed, simply from the sight and smell of food, and reinforced further as the stomach wall actually stretches. Without this reflex, eating a large meal would sharply raise intragastric pressure, forcing stomach contents back up toward the oesophagus; receptive relaxation is one of the quiet mechanisms that keeps ordinary eating comfortable, and its failure or exaggeration is implicated in conditions ranging from early satiety to functional dyspepsia.

4From Meal to Chyme

Mechanical churning, gastric acid and the enzyme pepsin (covered fully in Lesson 2.2) act together over roughly two to four hours to convert a solid meal into chyme — a thick, acidic, semi-liquid slurry with the consistency of a milkshake. Chyme formation is not optional groundwork; the small intestine downstream is built to process liquid, not solid, food, and its digestive enzymes work far more efficiently on a homogenised slurry with a huge exposed surface area than on chunks. A meal that leaves the stomach only partially liquefied — as can happen when chewing is inadequate or gastric function is impaired — burdens the small intestine with digestive work it is poorly equipped to finish, one of several reasons thorough chewing measurably improves digestive comfort.

Hydrochloric Acid and Pepsin

Learning Goal: Explain how and why the stomach produces strong acid, how pepsin is activated safely, and how the stomach protects itself from its own secretions.

1Parietal Cells and Acid Production

Hydrochloric acid (HCl) is manufactured by parietal cells embedded in the gastric glands of the stomach lining. These cells contain a specialised pump called the H+/K+ ATPase (the "proton pump") that actively transports hydrogen ions into the gastric lumen against a colossal concentration gradient — gastric juice can reach a pH of 1.5 to 3.5, roughly a million times more acidic than blood. This is the same proton pump that proton-pump-inhibitor medications (omeprazole and similar) block, which is why those drugs are so effective at reducing acid output for reflux and ulcer management, a topic returned to in the myth box below.

Parietal cells are stimulated to secrete acid by three separate signals acting together: the vagus nerve (via acetylcholine, particularly during the cephalic phase covered in Lesson 1.9 of Chapter 1), the hormone gastrin released by G-cells in the antrum in response to protein and stomach stretch, and histamine released locally by nearby cells that amplifies the effect of the other two. This three-way redundancy is precisely why single-mechanism acid blockers are sometimes only partially effective and why some conditions require combination therapy.

2Why Such Strong Acid?

Gastric acid performs three distinct jobs, and understanding all three explains why the stomach cannot simply run at a gentler pH. First, it denatures protein — unfolding the complex three-dimensional structure of dietary protein so that pepsin can access the peptide bonds inside, much as heat denatures egg white during cooking. Second, it provides powerful antimicrobial defence, killing the majority of bacteria, parasites and their eggs that arrive with food and water, which is one reason acid-suppressing medication is associated with a modestly increased risk of certain gut infections. Third, it activates pepsinogen into pepsin, the mechanism covered next, and it also assists mineral absorption downstream by keeping iron and calcium in a more soluble, absorbable form as chyme leaves the stomach.

3Chief Cells: Pepsinogen to Pepsin

Chief cells, also located in the gastric glands, secrete pepsinogen — an inactive precursor, or zymogen, of the enzyme pepsin. This is the same zymogen-activation safety pattern introduced in Lesson 1.4 for pancreatic enzymes: pepsinogen is manufactured and stored in a folded, inactive shape so that chief cells themselves are not digested by their own product. Once pepsinogen is released into the strongly acidic gastric lumen, the low pH triggers a conformational change that exposes pepsin's active site, converting it into active pepsin. Newly activated pepsin then accelerates the activation of further pepsinogen molecules nearby — a self-amplifying cascade very similar in logic to the trypsinogen cascade covered in Chapter 1, ensuring the enzyme supply ramps up quickly once the first trigger fires.

Active pepsin is an endopeptidase: it cuts protein chains at internal points rather than only from the ends, breaking long polypeptides into shorter fragments. It does not complete protein digestion — that finishing work happens in the small intestine, covered in Lesson 2.8 — but it gives protein digestion a substantial head start before the meal ever leaves the stomach.

4The Mucous Barrier: Protecting the Stomach from Itself

Given that gastric juice is acidic enough to dissolve metal and contains an active protein-cutting enzyme, an obvious question is why the stomach does not digest itself. The answer is a dedicated defence system. Mucous cells throughout the gastric lining secrete a thick layer of alkaline mucus that coats the stomach wall, trapping a thin layer of near-neutral fluid against the cell surface even while the bulk lumen remains strongly acidic. The epithelial cells beneath are also rapidly regenerating (as noted in Lesson 2.1) and are joined by tight junctions that limit acid from leaking backward into the tissue. When this barrier is breached — by Helicobacter pylori infection, chronic NSAID use, heavy alcohol intake or severe stress — the underlying tissue is exposed to acid and pepsin directly, which is the mechanism behind gastritis and peptic ulcers.

Gastric Emptying

Learning Goal: Explain the reflex loop that controls how fast the stomach empties, what makes food leave faster or slower, and how emptying can go wrong clinically.

1The Antral Pump

Gastric emptying begins with the powerful, ring-like contractions of the antrum described in Lesson 2.1. Each contraction squeezes a small volume of chyme toward the pylorus; most of it is forced back into the body of the stomach for further grinding because the pyloric opening only allows a few millilitres through per contraction, while larger, undigested particles are retropelled and mixed again. This retropulsion is deliberate — it is the mechanical grinding action itself, not a malfunction — and it means only chyme that has already been reduced to a fine, liquid consistency is likely to pass through on a given contraction. Only a small amount of chyme is released into the duodenum with each antral contraction, roughly three times per minute.

2The Enterogastric Reflex: The Duodenum Talks Back

The duodenum — the first, short segment of the small intestine, examined fully in Lesson 2.4 — is not a passive receiver of whatever the stomach sends it. Stretch receptors, acid (pH) receptors, and fat and osmolarity receptors in the duodenal wall constantly monitor the chyme arriving, and when conditions there are unfavourable — too acidic, too much fat, too concentrated, or simply too much volume too fast — they trigger the enterogastric reflex, a nerve- and hormone-mediated signal (involving the hormone CCK, covered in Lesson 1.8) that slows gastric motility and tightens the pyloric sphincter. This feedback loop is the single most important control on gastric emptying rate: the stomach effectively waits for the "all clear" from the intestine before releasing its next batch, which prevents the small intestine from being overwhelmed with more chyme than it can neutralise and digest at once.

3Liquids, Solids and Composition

Emptying rate depends heavily on what was eaten. Liquids empty fastest and follow a roughly linear, first-order decline once past the initial lag; solids must first be reduced to particles smaller than about 1–2 mm before they can pass the pylorus, which is why solid emptying shows a lag phase followed by a steadier decline. Composition matters at least as much as physical state: carbohydrate-dominant meals empty fastest, protein more slowly, and fat slowest of all — fat is the most powerful trigger of the enterogastric reflex of the three macronutrients, since fat digestion downstream requires bile and pancreatic lipase to be mobilised, a process that takes time to ramp up.

The Duodenum

Learning Goal: Describe the duodenum's anatomy and explain why it functions as the digestive system's central mixing and neutralisation chamber.

1Anatomy: A Short, C-Shaped Curve

The duodenum is the first and shortest of the three segments of small intestine, roughly 25 cm long, curving in a C-shape around the head of the pancreas. Unlike the rest of the small intestine, most of the duodenum is retroperitoneal — fixed against the back wall of the abdomen rather than hanging free on a mesentery — which anchors it firmly in place directly beneath the stomach's exit and alongside the pancreas and bile duct, the two organs it depends on most.

Two ducts empty into the duodenum's midsection at a structure called the hepatopancreatic ampulla (ampulla of Vater): the common bile duct, carrying bile from the liver and gallbladder, and the main pancreatic duct, carrying pancreatic juice. Both are controlled by a single ring of smooth muscle, the sphincter of Oddi, which relaxes on hormonal signal (CCK, triggered by fat and protein in the duodenum) to release bile and pancreatic secretions together, precisely timed to arrive alongside the acidic chyme they are needed to process.

2Neutralising the Acid

Chyme leaving the stomach is intensely acidic — pH as low as 1.5 to 2 — and this acidity must be neutralised almost immediately, both to protect the duodenal lining (which lacks the stomach's thick protective mucous barrier) and because the pancreatic enzymes about to be added only function near neutral pH. This neutralisation is achieved chiefly by bicarbonate, secreted in large volume by the pancreas (and, to a lesser degree, by Brunner's glands within the duodenal wall itself) in direct response to the hormone secretin, released when acid contacts the duodenal lining. Within centimetres of the pylorus, chyme is typically brought from a pH near 2 up toward a near-neutral pH of 6 to 7 — one of the fastest and most dramatic pH swings anywhere in the body.

3Bile: Emulsifying Fat

Bile, manufactured continuously by the liver and concentrated and stored in the gallbladder between meals, is released into the duodenum on the same CCK signal that opens the sphincter of Oddi. Bile contains no digestive enzymes; its job is purely physical — its bile salts act as a detergent, breaking large fat globules into much smaller droplets (a process called emulsification) so that pancreatic lipase, which can only act at a droplet's surface, has vastly more surface area to work on. This mechanism is examined in full in Lesson 2.9, but its starting point is here, in the duodenum, the moment bile and fatty chyme first meet.

4Pancreatic Enzymes: Finishing the Job

Alongside bicarbonate, the pancreas delivers the enzyme cocktail introduced in Chapter 1 — pancreatic amylase for remaining starch, trypsin and chymotrypsin (activated from their zymogen forms, as covered in Lesson 1.4) and other proteases for protein, and pancreatic lipase for fat — directly into the duodenum via the same duct system. Together, bicarbonate neutralisation, bile emulsification and the full pancreatic enzyme set mean the duodenum, though physically short, is where the overwhelming majority of chemical digestion is actually completed. By the time chyme leaves the duodenum for the jejunum, most macronutrients have already been broken down to (or very close to) their final absorbable form.

The Jejunum and Ileum

Learning Goal: Distinguish the jejunum from the ileum structurally and functionally, and explain why absorption is distributed unevenly along the small intestine's remaining length.

1The Jejunum: The Main Absorptive Workhorse

The jejunum makes up roughly the next 2.5 metres of small intestine after the duodenum. Its wall is thicker and more richly vascularised than the ileum's, and its lining bears the tallest, most densely packed villi (Lesson 2.6) anywhere in the small intestine, giving it the single greatest absorptive capacity per unit length in the entire gut. The majority of carbohydrate, protein, fat, and most vitamins and minerals are absorbed in the jejunum — by the time chyme reaches the far end of the jejunum, in a healthy gut, the great majority of macronutrients have already crossed into the bloodstream or lymphatic system.

2The Ileum: Shorter Villi, Specialist Jobs

The ileum, the final and longest segment at roughly 3–3.5 metres, has shorter, less densely packed villi and a correspondingly lower general absorptive capacity per unit length — most of the "easy" nutrients have already been absorbed upstream by the time chyme arrives. But the ileum is not simply a mopping-up zone; it hosts two absorption processes that occur nowhere else in the gut. Vitamin B12, bound to intrinsic factor secreted by gastric parietal cells (Lesson 2.2), is absorbed almost exclusively in the terminal ileum via a dedicated receptor, and bile salts are reabsorbed here in bulk as part of the enterohepatic circulation described below. Peyer's patches — dense clusters of lymphoid tissue that sample gut contents for immune surveillance — are also concentrated in the ileum, reflecting its role as one of the last checkpoints before material passes into the large intestine.

3Enterohepatic Circulation: Recycling Bile

Roughly 95% of bile salts released into the duodenum are not lost with the stool; they are reabsorbed in the terminal ileum, returned to the liver via the portal blood, and re-secreted in bile for the next meal — a cycle called enterohepatic circulation that may repeat several times per day and dramatically reduces how much new bile the liver must synthesise from scratch. This recycling loop explains why surgical removal of the terminal ileum (sometimes necessary in severe Crohn's disease) commonly causes fat malabsorption and diarrhoea even when the rest of the small intestine is intact: without ileal reabsorption, bile salts are lost in the stool faster than the liver can replace them, eventually leaving too little bile available to emulsify dietary fat.

4A Gradient, Not a Switch

It is more accurate to describe absorptive capacity along the jejunum and ileum as a gradual gradient than a hard boundary — there is no visible anatomical marker where the jejunum ends and the ileum begins; the distinction is functional and histological. Villus height, digestive enzyme concentration and blood supply density all decline gradually from the duodenal end toward the ileocaecal valve, where the small intestine finally joins the large intestine (Volume 1 introduced this valve; it prevents backflow of bacteria-laden colonic contents into the comparatively sterile small intestine).

Villi and Microvilli

Learning Goal: Describe the three levels of folding that create the small intestine's absorptive surface, and explain how each villus is structured to absorb nutrients into blood and lymph.

1Three Levels of Amplification

The small intestine amplifies its absorptive surface area through three nested structures. Plicae circulares (circular folds) are large, permanent ridges of mucosa and submucosa that spiral around the intestinal lumen, visible to the naked eye, increasing surface area roughly threefold and also slowing the passage of chyme to allow more contact time. Covering these folds are villi — finger-like projections about 0.5–1.5 mm tall, visible under low magnification, that increase surface area a further tenfold. Covering the surface of every individual villus, in turn, are microvilli — microscopic, densely packed projections on the apical (lumen-facing) membrane of each absorptive cell, visible only under electron microscopy, adding another twentyfold increase and collectively forming what is often called the brush border because of its bristled appearance.

Multiplied together, these three levels of folding increase the small intestine's absorptive surface area by a factor of roughly 600 compared with a smooth tube of the same length and diameter — the anatomical basis for describing the small intestine's absorptive surface as being on the order of 30–40 square metres in a living adult.

2Inside a Single Villus

Each villus is far more than a passive bump of tissue. It is covered by a single layer of absorptive epithelial cells (enterocytes) interspersed with mucus-secreting goblet cells, and its core contains a dense capillary network plus a single central lymphatic vessel called a lacteal. This dual transport system matters because different nutrients leave the gut by different routes: water-soluble nutrients (glucose, amino acids, most vitamins and minerals) enter the capillary network and travel via the portal vein straight to the liver, while most absorbed fat, packaged into particles called chylomicrons (Lesson 2.9), is too large to enter capillaries directly and instead enters the lacteal, travelling via the lymphatic system before eventually draining into the bloodstream near the heart, bypassing the liver on its first pass.

3The Brush Border: Digestion's Final Step

The microvillus membrane is not just a passive absorptive surface; it is studded with brush-border enzymes — including lactase, sucrase, maltase and various peptidases — anchored directly to the enterocyte membrane. These enzymes perform the very last step of carbohydrate and protein digestion, converting disaccharides to monosaccharides and short peptides to single amino acids at the exact membrane surface where the appropriate transporter is waiting to absorb the product immediately. This tight physical coupling of final-step digestion and absorption — sometimes called contact digestion — is far more efficient than releasing an enzyme into the open lumen and hoping the product diffuses to the right location; lactase deficiency, covered in Lesson 1.4, is a brush-border enzyme deficiency specifically.

4Villus Turnover and Damage

Enterocytes are produced continuously from stem cells in structures called crypts of Lieberkühn at the base of each villus, migrate up the villus over roughly three to five days performing absorption, and are shed from the tip — one of the fastest cell turnover rates anywhere in the body. This rapid renewal is a double-edged feature: it allows quick repair after minor injury, but it also means the villi are highly vulnerable to any condition that damages the crypts or disrupts the cell cycle, since replacement cells cannot be produced fast enough. Coeliac disease (introduced in Lesson 1.2) causes immune-mediated villous flattening (atrophy) specifically because of this vulnerability, drastically reducing the very surface area this lesson describes and producing widespread malabsorption despite otherwise normal digestion.

Carbohydrate Absorption

Learning Goal: Trace dietary carbohydrate from disaccharide to bloodstream, naming every enzyme and transporter involved.

1From Starch to Disaccharides

Dietary carbohydrate arrives mostly as starch (long glucose chains), plus smaller amounts of the disaccharides sucrose (table sugar) and lactose (milk sugar), and traces of free monosaccharides. Digestion begins with salivary amylase in the mouth and continues with pancreatic amylase in the duodenum (Lesson 2.4), both of which cut starch down to short chains and the disaccharide maltose — but neither enzyme can produce free monosaccharides on its own. That final cut happens at the brush border.

2Brush-Border Disaccharidases: The Final Cut

Anchored to the microvillus membrane described in Lesson 2.6 are three key disaccharidases, each specific to one substrate: maltase splits maltose into two glucose molecules, sucrase splits sucrose into one glucose and one fructose molecule, and lactase splits lactose into one glucose and one galactose molecule. This is contact digestion at its clearest — the enzyme, the product, and the transporter that will absorb that exact product all sit within nanometres of each other on the same membrane, so the freed monosaccharide is absorbed almost the instant it is produced rather than diffusing away into the lumen.

3Three Transporters for Three Sugars

Glucose and galactose share the same absorption route: the SGLT1 transporter (sodium-glucose linked transporter 1), which actively co-transports one glucose (or galactose) molecule together with sodium ions, using the sodium gradient maintained by a separate pump as its energy source — this is the same active-transport mechanism introduced in Lesson 1.2 and is why glucose and galactose can be absorbed even against their own concentration gradient. Fructose, by contrast, is absorbed via GLUT5, a facilitated-diffusion transporter that requires no sodium co-transport and no direct energy input, but which has a comparatively limited transport capacity — a large fructose load delivered without accompanying glucose can outpace GLUT5's absorption rate, leaving unabsorbed fructose to travel to the colon, where bacteria ferment it, producing gas, bloating and osmotic diarrhoea. This is the physiological basis of fructose malabsorption, distinct from hereditary fructose intolerance, which is a separate metabolic disorder.

Once inside the enterocyte, all three monosaccharides exit on the far (basolateral) side into the capillary network via a fourth transporter, GLUT2, entering the portal blood for direct transport to the liver.

Protein Absorption

Learning Goal: Trace dietary protein from polypeptide to bloodstream, explaining why most protein is absorbed as small peptides rather than single amino acids.

1From Polypeptide to Small Peptides

Recall from Lesson 2.2 that pepsin begins protein digestion in the stomach, cutting long polypeptide chains into shorter fragments. In the duodenum, pancreatic proteases continue the work: trypsin and chymotrypsin (activated from trypsinogen and chymotrypsinogen, as covered in Lesson 1.4's trypsinogen cascade) are endopeptidases that cut chains at internal points, while carboxypeptidase, an exopeptidase, trims single amino acids off the end of the chain. Together these enzymes reduce most dietary protein to a mixture of small peptides (mostly two to eight amino acids long) and a smaller proportion of free amino acids — but critically, very little protein is reduced all the way to single amino acids by this stage alone.

2Brush-Border Peptidases: The Final Trim

As with carbohydrate, the final digestive step happens at the brush border. A battery of brush-border peptidases further cleaves the small peptides arriving from the duodenum, converting many (though not all) into free amino acids right at the absorptive membrane. What is absorbed, therefore, is a mixture: free amino acids via dedicated amino acid transporters, and a substantial proportion of di- and tripeptides via a completely separate transporter, described below — an important correction to the older assumption that all protein is absorbed only as single amino acids.

3Two Absorption Routes: Amino Acids and Small Peptides

Free amino acids are absorbed via several distinct sodium-dependent active transporters, each somewhat specific to a class of amino acid (neutral, acidic, basic) — a redundancy that means a defect in one transporter (as occurs in the genetic disorder Hartnup disease, affecting neutral amino acid transport) does not necessarily block absorption of all amino acids, only the affected class. Di- and tripeptides, meanwhile, are absorbed largely intact via a single transporter called PepT1, which uses a hydrogen-ion gradient rather than a sodium gradient as its energy source. Once inside the enterocyte, any remaining small peptides are typically broken down the rest of the way to free amino acids by intracellular peptidases before being released into the capillary blood, so that what ultimately reaches the portal vein is overwhelmingly free amino acids regardless of which route was used to get inside the cell.

4Why This Two-Route System Matters

The existence of PepT1 as a separate, sodium-independent route is not a minor technical detail — it is the reason di- and tripeptide-based protein supplements and enteral (tube) feeding formulas can be absorbed efficiently even in patients with impaired amino acid transport, and it partly explains why whey protein hydrolysates (pre-digested into small peptides) are sometimes absorbed slightly faster than intact protein or even free amino acid mixtures, since PepT1 can carry a peptide representing two or three amino acids' worth of nitrogen in a single transport event rather than requiring three separate transporter cycles.

Fat Absorption

Learning Goal: Explain the full journey of dietary fat from large globule to bloodstream, including emulsification, micelle formation, resynthesis and chylomicron transport.

1Emulsification: Breaking Fat Into Droplets

Dietary fat arrives in the duodenum as large globules of triglyceride, which is a problem because pancreatic lipase, the main fat-digesting enzyme, is water-soluble and can only act at the surface of a fat droplet, not inside it. Bile salts, released from the gallbladder as covered in Lesson 2.4, solve this by acting as an emulsifier — their structure has both a water-loving and a fat-loving end, allowing them to coat fat droplets and, together with the mechanical churning of intestinal contractions, break large globules into a fine suspension of much smaller droplets. This dramatically increases the total surface area exposed to lipase without changing the total amount of fat present.

2Lipase Digestion: Triglyceride to Free Fatty Acids and Monoglycerides

Pancreatic lipase, working with a helper protein called colipase that anchors it to the emulsified droplet surface against the displacing action of bile salts, cleaves dietary triglycerides — each made of a glycerol backbone with three fatty acids attached — into two free fatty acids and one monoglyceride (glycerol with one fatty acid still attached), rather than breaking them all the way down to free glycerol. This is the primary digestive product of fat: not glycerol and three separate fatty acids, but a 2-monoglyceride plus two free fatty acids.

3Micelles: Ferrying Fat Through a Watery Environment

Free fatty acids and monoglycerides are not water-soluble, which creates a transport problem: the intestinal lumen is a watery environment, but these digestion products need to reach the enterocyte membrane to be absorbed. Bile salts solve this second problem too, by assembling around the fatty acids and monoglycerides into tiny spherical structures called micelles — with the fat-loving digestion products tucked inside and the water-loving bile salt surfaces facing outward, allowing the whole package to travel through the watery lumen and ferry its cargo directly to the enterocyte's brush-border surface. There, the fatty acids and monoglycerides diffuse out of the micelle and across the enterocyte membrane (mostly by passive diffusion, since they are lipid-soluble), while the bile salts themselves are left behind in the lumen to pick up more digestion products — and are ultimately reabsorbed in the ileum, as covered in Lesson 2.5.

4Resynthesis and Chylomicron Formation

Once inside the enterocyte, free fatty acids and monoglycerides are not simply released into the blood as-is. They are transported to the smooth endoplasmic reticulum and reassembled back into triglycerides, then packaged together with cholesterol and phospholipids into a lipoprotein particle, coated with proteins, called a chylomicron. Chylomicrons are too large to cross into blood capillaries, so — exactly as introduced in Lesson 2.6 — they exit the enterocyte and enter the villus's central lacteal, travelling via the lymphatic system, up through the thoracic duct, and into the bloodstream near the heart, entirely bypassing the liver's portal circulation on this first pass. This is a fundamentally different route from every water-soluble nutrient covered so far, and it is the reason a very high-fat meal can visibly cloud a blood sample (lipaemia) for several hours afterward, as chylomicrons circulate before being cleared by peripheral tissues.

Vitamin, Mineral and Water Absorption

Learning Goal: Explain how fat-soluble and water-soluble vitamins differ in absorption, identify the special mechanisms for iron, calcium and B12, and describe how water and electrolytes are absorbed.

1Fat-Soluble Vitamins Travel With Fat

Vitamins A, D, E and K are fat-soluble, and their absorption is directly tied to the fat absorption pathway described in Lesson 2.9: they are incorporated into bile salt micelles alongside dietary fat, absorbed largely by passive diffusion into the enterocyte, then packaged into chylomicrons and exported via the lacteal. A direct consequence is that fat-soluble vitamin absorption depends on adequate dietary fat and normal bile flow — a very low-fat meal, or any condition that impairs bile secretion (Lesson 2.9's expert box), reduces absorption of all four fat-soluble vitamins simultaneously, which is why fat-soluble vitamin supplements are typically recommended to be taken with a meal containing some fat.

2Water-Soluble Vitamins Use Dedicated Transporters

The B-complex vitamins and vitamin C are water-soluble and, with the singular exception of B12 described below, are absorbed largely via specific active-transport carriers in the small intestine, mostly in the jejunum, independent of fat intake. Because they do not require micelle incorporation, water-soluble vitamin absorption is generally more robust to variation in dietary fat, though it can still be impaired by damage to the absorptive lining itself (as in coeliac disease) or by competition — very high-dose supplementation of one B vitamin can sometimes reduce absorption of another that shares part of the same transport pathway.

3Vitamin B12: A Uniquely Complex Journey

B12 absorption deserves special attention because it involves more steps than any other micronutrient. Dietary B12, bound to protein in food, is first freed by gastric acid and pepsin (Lesson 2.2) and briefly binds a salivary protein called haptocorrin. In the duodenum, pancreatic proteases break down haptocorrin, releasing B12 to bind intrinsic factor, a protein secreted by gastric parietal cells specifically for this purpose. The B12–intrinsic factor complex survives intact all the way to the terminal ileum (Lesson 2.5), where a dedicated receptor recognises and absorbs the complex as a unit — a journey requiring functioning stomach acid, functioning pepsin, functioning intrinsic factor secretion, functioning pancreatic enzymes, and a healthy terminal ileum, all in sequence. A defect at any single step — atrophic gastritis reducing acid and intrinsic factor, pancreatic insufficiency, or ileal disease — can independently cause B12 deficiency, which is why B12 deficiency has an unusually long list of possible underlying causes compared with most other micronutrient deficiencies.

4Iron and Calcium: Tightly Regulated at the Gate

Iron absorption is unusual in being actively regulated according to the body's need, chiefly by the hormone hepcidin, which reduces intestinal iron absorption when body iron stores are already adequate and increases it during deficiency — a feedback control that most other nutrients lack. Iron absorption is also strongly influenced by chemical form: haem iron (from animal sources) is absorbed considerably more efficiently than non-haem iron (from plant sources such as dal and leafy greens), and non-haem iron absorption is further boosted by vitamin C consumed in the same meal (which converts iron to a more absorbable form) and inhibited by tannins in tea and phytates in whole grains and legumes when consumed at the same sitting — the basis of the common advice to avoid tea immediately with or right after an iron-rich meal. Calcium absorption similarly depends on active transport that requires vitamin D (which upregulates the calcium-binding proteins involved) and is likewise inhibited by phytates and oxalates found in some vegetables.

5Water and Electrolyte Absorption

Water absorption is passive throughout the small and large intestine, following osmotic gradients created by the active absorption of solutes — principally sodium, glucose and amino acids — a mechanism directly connected to the SGLT1-driven sodium-glucose co-transport introduced in Lesson 1.2. As sodium and glucose are actively pulled into the enterocyte, water follows osmotically through and between the cells. The vast majority of the roughly 9 litres of fluid that enters the gut daily (from both food/drink and digestive secretions) is reabsorbed in the small intestine, with the large intestine reclaiming most of the remainder and typically only about 100–200 mL being lost in stool under normal conditions — which is why diarrhoeal illness, which disrupts this reabsorption, can cause dangerously rapid fluid loss.

Chapter Revision

Learning Goal: Consolidate the whole stomach-to-absorption pathway into a single mental model, and connect symptoms to the specific mechanism most likely responsible.

1The Consolidated Model

Follow a mixed meal — say, dal, rice, a small amount of ghee and a glass of buttermilk — through everything covered in this chapter. In the stomach (Lessons 2.1–2.3), acid and pepsin begin protein digestion while mechanical churning liquefies the meal into chyme, released into the duodenum only as fast as the enterogastric reflex allows, more slowly given the ghee's fat content. In the duodenum (Lesson 2.4), pancreatic bicarbonate neutralises the acid, bile emulsifies the ghee's fat, and pancreatic enzymes attack all three macronutrients at once. Across the jejunum and ileum (Lessons 2.5–2.6), the amplified villous surface absorbs the products: glucose and galactose from the rice's starch and the buttermilk's lactose via SGLT1, fructose (if any) via GLUT5, amino acids and small peptides from the dal's protein via sodium transporters and PepT1, and the ghee's fatty acids via the bile-salt-micelle-to-chylomicron pathway (Lessons 2.7–2.9). Meanwhile, B12 and any remaining bile salts are captured in the terminal ileum, iron absorption is throttled or boosted according to the body's current stores, and water follows the sodium and glucose gradients passively throughout (Lesson 2.10).

2Symptoms Mapped to Mechanism

A recurring theme across Volume 2 is that a symptom rarely has only one possible cause, but tracing it through this chapter's mechanisms usually narrows the list considerably. Persistent bloating after dairy points first toward lactase deficiency (Lesson 2.7); heavy, greasy stools point toward a fat-absorption failure somewhere in the bile-emulsification-micelle-chylomicron chain (Lesson 2.9); unexplained fatigue with normal iron studies but macrocytic red blood cells points toward B12 absorption specifically, and therefore toward the stomach-to-ileum chain described in Lesson 2.10 rather than diet alone; and a sensation of food "sitting like a stone" after fried meals points toward the fat-triggered enterogastric reflex slowing gastric emptying (Lesson 2.3), not toward a stomach disorder.

Assessment and Absorption Problems

Learning Goal: Demonstrate integrated command of gastric and small-intestinal physiology through recall, explanation and applied clinical reasoning.

AMultiple Choice

BShort Answer

CApplied Case Studies

DProfessional Judgement