Next Lesson - Upper Gastrointestinal Pathology
Core
Following a Mixed Meal
Think of one plate that holds starch, protein and fat, with water and a few micronutrients (vitamins and minerals needed in small amounts). Digestion is not a single chemical bath. At each station ask three questions: what is being cut, which enzyme or physical process acts, and what finally crosses the enterocyte (the absorptive cell lining the small bowel). Motility mixes and propels the meal, as covered in Gastrointestinal Motility and the Enteric Nervous System. Bile salts and pancreatic juice that arrive in the duodenum are produced and controlled as taught in Liver, Biliary and Exocrine Pancreatic Physiology. This lesson follows the substrates themselves from mouth to blood or lymph.
Each step in the journey sets up the next. The mouth increases surface area and starts starch. The stomach denatures protein (unfolds it) and begins proteolysis (protein breakdown). The duodenum receives acid chyme (the semi-digested mix of food and gastric juice), bicarbonate, bile and pancreatic enzymes so luminal digestion can finish safely. The jejunum and ileum present a huge brush-border surface (covered in microscopic finger-like projections called microvilli) with transporters that take monosaccharides (single sugars), amino acids, small peptides and, after micelle delivery, lipid building blocks. The colon reclaims water and electrolytes that escaped proximal absorption. Regional gross anatomy stays in the lessons on the foregut, midgut and hindgut (the embryological upper, middle and lower parts of the gut); the physiology here is the hand-off sequence for one meal.
Place that sequence onto the regional maps in Anatomy and Physiology of the Foregut, Anatomy and Physiology of the Midgut and Anatomy and Physiology of the Hindgut. The links prevent regional anatomy and integrated transport physiology from becoming separate facts.
Meal journey by segment: surface-area and early enzyme work, gastric protein attack, duodenal enzyme and bile mixing, small-bowel absorption, then colonic water salvage.
SimpleMed original educational diagram
Cephalic, Gastric and Intestinal Phases
Secretion is phased so enzymes and acid appear when food is expected or present, without flooding an empty lumen. The cephalic phase (it starts in the brain, before food reaches the stomach) is anticipatory. Sight, smell, taste and thought of food increase salivary flow (through parasympathetic fibres in the facial and glossopharyngeal nerves) and, through the vagus nerve, prepare gastric acid and pepsinogen (the inactive precursor of pepsin) output before the bolus (the chewed ball of food) arrives. It is neural preparation, not yet full chemical feedback from the gut lumen.
The gastric phase is driven by what is in the stomach. Wall distension and the sensing of peptides and amino acids stimulate the release of gastrin (a hormone that promotes acid secretion) from G cells in the lower part of the stomach (the antrum). Gastrin supports acid secretion from parietal cells (the acid-secreting cells of the stomach lining) and promotes pepsinogen release. Local reflexes and vagovagal reflexes (reflexes in which the vagus nerve carries both the sensory and the motor signal) amplify the same programme. Acid itself later restrains gastrin, so the phase is self-limiting once the meal is ground and emptied.
The intestinal phase is dominated by duodenal signals once acidic, fatty or hypertonic chyme arrives. Secretin (a hormone from S cells in the duodenal wall) is released when luminal pH falls and drives bicarbonate-rich fluid from pancreatic ducts and biliary epithelium. Cholecystokinin (CCK) from I cells, released by fat and protein digestion products, stimulates pancreatic enzyme secretion. It also contracts the gallbladder and helps relax the sphincter of Oddi, so bile and juice enter the lumen. The same nutrient signals slow gastric emptying so the small bowel is not overwhelmed. Do not treat the intestinal phase as simply another strong driver of gastric acid; its main job is to coordinate accessory secretions and pace delivery. Motility detail remains with the enteric nervous system (ENS) lesson; enzyme packaging and zymogen (inactive enzyme precursor) safety remain with the liver and pancreas lesson.
Mouth and Stomach Preparation
Mastication (chewing) reduces particle size and mixes food with saliva. Saliva lubricates the bolus for safe swallowing and supplies salivary amylase (a starch-digesting enzyme), which begins starch digestion by cleaving internal alpha-1,4 glycosidic bonds (the links between sugar units) to yield maltose, maltotriose and limit dextrins (short branched fragments of starch). That work continues until acid inactivates the enzyme in the stomach. Lingual lipase (a fat-digesting enzyme secreted in the mouth), and later gastric lipase, make a modest contribution to triglyceride hydrolysis (splitting by water) in adults: at the pH of the fed stomach, gastric lipase releases roughly 10 to 25 per cent of the fatty acid chains from dietary triglyceride. They matter more as early surface attack than as the main fat pathway.
In the stomach, acid does three useful things for digestion of a mixed meal. First, it creates a hostile environment for many swallowed microbes. Second, it denatures protein (unfolds it) so peptide bonds become more accessible. Third, it supports activation of pepsinogen to pepsin. Pepsin starts internal proteolysis (protein breakdown), producing large peptides rather than free amino acids ready for absorption. Carbohydrate digestion largely pauses in the acid environment. Intrinsic factor (a protein that binds vitamin B12), also secreted by parietal cells, travels onwards for later B12 handling rather than enabling gastric B12 absorption. Parietal-cell pharmacology and acid-suppressing drugs belong in their dedicated articles; here the point is functional preparation of the meal for the duodenum.
Carbohydrate Digestion and Transport
Once chyme is neutralised in the duodenum, pancreatic amylase continues starch breakdown. Its products are still oligosaccharides (short chains of sugar units) and disaccharides (two-sugar molecules), not free glucose ready for bulk absorption. Final liberation of monosaccharides (single sugars) is a brush-border job. Maltase-glucoamylase, sucrase-isomaltase and lactase cleave maltose, maltotriose, sucrose, limit dextrins and lactose into glucose, galactose (the other sugar in lactose) and fructose at the apical membrane (the lumen-facing surface of the cell). Dietary fibre that human enzymes cannot hydrolyse (split by adding water) continues distally and may be fermented by colonic bacteria. Fermentation is not absorption of the fibre itself.
Monosaccharide entry is transporter-specific. SGLT1 (sodium-glucose linked transporter 1) cotransports glucose or galactose with sodium across the apical membrane. It is driven by the sodium gradient that the Na+/K+-ATPase (the sodium-potassium pump) maintains on the basolateral side, the blood-facing surface of the cell. GLUT5 (glucose transporter 5) facilitates fructose entry at the apex. All three monosaccharides leave the enterocyte toward the interstitium (the tissue fluid between cells) and portal blood mainly through basolateral GLUT2 (glucose transporter 2). Remember the division of labour: luminal enzymes cut polymers; brush-border enzymes finish disaccharides; SGLT1 and GLUT5 take monomers in; GLUT2 sends them out.
Apical uptake map: SGLT1 for glucose and galactose with sodium, GLUT5 for fructose, PepT1 for di- and tripeptides with H+, and representative amino-acid carriers. Basolateral exit: GLUT2 exports monosaccharides and amino-acid carriers export amino acids toward portal blood.
SimpleMed original educational diagram
Protein Digestion and Peptide Uptake
Protein digestion is staged. Pepsin in the stomach starts the process. In the duodenum, enteropeptidase on the brush border activates trypsinogen to trypsin; trypsin then activates other pancreatic protease zymogens. Endopeptidases (trypsin, chymotrypsin, elastase) cut bonds inside the protein chain; carboxypeptidases trim free amino acids from the carboxyl end of the chain. The lumen therefore contains free amino acids plus oligopeptides (short chains of a few amino acids), not only single amino acids.
Absorption uses two complementary routes. Free amino acids ride several apical carriers, many sodium-coupled, with overlapping specificities for uncharged (neutral), positively charged (cationic) or negatively charged (anionic) side chains. Dipeptides and tripeptides take a high-capacity path through PepT1 (peptide transporter 1), which couples peptide entry to the inward hydrogen ion (H+) gradient. Inside the enterocyte, cytosolic peptidases (peptide-splitting enzymes in the cell fluid) often complete hydrolysis to free amino acids. Absorbed products leave across the basolateral membrane into interstitial fluid and then portal blood. Intact dietary protein does not need to cross as whole molecules for nutrition; what the body mainly absorbs is amino acids and small peptides, which are then used around the body.
Fat: Emulsification, Micelles and Chylomicrons
Dietary triglyceride is insoluble in water. Bile salts emulsify large fat droplets into smaller ones, increasing the surface available to pancreatic lipase and colipase (a small protein that anchors lipase to the fat droplet). Lipase yields mainly 2-monoglycerides (glycerol with one fatty acid attached) and free fatty acids; cholesterol and fat-soluble vitamins travel with the lipid phase. These lipids are then packaged into mixed micelles: small, bile-salt-stabilised aggregates that ferry long-chain fatty acids, monoglycerides, cholesterol and vitamins A, D, E and K through the aqueous unstirred layer (the thin film of water next to the cell surface) to the brush border.
Critical geometry: micelles do not enter the enterocyte intact. Lipid leaves the micelle at the apical membrane and is taken up; bile salts remain largely in the lumen for reuse until they are reclaimed in the terminal ileum (the last part of the small bowel). Inside the cell, long-chain products are re-esterified (rebuilt) into triglyceride in the smooth endoplasmic reticulum. They are then assembled with phospholipid, cholesterol and apolipoprotein B-48 into chylomicrons (fat-carrying particles), which are released into lacteals (lymph vessels in the gut lining) rather than portal capillaries. Short- and medium-chain fatty acids can reach portal blood more directly with less obligatory chylomicron packaging. Systemic lipoprotein metabolism after chylomicrons enter blood is taught with lipid transport articles, not here.
Fat path: emulsion to mixed micelle, monomer uptake at the brush border without whole-micelle entry, re-esterification and apoB-48 chylomicron assembly, then lacteal export.
SimpleMed original educational diagram
Water, Electrolytes and Regional Micronutrients
About 8 to 9 litres of fluid enter the gut each day from diet and secretions; most is reabsorbed (only about 100 to 200 mL is lost in stool), so stool remains formed. Water is not pumped by a dedicated water ATPase. It follows net solute absorption osmotically, either through the cells (transcellular) or between the cells (paracellular), by routes that vary by segment. Sodium uptake includes nutrient cotransport (as with SGLT1) and other segment-specific pathways; chloride and water follow when net solute leaves the lumen. The small bowel absorbs the bulk of the fluid load; the colon salvages remaining water and electrolytes. These volumes vary with diet and secretion, so treat them as typical figures. The principle is that water recovery follows solute recovery, and the colon adjusts the final amount.
Location matters for selected micronutrients. Iron is absorbed chiefly in the duodenum and proximal jejunum after reduction of ferric iron (Fe3+, the oxidised form) and apical uptake of ferrous iron (Fe2+, the reduced form), with regulated basolateral export. Folate (vitamin B9) is taken up mainly in the proximal small bowel. Vitamin B12 is freed from food protein and bound first to haptocorrin (a B12-binding protein found in saliva). Pancreatic proteases then digest haptocorrin, and the B12 is transferred to intrinsic factor. The intrinsic-factor-B12 complex is absorbed by receptor-mediated endocytosis (uptake in a membrane-bound vesicle after binding to a receptor) in the terminal ileum. Conjugated bile salts (bile salts joined to glycine or taurine) are actively reclaimed in the terminal ileum and returned to the liver (enterohepatic circulation), as mapped in the previous lesson, Liver, Biliary and Exocrine Pancreatic Physiology. Major macronutrient (carbohydrate, protein and fat) absorption is predominantly small-bowel work along the duodenum, jejunum and ileum, not a single exclusive centimetre. These are predominance statements, not absolute bans on minor uptake elsewhere.
Predominant locations: proximal iron, small-bowel macronutrients with bulk solute-led water, terminal-ileal B12-intrinsic-factor and bile-salt reclaim, residual water and ion colonic salvage.
SimpleMed original educational diagram
Two brief failures illuminate the normal map. If lactase is deficient, lactose remains an osmotically active, fermentable sugar in the lumen, drawing water and generating gas after dairy. If coeliac injury flattens villi (the finger-like folds that line the small bowel), surface area and brush-border enzyme capacity fall, so several absorption steps fail together. Diagnosis and treatment stay in clinical articles; the physiology lesson is that missing enzymes or missing surface both break the cut-and-cross sequence.
The next lesson, Upper Gastrointestinal Pathology, applies these mechanisms when structure or regulation fails in the upper gut.
Reviewed by: Dr. Marcus Judge
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