2.2 The Digestive System: Organs, Enzymes, GI Hormones, and the Liver
Key Takeaways
Gastric parietal cells secrete hydrochloric acid and intrinsic factor, chief cells secrete pepsinogen, and G cells secrete gastrin.
Secretin, released when acid enters the duodenum, stimulates pancreatic bicarbonate; cholecystokinin, released by fat and protein, stimulates gallbladder contraction and pancreatic enzyme secretion and slows gastric emptying.
Iron and calcium are absorbed mainly in the duodenum, most nutrients in the jejunum, and vitamin B12 (with intrinsic factor) and bile salts in the terminal ileum.
About 95% of bile salts are reabsorbed in the terminal ileum and returned to the liver through the enterohepatic circulation.
The colon absorbs water and electrolytes and ferments fiber into short-chain fatty acids, and colonic bacteria synthesize vitamin K and biotin.
The table of specifications asks examinees to identify the function of the digestive system, the gastrointestinal hormones and enzymes, and the role of the liver and other organs in metabolism. These facts underlie clinical questions on gastrectomy, short bowel syndrome, pancreatitis, and liver disease.
Organs of the Digestive Tract
| Organ | Key structures and secretions | Main functions |
|---|---|---|
| Mouth | Teeth, tongue, salivary glands | Chewing; salivary amylase begins starch digestion; lingual lipase begins fat digestion |
| Esophagus | Upper and lower esophageal sphincters | Peristalsis carries the bolus; the lower sphincter prevents reflux |
| Stomach | Fundus, body, antrum, pylorus; parietal, chief, and G cells | Stores and mixes food into chyme; HCl denatures protein and kills microbes; pepsin starts protein digestion; intrinsic factor binds vitamin B12 |
| Small intestine | Duodenum (about 25 cm), jejunum (about 2.5 m), ileum (about 3.5 m); villi and microvilli | Main site of digestion and absorption; brush border enzymes finish digestion |
| Large intestine | Cecum, colon, rectum | Absorbs water and electrolytes; bacterial fermentation; forms and stores feces |
| Liver | Hepatocytes, bile ducts | Makes bile; processes absorbed nutrients; stores glycogen, vitamins A, D, and B12, and iron; makes plasma proteins; forms urea; detoxifies drugs and alcohol |
| Gallbladder | Stores and concentrates bile | Contracts in response to cholecystokinin |
| Pancreas | Acinar cells (exocrine); islets (endocrine) | Secretes digestive enzymes and bicarbonate; islets secrete insulin and glucagon |
Phases of Digestive Control
- Cephalic phase: the sight, smell, or thought of food stimulates the vagus nerve, increasing saliva and gastric secretion.
- Gastric phase: stomach distension and protein stimulate gastrin and acid secretion.
- Intestinal phase: acid, fat, and protein entering the duodenum release secretin and cholecystokinin, which slow gastric emptying and stimulate the pancreas and gallbladder.
Gastrointestinal Hormones
| Hormone | Source | Main stimulus | Main actions |
|---|---|---|---|
| Gastrin | G cells of the gastric antrum | Protein in the stomach, distension, vagal stimulation | Increases HCl secretion and gastric motility |
| Secretin | S cells of the duodenum | Acid in the duodenum | Increases pancreatic bicarbonate and bile flow; decreases acid secretion |
| Cholecystokinin (CCK) | I cells of the duodenum and jejunum | Fat and protein in the duodenum | Gallbladder contraction, pancreatic enzyme secretion, slower gastric emptying, satiety |
| GIP | K cells of the duodenum | Glucose and fat | Stimulates insulin release (incretin) |
| GLP-1 | L cells of the ileum and colon | Nutrients in the distal gut | Stimulates insulin, inhibits glucagon, slows gastric emptying, promotes satiety |
| Ghrelin | Stomach | Fasting | Stimulates hunger |
| Peptide YY | Ileum and colon | Food in the distal gut | Promotes satiety |
| Motilin | Duodenum | Fasting | Triggers the migrating motor complex that sweeps the gut between meals |
| Somatostatin | D cells | Various | Inhibits most GI secretions and hormones |
Tip
A classic question links fat to cholecystokinin and acid to secretin. A high-fat meal causes gallbladder contraction, which is why fatty meals trigger pain in gallstone disease.
Digestive Enzymes by Site
| Site | Enzyme | Substrate and product |
|---|---|---|
| Mouth | Salivary amylase | Starch to dextrins and maltose |
| Stomach | Pepsin (from pepsinogen) | Proteins to polypeptides |
| Stomach | Gastric lipase | Short- and medium-chain triglycerides |
| Pancreas | Pancreatic amylase | Starch to maltose, maltotriose, limit dextrins |
| Pancreas | Trypsin, chymotrypsin, elastase, carboxypeptidases | Proteins to peptides and amino acids |
| Pancreas | Pancreatic lipase with colipase | Triglycerides to 2-monoglycerides and fatty acids |
| Brush border | Maltase, sucrase-isomaltase, lactase | Disaccharides to monosaccharides |
| Brush border | Enteropeptidase (enterokinase), aminopeptidases, dipeptidases | Activates trypsinogen; finishes protein digestion |
Where Nutrients Are Absorbed
| Segment | Main nutrients absorbed |
|---|---|
| Duodenum | Iron, calcium, magnesium, zinc, copper, folate (also jejunum) |
| Jejunum | Most carbohydrates, amino acids, fats, water-soluble vitamins |
| Terminal ileum | Vitamin B12 bound to intrinsic factor, bile salts |
| Colon | Water, sodium, potassium, short-chain fatty acids |
Clinical consequences follow directly. Loss of the stomach removes intrinsic factor, so B12 must be given by injection. Resection of the terminal ileum causes B12 deficiency and bile acid loss, which leads to fat malabsorption. Duodenal bypass increases the risk of iron and calcium deficiency.
Bile and the Enterohepatic Circulation
The liver makes bile from cholesterol, bile salts, phospholipids, bilirubin, and water. Bile salts emulsify fat and form micelles. About 95% of bile salts are reabsorbed in the terminal ileum and return to the liver through the portal vein, so the same bile salt pool is reused several times a day. Soluble fiber binds bile acids and increases their excretion, which forces the liver to use cholesterol to make new bile acids and lowers blood LDL.
The Liver's Metabolic Roles
- Carbohydrate: stores glycogen, releases glucose by glycogenolysis and gluconeogenesis, and converts galactose and fructose to glucose.
- Protein: deaminates amino acids, forms urea, and synthesizes albumin, transport proteins, and clotting factors.
- Fat: makes lipoproteins (VLDL), cholesterol, bile salts, and ketone bodies.
- Vitamins and minerals: activates vitamin D (25-hydroxylation) and stores vitamins A, D, and B12, iron, and copper.
- Detoxification: metabolizes alcohol, drugs, and ammonia.
The Colon and Gut Microbiota
Colonic bacteria ferment fiber and resistant starch into short-chain fatty acids (acetate, propionate, butyrate); butyrate is the main fuel of colonocytes. Bacteria also synthesize vitamin K and biotin. Prebiotics are non-digestible food components that feed beneficial bacteria, and probiotics are live beneficial microorganisms.
A patient with gallstones has severe pain after eating a fatty lechon kawali meal. Which hormone is most directly responsible for the gallbladder contraction that causes the pain?
Gastrin
Gastric inhibitory peptide
Cholecystokinin
Ghrelin
After resection of the terminal ileum, which pair of problems should the nutritionist-dietitian anticipate?
Iron deficiency and lactose intolerance only, from loss of the duodenum
Vitamin B12 deficiency and fat malabsorption from bile salt loss
Calcium deficiency from loss of the duodenum
Excess bile salt absorption and constipation
Which statement about the stomach's secretory cells is correct?
Parietal cells secrete hydrochloric acid and intrinsic factor
Chief cells secrete gastrin and intrinsic factor
G cells secrete pepsinogen and gastric lipase
Parietal cells secrete pepsinogen and bicarbonate into the gastric lumen
Sections you finish are checked off in the contents.