Human Digestive System: MDCAT Biology notes
Human Digestive System for MDCAT: mouth, stomach cells, enzyme activation, bile, pancreatic and intestinal juice, gut hormones, absorption and disorders.
Wall of the gut
From the lumen outward the layers are mucosa (innermost), submucosa, muscularis and serosa. Food is moved by peristalsis, waves of involuntary muscle contraction; hunger pangs are peristaltic contractions of the empty stomach.
Mouth and swallowing
There are three pairs (six) of salivary glands opening into the buccal cavity. Saliva contains salivary amylase, which starts starch digestion in the mouth, breaking it to maltose; mucus to lubricate; and sodium bicarbonate to keep the pH near neutral. Amylase is secreted in active form and stops working in the acid of the stomach. In the pharyngeal stage of swallowing the epiglottis closes the larynx so food does not enter the lungs.
Stomach
Food passes the cardiac sphincter into the stomach. Gastric glands contain:
| Cell | Secretion |
|---|---|
| Parietal (oxyntic) cells | HCl and intrinsic factor (needed to absorb vitamin B12 in the ileum) |
| Chief (zymogen, peptic) cells | Pepsinogen, the enzymatic part of gastric juice |
| Mucus cells | Mucin, which protects the wall from acid |
| Endocrine (G) cells | Gastrin, which stimulates parietal and chief cells |
HCl converts pepsinogen to pepsin, and protein digestion begins in the stomach. The acid also kills microbes in food (an antiseptic role). Churned food becomes semi-fluid chyme, released through the pyloric sphincter into the duodenum.
Duodenum: bile and pancreatic juice
Most chemical digestion is completed in the duodenum.
- Bile is made in the liver and stored and concentrated in the gall bladder. It contains bile salts, bile pigments, lecithin, cholesterol and mucus but no digestive enzymes. Bile salts emulsify fats. Without a gall bladder, bile trickles in continuously rather than being released with meals, so fat digestion suffers.
- Pancreatic juice contains amylase, lipase (fats to fatty acids and glycerol), and the protein-digesting zymogens trypsinogen, chymotrypsinogen and procarboxypeptidase. Carboxypeptidase removes amino acids from the carboxyl end of peptides. Zymogens are activated only in the small intestine so the pancreas does not digest itself.
Enzyme activation
Enterokinase (from intestinal glands) is an enzyme activator: it converts trypsinogen to trypsin, and trypsin then activates the other pancreatic zymogens. Intestinal juice also contains erepsin (peptidases, secreted active), maltase, sucrase and lactase (sugar-digesting, unlike pepsin, trypsin and chymotrypsin). Protein digestion ends with amino acids and small peptides, which are absorbed.
Gut hormones
- Gastrin: stimulates gastric juice.
- Secretin: stimulates the pancreas to release bicarbonate-rich juice; it inhibits gastric secretion.
- Cholecystokinin (CCK): makes the gall bladder contract to release bile and stimulates pancreatic enzymes.
In the islets of Langerhans, beta cells secrete insulin (lowers blood glucose), alpha cells glucagon and delta cells somatostatin.
Absorption, liver and large intestine
Digested food is absorbed through villi of the small intestine (jejunum and ileum). Absorbed food goes to the liver by the hepatic portal vein. Kupffer cells in the liver are phagocytes. The large intestine begins with the caecum, which bears the appendix; then colon and rectum. It absorbs water and electrolytes, and its bacteria produce vitamins; it does not absorb amino acids.
Disorders
Dyspepsia: incomplete digestion. Anorexia nervosa: loss of appetite and refusal of food. Bulimia nervosa: bouts of overeating. Botulism is a form of food poisoning.
Common MDCAT traps
- Pepsinogen is activated by HCl; trypsinogen by enterokinase.
- Erepsin comes from intestinal glands, not the pancreas.
- Secretin stimulates pancreatic juice; it does not inhibit it.
- Bile is made in the liver, stored in the gall bladder, and has no enzymes.
- Intrinsic factor comes from the stomach (parietal cells).
Quick revision
- Starch digestion starts in the mouth; protein in the stomach.
- Parietal cells: HCl and intrinsic factor.
- CCK releases bile.
- Hepatic portal vein carries absorbed food to the liver.
- Appendix arises from the caecum.