Memory tricks for digestion from start to finish

Food enters the mouth (mechanical and chemical digestion begin) → pharynx → esophagus (peristalsis, no digestion) → stomach (protein digestion, chyme formation) → small intestine: duodenum → jejunum → ileum (90% absorption occurs here) → ileocecal valve → large intestine: cecum → ascending → transverse → descending → sigmoid colon (water absorption, stool formation) → rectum → anal canal → anus. Total length approximately 30 feet.

Duodenum (12 finger widths long): receives chyme from stomach plus bile from liver/gallbladder and pancreatic enzymes. Most digestion occurs here. Jejunum: most absorption of nutrients — carbohydrates, proteins, fats, vitamins. Ileum: absorbs vitamin B12 (with intrinsic factor) and bile salts — the only place these are absorbed. Ileum connects to large intestine at ileocecal valve. Villi and microvilli (brush border) dramatically increase surface area.

The large intestine frames the small intestine in the abdominal cavity. Cecum (with appendix attached) → Ascending colon (right side, goes up) → Transverse colon (crosses the abdomen) → Descending colon (left side, goes down) → Sigmoid colon (S-shaped, most common site of diverticulitis) → Rectum → Anal canal. Haustra (pouches) give colon its segmented appearance. Water is the primary substance absorbed — no digestion occurs.

Liver: produces bile (1 L/day), detoxifies blood, metabolizes nutrients, produces clotting factors and albumin. Gallbladder: stores and concentrates bile — releases it when fat is detected in duodenum. Pancreas: dual function — exocrine (digestive enzymes: lipase, amylase, proteases) and endocrine (insulin and glucagon). All three deliver secretions to the duodenum via the ampulla of Vater — controlled by the sphincter of Oddi.

Salivary amylase begins starch digestion in mouth. Pancreatic amylase continues in small intestine. Lipase (pancreatic) breaks fats into fatty acids and monoglycerides — requires bile for emulsification first. Proteases (pepsin in stomach, trypsin and chymotrypsin from pancreas) break proteins into peptides and amino acids. Brush border enzymes (maltase, sucrase, lactase) complete carbohydrate digestion at the intestinal surface.

Gastrin: released by stomach G cells when protein enters stomach → stimulates HCl secretion and gastric motility. Secretin: released by duodenal S cells when acid enters → stimulates pancreas to release bicarbonate (neutralizes acid). CCK (Cholecystokinin): released by duodenal I cells when fat and protein enter → stimulates gallbladder contraction and pancreatic enzyme release. Memory: Gastrin = stomach acid, Secretin = neutralize, CCK = fat digestion.

Parietal cells secrete HCl (makes stomach pH 1.5-3.5 — kills bacteria and activates pepsin) and intrinsic factor (essential for vitamin B12 absorption in ileum). Loss of parietal cells → pernicious anemia. Chief cells secrete pepsinogen (inactive) → activated to pepsin by HCl. Mucous cells secrete mucus that coats the stomach lining — prevents self-digestion. Ulcers occur when this protection fails (H. pylori or NSAIDs).

Bile is produced by the liver and stored in the gallbladder — it is NOT an enzyme. It is a detergent-like substance that emulsifies fat (breaks large fat globules into smaller droplets), dramatically increasing surface area for lipase to work. Bile also carries bilirubin (breakdown product of hemoglobin) and cholesterol for excretion. Jaundice = bilirubin accumulates in blood when bile flow is obstructed or liver is damaged. Gallstones = crystallized cholesterol or bilirubin in gallbladder.

The liver is the most metabolically active organ in the body. Metabolism: processes carbs, fats, and amino acids. Albumin: produces the main plasma protein — maintains osmotic pressure. Detoxification: metabolizes drugs, alcohol, hormones, ammonia → urea. Enzymes: produces clotting factors (I, II, V, VII, IX, X). Bile: 1 L/day for fat digestion. Immunity: Kupffer cells phagocytize bacteria from portal blood. Glycogen: stores glucose as glycogen — releases when blood sugar drops.