30 feet of GI tract, accessory organs, enzymes, hormones, and absorption sites — the digestive system covers a lot of ground. These memory tricks help you follow food from mouth to rectum and understand what happens at each stop.
Water and electrolyte absorption — no digestion. Bacteria produce vitamin K.
Small Intestine Regions
Dirty Jobs In — Duodenum · Jejunum · Ileum
Duodenum (25cm) · Jejunum (2.5m) · Ileum (3.5m)
Three regions of the small intestine — length, function, and what each absorbs
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.
Duodenum
25 cm — receives bile + pancreatic enzymes. Ampulla of Vater = entry point.
Jejunum
2.5 m — most nutrient absorption. Carbs, proteins, fats, fat-soluble vitamins.
Ileum
3.5 m — vitamin B12 + bile salt absorption only here. Terminal ileum.
Villi/microvilli
Surface area = tennis court. Brush border enzymes complete digestion.
Large Intestine Regions
Cecum Ascends, Travels, Descends, Sigmoidly to Rectum
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.
Cecum
Blind pouch — appendix attached here. Ileocecal valve above it.
Ascending
Right side — hepatic flexure at liver.
Transverse
Crosses abdomen — most mobile segment.
Descending
Left side — splenic flexure at spleen.
Sigmoid
S-shaped — most common site of diverticulitis and colorectal cancer.
Accessory Organs
LGP — Liver · Gallbladder · Pancreas
Three accessory digestive organs that contribute secretions to the duodenum
Three accessory organs — none is part of the GI tract but all are essential
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.
Liver
Bile production, detoxification, glycogen storage, clotting factors, albumin.
Gallbladder
Stores/concentrates bile — CCK triggers release when fat enters duodenum.
Where bile duct + pancreatic duct join and enter duodenum.
Digestive Enzymes
ALPS — Amylase · Lipase · Proteases · Sucrase
Starch → Fats → Proteins → Sucrose — each enzyme breaks one macromolecule
Key digestive enzymes — what they digest and where they work
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.
Three key digestive hormones — what triggers each and what it does
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.
Gastrin
G cells in stomach — protein triggers → stimulates HCl + gastric motility.
Secretin
S cells in duodenum — acid triggers → pancreatic bicarbonate (neutralizes).
CCK
I cells in duodenum — fat triggers → gallbladder contraction + pancreatic enzymes.
Three gastric gland cell types — what each secretes and why it matters
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).
What bile actually does — and why it is not an enzyme
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.
Emulsification
Breaks fat globules into micelles — increases surface area for lipase.
Bilirubin
Hemoglobin breakdown → excreted in bile → gives stool its brown color.
Jaundice
Bilirubin in blood → yellow skin/eyes. Pre-hepatic, hepatic, or post-hepatic causes.
Gallstones
Cholesterol or bilirubin crystals — right upper quadrant pain after fatty meals.
Liver Functions
MADE BIG — Metabolism · Albumin · Detox · Enzymes · Bile · Immunity · Glycogen
Seven major liver functions
The liver does more than you think — seven functions to know
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.
Metabolism
Processes all absorbed nutrients from portal blood.
Albumin
Main plasma protein — low albumin → edema (no oncotic pressure).
✅ The pyloric sphincter controls the rate at which chyme (partially digested food) empties from the stomach into the duodenum. It prevents too-rapid entry of acidic chyme.
❓ Where does most nutrient absorption occur in the GI tract?
✅ The small intestine — specifically the jejunum. The duodenum handles most chemical digestion. The ileum absorbs vitamin B12 and bile salts.
❓ What is the role of the liver in digestion?
✅ The liver produces bile (stored in gallbladder), metabolizes nutrients absorbed from the gut, detoxifies drugs and toxins, produces clotting factors, and metabolizes bilirubin from old RBCs.
❓ Which enzyme breaks down carbohydrates and where is it produced?
✅ Salivary amylase (mouth) and pancreatic amylase (duodenum) break down complex carbohydrates into disaccharides. Brush border enzymes (maltase, sucrase, lactase) complete digestion in the small intestine.