Digestive System

The digestive system breaks food into absorbable nutrients, and it is one of the highest-yield organ systems on the MCAT. This guide walks the tract from mouth to rectum at review depth — concepts and the logic behind them, not memorize-the-table detail.

Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.

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Overview: The Big Picture

Must know

Think of the GI tract as a disassembly line: large macromolecules (carbs, proteins, fats, nucleic acids) must be hydrolyzed into monomers small enough to cross the epithelium. Two modes work together:

• Mechanical digestion: chewing, churning, peristalsis — increases surface area
• Chemical digestion: enzymatic hydrolysis of covalent bonds

Nutrients absorbed by the small intestine (sugars, amino acids, short-chain fatty acids) enter the hepatic portal vein and pass through the liver before systemic circulation. Long-chain fatty acids take a separate route through lacteals → lymph → thoracic duct, bypassing the liver initially. The MCAT tests this distinction repeatedly.

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Ingestion and the Oral Cavity

Must know

Digestion begins before swallowing. Teeth break food mechanically; the tongue forms a bolus; and saliva starts chemical digestion. Saliva's roles:

• Lubrication via the glycoprotein mucin, easing swallowing and protecting the epithelium
• Salivary amylase (ptyalin): cleaves $\alpha$-1,4 glycosidic bonds in starch → maltose/oligosaccharides. Inactivated by stomach acid, so it only partially digests starch.
• Lingual lipase: minor fat hydrolysis that continues briefly in the stomach

Saliva secretion is autonomically driven (parasympathetic dominant).

Optional

Lysozyme and IgA add antimicrobial defense.

Quick check: If a patient's salivary glands were removed, what happens to starch digestion? Answer: Delayed but not eliminated — pancreatic amylase fully compensates. The bigger problem is impaired lubrication and swallowing.

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The Esophagus: Transport, Not Digestion

Must know

A ~25 cm muscular tube connecting pharynx to stomach. Transport only — no enzymes, no absorption.

Swallowing (deglutition) is a reflex: the soft palate seals the nasopharynx, the epiglottis covers the trachea, and the bolus is driven by peristalsis (circular muscle contracts behind, relaxes ahead). The upper esophagus is skeletal muscle (voluntary), transitioning to smooth muscle below.

The lower esophageal sphincter (LES) prevents reflux of acidic gastric contents; its failure causes GERD.

Quick check: Peristalsis in the esophagus is coordinated by which nervous system? Answer: The enteric nervous system (myenteric/Auerbach's plexus), modulated by the vagus. It is involuntary once a swallow begins.

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The Stomach

Must know

The stomach stores, churns (mixing food into semi-liquid chyme), and begins protein digestion.

Passage-level

Gross structure: four regions — cardia (entry), fundus, body, pylorus (exits via the pyloric sphincter). The lining folds into rugae that flatten as the stomach fills.

Gastric Glands

Know the logic

Cell	Secretion	Function
Chief cells	Pepsinogen	Zymogen → pepsin
Parietal cells	HCl + intrinsic factor	Acid; B₁₂ absorption
G cells	Gastrin	Hormone → ↑ acid/pepsin
Mucous cells	Mucus	Protects lining from self-digestion

Optional

ECL cells release histamine to stimulate parietal cells.

How HCl Is Made

Know the logic

Inside the parietal cell, $\ce{CO2 + H2O ->[carbonic anhydrase] H2CO3 -> H+ + HCO3-}$. The $\ce{H+}$ is pumped into the lumen by the $\ce{H+/K+}$-ATPase (the proton pump targeted by PPIs like omeprazole), while $\ce{HCO3-}$ exits into blood in exchange for $\ce{Cl-}$, which then enters the lumen to pair with $\ce{H+}$ as $\ce{HCl}$. The $\ce{HCO3-}$ entering blood after a meal is the "alkaline tide."

Pepsin and the Zymogen Concept

Must know

Chief cells secrete pepsinogen, an inactive zymogen. The acidic stomach cleaves an inhibitory peptide to expose active pepsin, an endopeptidase that cleaves peptide bonds next to aromatic/hydrophobic residues. The zymogen strategy plus the mucus layer protect the stomach from self-digestion.

Low pH: Multiple Functions

Must know

Stomach acid (pH ~1–2) activates pepsinogen, denatures proteins (exposing peptide bonds), kills microbes, and inactivates salivary amylase.

Phases of Gastric Secretion

Passage-level

1. Cephalic: sight/smell/thought of food; via the vagus
2. Gastric: protein and distension → G cells release gastrin → ↑ HCl + pepsinogen
3. Intestinal: chyme in the duodenum sends both stimulatory and (mostly) inhibitory signals back to the stomach

Quick check: Why secrete pepsinogen rather than pepsin? Answer: Active pepsin would digest the cells that made it. The zymogen stays inactive until it reaches the acidic lumen, where both activation and substrate are present.

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The Small Intestine: Primary Site of Digestion and Absorption

Structure: Maximizing Surface Area

Must know

Three nested adaptations expand surface area from ~tube-like to roughly a tennis court (~250 m²):

• Plicae circulares: large circular folds
• Villi: finger-like projections, each with a lacteal (for fat) and a capillary network (for sugars/amino acids)
• Microvilli (the brush border): apical projections housing brush-border enzymes

Anatomical Subdivisions

Must know

• Duodenum: receives chyme, bile, and pancreatic juice; site of most chemical digestion; iron absorption
• Jejunum: major site of nutrient absorption
• Ileum: absorbs vitamin B₁₂–intrinsic factor complex and recycles bile salts; ends at the ileocecal valve

Passage-level

Bile and pancreatic juice enter at the ampulla of Vater, gated by the sphincter of Oddi.

Neutralization of Stomach Acid

Must know

Chyme arrives at pH ~2, which would destroy pancreatic enzymes. Bicarbonate from the pancreas (driven by the hormone secretin) is the main neutralizer, raising duodenal pH to ~6–7.

Optional

Brunner's glands add alkaline mucus.

Brush Border Enzymes

Know the logic

The small intestine makes its own enzymes anchored in the brush border to finish digestion at the cell surface — disaccharidases (maltase, sucrase, lactase) → monosaccharides, and peptidases → amino acids. One special brush-border enzyme, enteropeptidase (enterokinase), activates pancreatic trypsinogen → trypsin.

Lactase deficiency → lactose intolerance: undigested lactose is fermented by colonic bacteria, causing gas, bloating, and osmotic diarrhea.

Absorption of Nutrients

Must know

• Carbohydrates → monosaccharides. Glucose/galactose enter via Na⁺-coupled cotransport (secondary active transport); fructose by facilitated diffusion → capillaries → portal vein.
• Proteins → amino acids/small peptides via Na⁺- or H⁺-coupled cotransport → portal vein.
• Lipids: fatty acids + monoglycerides ride micelles to the brush border, enter the enterocyte, are re-esterified into triglycerides, packaged into chylomicrons, and secreted into lacteals → lymph → systemic circulation, bypassing the liver initially.
• Water: absorbed osmotically following solutes.
• Vitamins/minerals: fat-soluble vitamins (A, D, E, K) follow micelles; B₁₂ needs intrinsic factor and is absorbed in the terminal ileum; iron in the duodenum.

Quick check: A patient has the terminal ileum removed. Which deficiency and consequence? Answer: Vitamin B₁₂ deficiency (sole site of B₁₂–intrinsic factor absorption). B₁₂ is needed for DNA synthesis → megaloblastic (macrocytic) anemia.

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The Pancreas: The Master Enzyme Factory

Exocrine vs. Endocrine

Must know

• Endocrine (islets of Langerhans): insulin (β), glucagon (α), somatostatin (δ) → blood
• Exocrine (the bulk): acinar cells make digestive enzymes; ductal cells secrete bicarbonate

This guide focuses on exocrine function, but know both exist.

Pancreatic Enzymes and the Zymogen Cascade

Must know

The pancreas digests all four macromolecule classes, secreting proteases as inactive zymogens so it doesn't digest itself. Enteropeptidase activates trypsinogen → trypsin, and trypsin then activates all the other zymogens (chymotrypsinogen, proelastase, procarboxypeptidases, and more trypsinogen autocatalytically).

Optional

Trypsin, chymotrypsin, and elastase are endopeptidases; carboxypeptidases A/B are exopeptidases removing C-terminal residues.

Other enzymes (secreted active): pancreatic amylase (starch → maltose), pancreatic lipase (TG → 2 fatty acids + monoglyceride; needs colipase to work at the lipid-water interface among bile salts), plus phospholipase, cholesterol esterase, and nucleases.

Transport to the Small Intestine

Must know

Pancreatic juice flows through the pancreatic duct → joins the common bile duct → empties into the duodenum at the sphincter of Oddi. Enzyme release is driven by CCK; bicarbonate by secretin.

Quick check: In acute pancreatitis, trypsinogen is activated inside the pancreas. Why so destructive? Answer: Active trypsin triggers the whole zymogen cascade inside the organ, turning the pancreas's own proteases against its tissue (elastase can erode vessels → hemorrhage).

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The Liver

Position in the GI System

Must know

Because blood from the GI tract drains through the hepatic portal vein to the liver, the liver is the first-pass organ for all absorbed nutrients except chylomicron fats (which arrive via lymph).

Optional

Its functional unit is the hepatic lobule.

Production of Bile

Must know

Bile is made continuously by hepatocytes and routed via bile ducts. Key components:

• Bile salts: amphipathic, cholesterol-derived — the active agents for fat digestion
• Bilirubin: waste from hemoglobin breakdown; gives bile/feces their color
• cholesterol, phospholipids, water, ions

Bile is not an enzyme — it emulsifies fat (breaks large droplets into small ones, increasing surface area for lipase). Think dish soap.

Passage-level

Enterohepatic circulation: most bile salts are reabsorbed in the terminal ileum and recycled to the liver, with little lost in feces.

Blood Glucose Regulation

Must know

The liver buffers blood glucose. After a meal, insulin drives glucose uptake → glycogenesis and lipogenesis. Between meals, glucagon drives glycogenolysis and gluconeogenesis, exporting glucose for the brain and RBCs.

Detoxification

Know the logic

The liver makes toxins water-soluble for excretion. It metabolizes drugs/xenobiotics (Phase I oxidation + Phase II conjugation), converts ammonia → urea (urea cycle), metabolizes alcohol, and conjugates bilirubin for secretion in bile.

Quick check: Liver failure prevents bilirubin conjugation. Blood and feces? Answer: Blood accumulates unconjugated (indirect) bilirubin → jaundice; feces are pale because no bilirubin-derived pigment reaches the gut.

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Bile and the Gallbladder

Storage and Concentration

Must know

Between meals, bile is diverted into the gallbladder and concentrated by water reabsorption. When fat enters the duodenum, CCK triggers gallbladder contraction and relaxation of the sphincter of Oddi (and stimulates pancreatic enzyme release).

Passage-level

Gallstones form from cholesterol- or bilirubin-supersaturated bile; a stone lodged at the ampulla can block both bile and pancreatic flow → jaundice + pancreatitis.

Function of Bile Salts

Must know

Bile salts are amphipathic (hydrophobic steroid face, hydrophilic face). They (1) emulsify fat globules and (2) form micelles that ferry fatty acids, monoglycerides, cholesterol, and fat-soluble vitamins to the brush border.

Quick check: Why does cholecystectomy cause problems with high-fat but not low-fat meals? Answer: Without storage, bile trickles in continuously instead of as a concentrated bolus. A large fatty meal overwhelms the trickle → fat malabsorption, steatorrhea; low-fat meals are handled fine.

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The Large Intestine

Water and Electrolyte Absorption

Must know

The colon's primary job is reabsorbing water and electrolytes (Na⁺, Cl⁻, then water osmotically), concentrating ~1.5 L of chyme into ~100–150 mL of feces. Impaired reabsorption → diarrhea. The colon has no villi; goblet cells secrete lubricating mucus.

Passage-level

Subdivisions: cecum → ascending → transverse → descending → sigmoid → rectum → anus; the ileocecal valve controls inflow.

Optional

The appendix is clinically notable for appendicitis.

Gut Microbiota

Must know

Colonic bacteria ferment fiber → short-chain fatty acids (energy for colonocytes) and synthesize vitamin K and some B vitamins; they also outcompete pathogens.

Optional

Antibiotic disruption lets C. difficile overgrow.

Rectum and Defecation

Must know

The rectum stores feces; distension triggers the defecation reflex — the internal anal sphincter (smooth muscle, involuntary) relaxes, while the external anal sphincter (skeletal muscle, voluntary) gives conscious control. Both must relax to defecate.

Quick check: A spinal cord injury above S2–S4 disrupts voluntary defecation but spares the reflex. Which sphincter is affected and why? Answer: Voluntary control of the external anal sphincter (skeletal, somatic via pudendal nerve, S2–S4) is lost; the internal sphincter reflex (autonomic) can remain intact.

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Peristalsis and Muscular Control

Must know

Peristalsis propels contents — circular muscle contracts behind the bolus and relaxes ahead of it (receptive relaxation), while longitudinal muscle ahead contracts. It is an intrinsic myenteric reflex, so it works even in isolated intestine.

Segmentation is different: alternating contractions slosh contents back and forth to mix and maximize mucosal contact (predominant in the small intestine after a meal).

Optional

Mass movements in the colon, often via the gastrocolic reflex, occur a few times a day.

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Endocrine Control: The GI Hormones

Must know

Hormone	Source	Stimulus	Major Actions
Gastrin	stomach (G cells)	protein, distension, vagus	↑ HCl, ↑ pepsinogen, ↑ motility
Secretin	duodenum	acid in duodenum	↑ pancreatic bicarbonate; ↓ gastric acid
CCK	duodenum	fats + protein in duodenum	↑ pancreatic enzymes, ↑ gallbladder contraction, ↓ gastric emptying

Quick check: A drug blocks CCK receptors. Predicted effects? Answer: ↓ pancreatic enzyme release and impaired gallbladder contraction → fat/protein maldigestion, steatorrhea; faster gastric emptying; loss of satiety signal.

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Nervous Control: The Enteric Nervous System

Must know

The enteric nervous system (ENS) — the "gut brain" — has ~100–500 million neurons and runs basic GI functions (motility, secretion, blood flow) independently of the CNS. Two plexuses:

• Myenteric (Auerbach's), between the muscle layers → controls motility
• Submucosal (Meissner's), in the submucosa → controls secretion and senses the lumen

Optional

Neurotransmitters: ACh and substance P are excitatory; VIP and NO are inhibitory; ~95% of body serotonin is in the gut.

Autonomic modulation: parasympathetic (vagus + sacral nerves) increases motility and secretion; sympathetic decreases them, constricts GI vessels, and raises sphincter tone.

Optional

Clinical: Hirschsprung disease — absent ENS ganglia in the rectosigmoid → functional obstruction in neonates.

Quick check: Why does morphine cause constipation? Answer: Opioid (μ) receptors on ENS neurons inhibit ACh release → ↓ peristalsis → slower transit → more water absorption → hard, dry stools.

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Summary of Digestion by Macromolecule

Must know

• Carbohydrates: salivary amylase (mouth, partial) → pancreatic amylase (duodenum) → brush-border disaccharidases → monosaccharides
• Proteins: pepsin (stomach) → pancreatic proteases trypsin/chymotrypsin/elastase + carboxypeptidases (duodenum) → brush-border peptidases → amino acids
• Fats: minor lingual/gastric lipase → bile-salt emulsification + pancreatic lipase (+colipase) in the duodenum → fatty acids + monoglycerides → micelles → enterocytes → chylomicrons → lacteals
• Nucleic acids: pancreatic DNase/RNase → brush-border nucleotidases/nucleosidases → nucleosides, bases, phosphate

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Common Confusions & Tricks

1. Bile is not an enzyme. It emulsifies fat (a detergent), never hydrolyzes — its role is physical (surface area for lipase), not chemical.

2. Zymogens vs. active enzymes. Pepsinogen, trypsinogen, chymotrypsinogen, etc. are zymogens; the rule is that any enzyme which would destroy its own cell is secreted inactive. Amylase and lipase don't, so they're secreted active.

3. Trypsin activates everything. Enteropeptidase activates only trypsinogen → trypsin; trypsin then activates all other pancreatic zymogens (and more trypsinogen).

4. Fat absorption bypasses the portal system initially. Long-chain FAs → chylomicrons → lacteals → lymph → systemic blood. Short-/medium-chain FAs are water-soluble enough to enter capillaries → portal vein directly.

5. Secretin vs. CCK. Both from the duodenum, different triggers: Secretin ← acid → bicarbonate. CCK ← fat/protein → enzymes + gallbladder contraction.

6. Gastric vs. intestinal phase. The stomach promotes its own secretion (gastrin, vagus); once chyme hits the duodenum, CCK and secretin slow gastric emptying — the inhibitory intestinal phase.

7. Myenteric vs. submucosal plexus. Myenteric = Movement (motility); Submucosal = Secretion.

8. Intrinsic factor is made by parietal cells, not the ileum. B₁₂ absorption is in the ileum; intrinsic factor production is in the stomach. Pernicious anemia (loss of parietal cells/IF) → B₁₂ deficiency → megaloblastic anemia.

9. Pepsin's optimal pH is ~2. Active in strong acid; destroyed at the alkaline duodenal pH — so it stops once chyme is neutralized.

10. Emulsification ≠ micelle formation. Bile salts first emulsify (let lipase work); then the products of lipase are carried in micelles (much smaller, water-stable) to the brush border. Two distinct steps.

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Key Takeaways

Organs and Their Key Functions

Organ	Key Contributions
Mouth	Salivary amylase, lingual lipase, mucin lubrication
Esophagus	Peristaltic transport only; LES prevents reflux
Stomach	HCl (activates pepsin, denatures, kills microbes); pepsin; intrinsic factor; three secretion phases
Liver	Bile production; glucose regulation; detox; urea synthesis; first-pass metabolism
Gallbladder	Bile storage/concentration; contracts to CCK
Pancreas (exocrine)	Bicarbonate; all enzyme classes; zymogens activated by enteropeptidase → trypsin cascade
Small intestine	Most digestion + absorption; duodenum (neutralization, iron), jejunum (absorption), ileum (B₁₂, bile salts)
Large intestine	Water/electrolyte reabsorption; bacterial fermentation (SCFAs, vitamins K/B)

Key Hormones

• Gastrin: protein → ↑ HCl + pepsinogen
• Secretin: acid in duodenum → ↑ pancreatic bicarbonate
• CCK: fat + protein → ↑ pancreatic enzymes + gallbladder contraction

Fat Absorption Route

$\text{Triglycerides} \xrightarrow{\text{bile + lipase}} \text{FA + MG} \xrightarrow{\text{micelles}} \text{enterocytes} \xrightarrow{\text{chylomicrons}} \text{lacteals} \to \text{lymph} \to \text{systemic circulation}$

Structural Adaptations of the Small Intestine

Plicae circulares → Villi (lacteals + capillaries) → Microvilli/brush border → ~250 m²

Nervous System

• Myenteric (Auerbach's): motility; Submucosal (Meissner's): secretion
• Parasympathetic (vagus + sacral): ↑ motility + secretion; Sympathetic: ↓ motility + secretion, ↑ sphincter tone