Digestive System and Nutrition

Digestive Tract Anatomy

The alimentary canal comprises the oral cavity, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (cecum, colon, rectum), and anus. The wall of the GI tract has four concentric layers from lumen outward: (1) mucosa -- epithelium (stratified squamous in esophagus for protection, simple columnar in stomach and intestines for secretion/absorption), lamina propria (loose connective tissue with blood/lymph vessels), and muscularis mucosae (thin smooth muscle layer that creates local folds); (2) submucosa -- dense connective tissue containing larger blood and lymphatic vessels, nerves (submucosal/Meissner's plexus), and glands (Brunner's glands in duodenum secrete alkaline mucus); (3) muscularis externa -- typically two layers of smooth muscle: inner circular (narrowing lumen) and outer longitudinal (shortening tube), with the myenteric (Auerbach's) plexus between them controlling peristalsis; the stomach has a third oblique layer for churning; (4) serosa (visceral peritoneum) or adventitia (in esophagus above diaphragm). Specialized structures include the lower esophageal sphincter (prevents acid reflux), pyloric sphincter (regulates gastric emptying), ileocecal valve (prevents colonic backflow), and internal/external anal sphincters. The small intestine maximizes surface area through plicae circulares (circular folds), villi (fingerlike projections), and microvilli (brush border) -- amplifying absorptive surface to approximately 200 sq meters.

Mechanical and Chemical Digestion

Digestion begins in the mouth with mastication (mechanical breakdown, increasing surface area for enzymatic action) and salivary amylase (initiates starch hydrolysis). The bolus is propelled by peristalsis through the esophagus. In the stomach, three muscle layers churn food into chyme while gastric glands secrete HCl (from parietal cells, denatures proteins and activates pepsinogen), pepsinogen (from chief cells, converted to active pepsin by HCl; begins protein digestion via endopeptidase activity), intrinsic factor (from parietal cells, essential for vitamin B12 absorption in the ileum), and mucus (from goblet cells and mucous neck cells, protects the gastric epithelium). The stomach's acidic environment (pH ~1.5 to 3.5) also kills most ingested pathogens. The pyloric sphincter controls the release of small amounts of chyme into the duodenum. In the small intestine, segmentation contractions (non-propulsive mixing movements) expose chyme to pancreatic enzymes and bile, while peristalsis slowly advances contents. Brush border enzymes anchored in enterocyte microvilli complete the final hydrolysis steps before absorption. The large intestine receives approximately 500 mL of indigestible residue daily; its primary functions are water and electrolyte absorption, microbial fermentation (producing vitamin K, biotin, and short-chain fatty acids), and compaction of feces for elimination.

Enzymatic Digestion

Digestive enzymes catalyze hydrolysis -- breaking macromolecules into absorbable monomers by adding water across chemical bonds. Each macromolecule class has a dedicated enzyme cascade. Carbohydrates: salivary alpha-amylase (starch to maltose, maltotriose, alpha-dextrins) is inactivated in the stomach by low pH. Pancreatic alpha-amylase resumes starch digestion in the duodenum. Brush border disaccharidases -- maltase (maltose to 2 glucose), sucrase-isomaltase (sucrose to glucose + fructose; isomaltose to glucose), and lactase (lactose to glucose + galactose) -- complete digestion. Proteins: pepsin (stomach, endopeptidase activated at low pH) produces large peptides. Pancreatic proteases are secreted as zymogens: trypsinogen is activated by enteropeptidase (enterokinase) on the brush border to trypsin, which then activates more trypsinogen, chymotrypsinogen to chymotrypsin, procarboxypeptidases to carboxypeptidases, and proelastase to elastase. These endopeptidases (trypsin, chymotrypsin, elastase; cleave internal peptide bonds) and exopeptidases (carboxypeptidase A/B; cleave terminal amino acids) produce oligopeptides and amino acids. Brush border aminopeptidases and dipeptidases complete the job. Lipids: lingual and gastric lipases have limited activity. The major player is pancreatic lipase, which hydrolyzes triglycerides at the sn-1 and sn-3 positions producing 2-monoglycerides and free fatty acids. Emulsification by bile salts is prerequisite -- lipase can only act at the lipid-water interface. Colipase anchors lipase to the emulsion droplet. Phospholipase A2 hydrolyzes phospholipids; cholesterol esterase releases free cholesterol. Nucleic acids: pancreatic nucleases (ribonuclease, deoxyribonuclease) digest RNA/DNA; brush border nucleosidases and phosphatases produce free bases, pentose sugars, and phosphate.

Nutrient Absorption

Absorption occurs predominantly in the jejunum and ileum, with each nutrient class following a distinct path. Carbohydrates: glucose and galactose enter enterocytes via SGLT1 (Na+-dependent secondary active transport on the apical membrane) and exit into the bloodstream via GLUT2 (facilitated diffusion on the basolateral membrane). Fructose uses GLUT5 (apical) and GLUT2 (basolateral), both facilitated diffusion. The Na+ gradient driving SGLT1 is maintained by the basolateral Na+/K+-ATPase. Proteins: dipeptides and tripeptides are absorbed via PepT1 (H+-coupled cotransport) and hydrolyzed intracellularly to amino acids. Free amino acids use several Na+-dependent and Na+-independent transporters grouped by amino acid class (neutral, basic, acidic, imino). Absorbed amino acids exit via basolateral facilitated transporters into the portal blood. Lipids: bile salt micelles ferry monoglycerides, free fatty acids, cholesterol, and fat-soluble vitamins (A, D, E, K) to the unstirred water layer adjacent to enterocytes. These lipids diffuse across the apical membrane. Inside the enterocyte, they are re-esterified into triglycerides in the smooth ER, combined with cholesterol, phospholipids, and apolipoproteins in the Golgi to form chylomicrons. Chylomicrons are packaged into secretory vesicles and exocytosed into the lacteals (lymphatic capillaries in villi), bypassing the liver initially -- this is the only nutrient class that enters the lymphatic system. Chylomicrons enter the bloodstream at the thoracic duct. Water and electrolytes: water follows osmotically (primarily paracellular); Na+ is absorbed by multiple mechanisms (Na+/H+ exchanger, Na+-glucose cotransport, epithelial Na+ channels in the colon). Iron is absorbed in the duodenum as Fe2+ via DMT1 and exported by ferroportin; hepcidin regulates iron homeostasis. Calcium absorption occurs primarily in the duodenum via vitamin D-dependent Ca2+ channels (TRPV6) and calbindin; vitamin D stimulates synthesis of these transport proteins. Vitamin B12 requires intrinsic factor (produced by gastric parietal cells) for absorption in the terminal ileum via receptor-mediated endocytosis.

Liver, Pancreas, and Gallbladder Function

These three accessory organs deliver secretions into the duodenum via the hepatopancreatic ampulla (ampulla of Vater), controlled by the sphincter of Oddi. Liver: hepatocytes perform over 500 metabolic functions. Key digestive roles include: (1) bile synthesis -- bile acids (cholic acid, chenodeoxycholic acid) are synthesized from cholesterol, conjugated to glycine or taurine to form bile salts, and secreted into bile canaliculi. Bile salts are amphipathic, acting as detergents to emulsify dietary fats and solubilize lipids in mixed micelles. (2) Bilirubin conjugation and excretion: unconjugated bilirubin (from heme breakdown) is conjugated to glucuronic acid by UDP-glucuronosyltransferase, making it water-soluble for biliary excretion. (3) Nutrient processing: the liver receives portal blood rich in absorbed nutrients. It extracts glucose and stores it as glycogen (glycogenesis), releases glucose via glycogenolysis and gluconeogenesis, synthesizes plasma proteins (albumin, clotting factors, transport proteins), deaminates amino acids for energy or conversion to glucose/fatty acids (the urea cycle converts ammonia to urea), and packages triglycerides into VLDL for export. (4) Detoxification: Phase I (cytochrome P450 oxidation/reduction/hydrolysis) and Phase II (conjugation -- glucuronidation, sulfation, glutathione conjugation) reactions. Gallbladder: stores and concentrates bile between meals. CCK stimulates gallbladder contraction and sphincter of Oddi relaxation simultaneously, delivering concentrated bile into the duodenum. Pancreas: the exocrine pancreas produces ~1.5 L/day of alkaline (HCO3- rich) secretion that neutralizes gastric acid. Acinar cells synthesize and secrete digestive enzymes (amylase, lipase, proteases as zymogens, nucleases). Duct cells secrete bicarbonate in exchange for Cl-; CFTR chloride channels on the apical membrane are crucial -- defects cause cystic fibrosis with pancreatic insufficiency. Secretin stimulates duct cells (HCO3- secretion); CCK stimulates acinar cells (enzyme secretion).

Fed and Fasting States

The body alternates between two metabolic modes governed by the insulin-to-glucagon ratio. Fed (absorptive) state (0 to 4 hours post-meal): elevated blood glucose stimulates pancreatic beta-cells to secrete insulin, which promotes glucose uptake into cells (GLUT4 translocation in muscle and adipose), glycogenesis (glucose to glycogen in liver and muscle), glycolysis, lipogenesis (excess glucose to fatty acids to triglycerides in liver and adipose tissue), and protein synthesis. The liver takes up approximately 60% of ingested glucose. The brain and RBCs take up glucose via insulin-independent GLUT1/GLUT3. Fasting (postabsorptive) state (over 4 hours post-meal, extending to days): declining blood glucose reduces insulin and stimulates pancreatic alpha-cells to secrete glucagon. Glucagon acts primarily on the liver to activate glycogenolysis (glycogen to glucose) and gluconeogenesis (lactate, glycerol, amino acids to glucose). After glycogen stores deplete (approximately 12 to 24 hours), fatty acids become the primary fuel. In prolonged fasting/starvation, the liver produces ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone) from fatty acid-derived acetyl-CoA -- these serve as alternative fuel for the brain, sparing muscle protein. Other hormonal contributions: epinephrine (acute stress/exercise) stimulates glycogenolysis via beta-adrenergic receptors and cAMP cascade. Cortisol (chronic stress) promotes gluconeogenesis and protein catabolism. Growth hormone is anti-insulin, promoting lipolysis and decreasing glucose uptake. The liver's metabolic flexibility -- switching between glucose storage/release, fatty acid synthesis/oxidation, and ketogenesis -- is the hallmark of metabolic homeostasis. Type 1 diabetes mellitus (insulin deficiency) mimics the fasting state pathologically: hyperglycemia from unopposed hepatic glucose output, ketogenesis, and muscle wasting, despite abundant circulating glucose.

Gut Hormonal Regulation

The GI tract is the body's largest endocrine organ, secreting hormones from specialized enteroendocrine cells in response to luminal contents. Gastrin: secreted by G cells of the gastric antrum in response to stomach distension, peptides/amino acids, and vagal stimulation (via gastrin-releasing peptide, GRP). Gastrin stimulates parietal cells to secrete HCl and promotes gastric motility and mucosal growth. Gastrin secretion is inhibited by somatostatin (paracrine from D cells) and by low antral pH (negative feedback -- when pH is less than 2, gastrin release is suppressed). Secretin: released by S cells of the duodenum in response to acidic chyme (pH less than 4.5). Secretin stimulates pancreatic duct cells to secrete bicarbonate-rich fluid (neutralizing acid) and inhibits gastric acid secretion and gastric emptying. Cholecystokinin (CCK): secreted by I cells of the duodenum and jejunum in response to fatty acids and amino acids. CCK stimulates gallbladder contraction (bile release), pancreatic enzyme secretion, and sphincter of Oddi relaxation -- coordinating fat digestion. CCK also slows gastric emptying (allowing more time for fat digestion) and promotes satiety. Glucose-dependent insulinotropic peptide (GIP, formerly gastric inhibitory peptide): released by K cells of the duodenum/jejunum in response to glucose and fat. GIP stimulates insulin secretion from pancreatic beta-cells (the incretin effect -- oral glucose triggers more insulin than IV glucose because gut hormones amplify the response). Ghrelin: produced by P/D1 cells of the gastric fundus during fasting; ghrelin is the only known orexigenic (appetite-stimulating) gut hormone. It acts on the hypothalamic arcuate nucleus to stimulate NPY/AgRP neurons, increasing hunger. Ghrelin levels rise before meals and fall after eating. Leptin: secreted by adipose tissue (not the gut), leptin signals long-term energy stores to the hypothalamus, suppressing appetite via POMC/CART neurons. The enteric nervous system (the 'gut brain') coordinates local reflexes independently of the CNS: the myenteric plexus controls motility, and the submucosal plexus regulates secretion and blood flow; both receive modulatory input from sympathetic (inhibitory, decreasing motility/secretion) and parasympathetic (excitatory, via vagus nerve, increasing motility/secretion) divisions.

How it works

Digestion is a cascade of structural disruption and enzymatic hydrolysis orchestrated by neural and hormonal signals. Food enters the mouth, where mechanical grinding and salivary amylase begin carbohydrate breakdown. Peristalsis delivers the bolus to the stomach, where HCl denatures proteins and activates pepsin. The resulting chyme is slowly released into the duodenum, triggering secretin and CCK release -- these hormones call in pancreatic bicarbonate and enzymes plus gallbladder bile. In the small intestine, brush border enzymes complete digestion, and enterocytes absorb monomers by specialized transporters. Water-soluble nutrients (sugars, amino acids) enter the portal vein directly; lipids are packaged into chylomicrons and travel via lymph. The liver processes portal blood, storing or distributing nutrients while detoxifying xenobiotics. Between meals, glucagon shifts metabolism to glycogenolysis and gluconeogenesis; prolonged fasting triggers ketogenesis. Gut hormones provide the feedforward and feedback signals that synchronize motility, secretion, and satiety with nutrient availability.

How it works

Digestion is a cascade of structural disruption and enzymatic hydrolysis orchestrated by neural and hormonal signals. Food enters the mouth, where mechanical grinding and salivary amylase begin carbohydrate breakdown. Peristalsis delivers the bolus to the stomach, where HCl denatures proteins and activates pepsin. The resulting chyme is slowly released into the duodenum, triggering secretin and CCK release -- these hormones call in pancreatic bicarbonate and enzymes plus gallbladder bile. In the small intestine, brush border enzymes complete digestion, and enterocytes absorb monomers by specialized transporters. Water-soluble nutrients (sugars, amino acids) enter the portal vein directly; lipids are packaged into chylomicrons and travel via lymph. The liver processes portal blood, storing or distributing nutrients while detoxifying xenobiotics. Between meals, glucagon shifts metabolism to glycogenolysis and gluconeogenesis; prolonged fasting triggers ketogenesis. Gut hormones provide the feedforward and feedback signals that synchronize motility, secretion, and satiety with nutrient availability.

Comparisons

• C/P (Enzyme kinetics): Digestive enzymes follow Michaelis-Menten kinetics; zymogen activation is proteolytic cleavage -- catalytic efficiency and specificity are testable. Brush border enzymes are membrane-bound, meaning enzyme concentration is fixed -- Vmax is determined by surface area (affected by villous atrophy in celiac disease).
• C/P (Bile chemistry): Bile salts are amphipathic cholesterol derivatives; their detergent action lowers surface tension at the lipid-water interface -- a colloidal chemistry concept with direct physiological relevance. Conjugation to glycine/taurine increases water solubility.
• B/B (Membrane transport): SGLT1 (secondary active transport), GLUT2/GLUT5 (facilitated diffusion), PepT1 (H+-coupled symport), and Na+/K+-ATPase (primary active transport) all converge on a single enterocyte -- a perfect exam scenario for comparing transport mechanisms.
• B/B (Hormonal cascades): The insulin-to-glucagon ratio is a classic endocrine feedback system; incretins (GIP) link gut sensing to pancreatic secretion. The MCAT loves asking how hormone X affects blood glucose -- trace the pathway from release to effect.
• B/B (Lipid transport): Chylomicrons travel lymphatically, bypassing the portal circulation -- this exception to the rule is a favorite trap. VLDL, LDL, and HDL are later steps in the lipoprotein cascade; understand why chylomicrons are the first postprandial lipoproteins.
• P/S (Hunger and satiety): Ghrelin (orexigenic) and leptin (anorexigenic) connect gut/adipose signals to hypothalamic feeding centers -- a behavioral-neuroscience bridge that tests understanding of drive states and homeostasis.

Common confusions

• Confusing where each macromolecule begins digestion. Carbohydrates start in the mouth (salivary amylase), proteins start in the stomach (pepsin), and lipids start in the duodenum (pancreatic lipase with bile). Don't say protein digestion begins in the mouth or lipid digestion in the stomach -- lingual/gastric lipase are minor contributors.
• Assuming all nutrients enter the portal vein. Lipids are packaged into chylomicrons and enter lacteals -> lymphatic system -> thoracic duct -> bloodstream. The liver doesn't see dietary triglycerides directly -- it receives portal blood with amino acids and monosaccharides, and later processes chylomicron remnants.
• Mixing up secretin and CCK. Secretin responds to acid -> HCO3- secretion (S for pH is too low, need Secretin to Stop the Sting). CCK responds to fat/protein -> enzyme secretion + gallbladder contraction (CCK = Contraction of gallbladder, Causing enzyme Koming-out).
• Forgetting that pepsin is activated by HCl, not by enteropeptidase. Enteropeptidase (enterokinase) activates trypsinogen -> trypsin in the duodenum. Pepsinogen -> pepsin is a gastric event mediated by acid, not by another protease. Trypsin then activates other pancreatic zymogens.
• Treating SGLT1 as a passive transporter. SGLT1 couples glucose (against its gradient) to Na+ (down its gradient) - this is secondary active transport. GLUT2 is facilitated diffusion. The MCAT may show inhibitor effects (phlorizin blocks SGLT1 -> glucose malabsorption).
• Thinking the stomach absorbs nutrients. The stomach absorbs almost nothing except alcohol and some lipid-soluble drugs (aspirin). Essentially all nutrient absorption occurs in the small intestine, with water and electrolytes absorbed in the large intestine.
• Confusing fed-state insulin effects with fasting-state glucagon effects. Insulin stimulates glycogenesis, lipogenesis, and protein synthesis (build it). Glucagon stimulates glycogenolysis, gluconeogenesis, and ketogenesis (break it/mobilize it). Type 1 diabetes mimics the fasting state despite hyperglycemia because insulin is absent.
• Overlooking that vitamin B12 absorption requires intrinsic factor. Pernicious anemia (autoimmune destruction of parietal cells) causes B12 deficiency not because B12 isn't in the diet but because intrinsic factor is absent -- the terminal ileum cannot absorb B12 without it.

Quick review

• Mouth: mastication + salivary amylase (starch -> maltose). Esophagus: peristalsis only, no digestion.
• Stomach: HCl (parietal cells) + pepsinogen (chief cells) -> pepsin at low pH. Also intrinsic factor for B12 absorption.
• Pancreatic zymogens: trypsinogen activated by enteropeptidase -> trypsin -> activates chymotrypsinogen, procarboxypeptidases, proelastase.
• Bile: made by liver, stored/concentrated in gallbladder, released by CCK. Emulsifies fats -- not an enzyme, a detergent.
• Carb absorption: glucose/galactose via SGLT1 (secondary active transport); fructose via GLUT5. All exit via GLUT2 (basolateral).
• Lipid absorption: micelles -> enterocyte -> re-esterified -> chylomicrons -> lacteals -> lymph -> thoracic duct -> blood.
• Secretin (S cells, responds to acid): increases HCO3- from pancreatic ducts. CCK (I cells, responds to fat/AA): increases enzymes + gallbladder contraction.
• Fed state: increased insulin -> glycogenesis, lipogenesis, protein synthesis. Fasted state: increased glucagon -> glycogenolysis, gluconeogenesis, ketogenesis.
• Gastrin (G cells): increases HCl secretion. Inhibited by low pH (negative feedback) and somatostatin.
• Ghrelin (stomach): hunger signal, increases before meals. Leptin (adipose): satiety signal, proportional to fat stores.
• B12 absorption requires intrinsic factor (from parietal cells) and occurs exclusively in terminal ileum.
• Liver: bile synthesis, bilirubin conjugation, nutrient processing (glycogen storage, gluconeogenesis, urea cycle, VLDL synthesis), detoxification (CYP450 Phase I + conjugation Phase II).