Digestion Unit

Gastrointestinal tract components

Mouth, esophagus, stomach, small intestine (20 ft), large intestine (3.5 ft), rectum, anus; coordinated for nutrient intake and absorption.

Histological structure of small intestine's mucosa

Villi (small folds) lined with a single layer of columnar epithelial cells, highly pleated for >600-fold increased surface area.

Function of villi in small intestine

Increase surface area for digestion/absorption, with a rich blood supply to transport nutrients to the bloodstream.

Cell types in small intestine's crypts

Enterocytes (digestion/absorption), Goblet cells (secrete mucin), Enteroendocrine cells (hormone secretion), Paneth cells (immune monitoring).

Epithelial cell turnover in small intestine

Every 3-5 days, with stem cells in crypts replenishing them.

Peristalsis

Sweeping muscular motion propelling food forward; occurs in esophagus, stomach, small/large intestine.

Segmentation in GI tract

Closely spaced contractions in the small intestine mixing chyme with secretions, increasing mucosal contact.

Mass movement in GI tract

Large contractions in the large intestine moving waste toward the rectum.

Secretions in the mouth for digestion

Saliva with mucus (lubrication) and salivary amylase (breaks alpha-1,4 glycosidic bonds in starches).

Secretions in the stomach for digestion

HCl (denatures proteins), pepsin (protease active at low pH), gastric lipase (hydrolyzes triglycerides), intrinsic factor (for vitamin B12 absorption).

Secretions aiding digestion in small intestine

Pancreatic enzymes (amylase, proteases, lipase), bicarbonate (neutralizes acid), bile acids (emulsify lipids, from liver via gallbladder).

Role of gastric emptying in digestion

Muscular contractions grind food into chyme, gradually released into the small intestine.

Carbohydrate digestion in mouth

Salivary amylase breaks alpha-1,4 glycosidic bonds in amylose/amylopectin, starting starch digestion.

Carbohydrate digestion in small intestine

Pancreatic amylase converts starch to dextrins; brush border enzymes (lactase, sucrase, isomaltase) break dextrins into glucose, galactose, fructose.

Carbohydrate absorption in small intestine

Glucose/galactose via SGLT1 (Na+-dependent active transport); fructose via GLUT5 (facilitated transport); all move to blood via GLUT2.

Protein digestion initiation in stomach

HCl denatures proteins; pepsin breaks proteins into oligopeptides.

Protein digestion completion in small intestine

Pancreatic proteases (activated from proenzymes) hydrolyze oligopeptides; brush border peptidases produce di-/tripeptides and amino acids.

brush border transporters

Di-/tripeptides and amino acids are absorbed via these transporters (active/facilitated); intracellular peptidases further break peptides; amino acids move to blood via basal transporters.

Gastric lipase

Hydrolyzes triglycerides (TG) to free fatty acids (FFA) and monoglycerides.

Pancreatic lipase

With co-lipase, hydrolyzes TG to FFA, monoglycerides, glycerol; bile acids/lecithins emulsify lipids for enzyme access.

micelles

FFA, monoglycerides, glycerol form these with bile acids, absorbed via diffusion/protein-mediated transport; reassembled into chylomicrons in enterocytes.

blood

Nutrients (glucose, amino acids) absorbed by villi enter this via a rich blood supply, transported to the liver through the hepatic portal vein.

chylomicrons

Lipids (FFA, monoglycerides) are reassembled into these in enterocytes, transported via lymphatics to the bloodstream.

enterohepatic circulation

~98% of bile acids are reabsorbed in the terminal ileum, returned to the liver via the hepatic portal vein for reuse.

gastroesophageal reflux disease (GERD)

A loose lower esophageal sphincter (LES) allows acidic stomach contents to reflux into the esophagus, causing irritation; distinct from occasional heartburn.

proton pump inhibitors

Drugs used to block acid secretion as a treatment for GERD.

lactose intolerance

Inability to produce lactase, leading to undigested lactose; more common in Asian, African, or Mediterranean populations.

GI hormones

Cholecystokinin, secretin, gastrin, gastric inhibitory peptide that regulate secretions (enzymes, mucus, bicarbonate) and motility.

carbohydrates metabolism

Starch → (salivary/pancreatic amylase) → dextrins → (brush border enzymes) → monosaccharides → absorbed (SGLT1/GLUT5) → blood (GLUT2) → liver (energy, glycogen storage).

proteins metabolism

Proteins → (pepsin, HCl) → oligopeptides → (pancreatic/brush border peptidases) → amino acids/di-/tripeptides → absorbed → blood → liver (protein synthesis, gluconeogenesis).

lipids metabolism

Triglycerides → (gastric/pancreatic lipase, bile acids) → FFA, monoglycerides, glycerol → micelles → absorbed → chylomicrons → lymph → blood → tissues (energy, storage).

dietary fiber

Digest by large intestine bacteria, producing short-chain fatty acids that are absorbed into the bloodstream for energy.

transit time in small intestine

3-10 hours, allowing for most nutrient digestion and absorption.

transit time in large intestine

Up to 3 days, for water/mineral absorption and fecal formation.

bile acids

Emulsify lipids, forming micelles to facilitate pancreatic lipase action and lipid absorption.