Digestive System: Lower GI Tract and Accessory Organs

This set of flashcards covers the anatomy, histology, and physiological processes of the lower gastrointestinal tract, accessory digestive organs, and nutrient absorption.

Small Intestine

A coiled, thin-walled tube extending from the pylorus to the cecum where most chemical digestion and nutrient absorption occurs.

Duodenum

The first segment of the small intestine that receives chyme from the stomach and secretions from accessory organs; contains submucosal glands producing alkaline mucus.

Jejunum

The middle region of the small intestine, making up two-fifths of its length, and acting as the primary site for chemical digestion and nutrient absorption.

Ileum

The final three-fifths of the small intestine that ends at the ileocecal valve; contains abundant aggregated lymphoid nodules and increasing goblet cells.

Circular folds

Internal mucosal and submucosal folds that act as $speed$ $bumps$ to slow chyme and increase surface area for nutrient absorption.

Villi

Fingerlike mucosal projections containing an arteriole, capillary network, venule, and lacteal to increase absorption surface area.

Lacteal

A specialized lymphatic capillary within a villus responsible for absorbing lipids and lipid-soluble vitamins.

Microvilli

Extensions of the simple columnar epithelial plasma membrane that form a microscopic brush border containing enzymes for final chemical digestion.

Segmentation

A motility pattern prevalent early in the intestinal phase that mixes chyme with gland secretions by moving it against the brush border.

Migrating Motility Complex

Successive waves of peristaltic contractions initiated by motilin late in the intestinal phase to move remaining content to the large intestine.

Gastroileal reflex

A reflex where the ileum contracts and the ileocecal sphincter relaxes to move contents into the cecum, typically triggered by food in the stomach.

Portal triads

Peripheral structures of hepatic lobules consisting of a bile ductule and branches of the hepatic portal circulation.

Hepatocytes

The functional cells of the liver located within hepatic lobules that absorb nutrients and oxygen and produce bile.

Stellate macrophages (Kupffer cells)

Phagocytic cells that line the hepatic sinusoids and clear harmful substances from the blood.

Bile

A liver secretion containing bicarbonate, bile salts, and lecithin used to mechanically emulsify lipids and neutralize acidic chyme.

Pancreatic juice

An alkaline fluid containing water, $HCO_3^-$, and various digestive enzymes like amylase, lipase, proteases, and nucleases.

Cholecystokinin (CCK)

A hormone released by the small intestine that stimulates gallbladder contraction, pancreatic enzyme release, and inhibits gastric activity.

Secretin

A hormone released in response to chyme acidity that triggers the release of an alkaline solution from the liver and pancreatic ducts.

Teniae coli

Thin bundles of longitudinal smooth muscle that bunch the large intestine into sacs called haustra.

Gut microbiome

The indigenous bacterial flora of the large intestine that breaks down nutrients and produces B vitamins and vitamin K.

Gastrocolic reflex

A reflex initiated by stomach distension that causes mass movements (powerful contractions) in the colon.

Enteropeptidase

An enzyme synthesized in the small intestine that activates trypsinogen into trypsin.

Emulsification

The mechanical process where bile salts and lecithin break large lipid droplets into smaller micelles to increase lipase access.

Chylomicrons

Lipid packages formed in epithelial cells wrapped in protein, which are released into lacteals via exocytosis.

Intrinsic factor

A substance produced by parietal cells required for the receptor-mediated endocytosis of Vitamin $B_{12}$ in the small intestine.

Hepcidin

A liver hormone that inhibits iron absorption when iron levels are sufficient.