Digestive System

Four essential functional phases of the digestive system

1. Ingestion (intake) → 2. Digestion (mechanical and chemical breakdown) → 3. Absorption (nutrient uptake into blood/lymph) → 4. Elimination (expulsion of indigestible waste).

Mechanical digestion vs. Chemical digestion

Mechanical digestion physically breaks food into smaller pieces to increase surface area (mastication, churning, emulsification);

Chemical digestion uses enzymes and acids to break covalent bonds.

Salivary amylase (ptyalin) function and site of origin

Secreted by the salivary glands (predominantly the parotid glands) into the oral cavity to initiate the chemical digestion of starches into maltose and polysaccharides.

Protective role of salivary buffers in the oral cavity

Bicarbonate and phosphate buffers maintain oral pH around 6.5 - 7.5, neutralizing dietary acids and inhibiting bacterial demineralization of tooth enamel.

Bolus definition and transition during deglutition

A soft, lubricated mass of chewed food and saliva formed by the tongue and teeth, prepared to pass down the pharynx during swallowing.

Epiglottis function during deglutition

A flap of elastic cartilage that fold inferiorly over the laryngeal inlet (glottis) during swallowing, routing the bolus into the esophagus and preventing aspiration into the trachea.

Peristalsis in the esophagus and alimentary canal

Involuntary, rhythmic wave-like contractions of alternating circular and longitudinal smooth muscle layers that propel the bolus forward along the digestive tract.

Lower esophageal sphincter (gastroesophageal/cardiac sphincter) function

A physiological muscular ring at the esophageal-gastric junction that relaxes to admit the bolus and constricts to prevent acidic gastric reflux into the esophagus.

Adult stomach storage capacity and mechanical role

Accommodates up to approximately 2 to 4 liters when distended, using three muscular layers (longitudinal, circular, oblique) to churn food into a semi-liquid suspension called chyme.

Chyme definition

The highly acidic, soupy, semi-fluid paste of partially digested food mixed with gastric juice produced by stomach churning.

Hydrochloric acid (HCl) production and gastric functions

Secreted by gastric parietal cells; denatures dietary proteins, kills ingested microbes, and establishes an acidic pH (1.5 - 2.5) that activates pepsinogen into pepsin.

Pepsinogen synthesis and activation to pepsin

Synthesized and secreted as an inactive zymogen by gastric chief cells; cleaved by gastric HCl into active pepsin to hydrolyze proteins into smaller peptide fragments.

Pyloric sphincter function

A muscular valve between the gastric pylorus and duodenal bulb that meters small aliquots (about 3 mL) of chyme into the small intestine while preventing duodenal backflow.

Three anatomical segments of the small intestine

Duodenum, Jejunum, and Ileum

Duodenum primary physiological roles

Receives pancreatic enzymes, bicarbonate, and bile to complete chemical digestion of all macromolecules classes; primary site for the absorption of iron and divalent minerals.

Jejunum anatomical features and primary absorption

Possesses prominent circular folds (plicae circulares), long villi, and microvilli brush border to maximize surface area for the bulk absorption of monosaccharides and amino acids.

Ileum specific absorption responsibilites

Absorbs vitamin B₁₂ (complexed with gastric intrinsic factor), reabsorbs conjugated bile salts for enterohepatic recycling, and absorbs fat-soluble vitamins (A, D, E, and K).

Villi and microvilli brush border significance

Increase small intestine absorptive surface area hundreds-fold (approx. 200 to 300 m²) and house brush border enzymes (lactase, sucrase, maltase, peptidases).

Large intestine anatomical segments in order

Cecum (with vermiform appendix) → Ascending colon → Transverse colon → Descending colon → Sigmoid colon → Rectum → Anal canal.

Large intestine primary physiological functions

Absorbs residual water, electrolytes (Na⁺, Cl^-), and bacterial vitamins (vitamin K and biotin); compacts and stores indigestible residues as solid feces.

Liver digestive and metabolic roles

Synthesis bile, processes absorbed nutrients delivered via the hepatic portal vein, stores glycogen and fat-soluble vitamins, detoxifies xenobiotics, and synthesizes plasma proteins.

Gallbladder function and bile emulsification mechanism

Stores and concentrates bile produces by the liver; contracts to release bile salts that mechanically emulsify large lipid droplets into smaller micelles to accelerate pancreatic lipase action.

Exocrine pancreas secretions in digestion

Acinar cells secrete digestive enzymes (pancreatic amylase, lipase, proteases like trypsinogen); duct cells secrete aqueous sodium bicarbonate (NaHCO₃) to neutralize acidic chyme.

Cholecystokinin (CCK) secretion site and physiological actions

Secreted by enteroendocrine I cells of the duodenum and jejunum in response to fatty acids and amino acids; triggers gallbladder contraction, pancreatic enzyme secretion, and slows gastric motility.

Secretin secretion site and physiological actions

Secreted by enteroendocrine S cells of the duodenal mucosa in response to low pH (<4.5); stimulates pancreatic ductal bicarbonate release to neutralize acidic chyme entering from the stomach.