Digestive System Structure, Absorption, Regulation (3B) - MCAT Biological and Biochemical Foundations of Living Systems

Begins carbohydrate digestion by hydrolyzing starch to smaller sugars. Salivary amylase initiates the breakdown of complex carbohydrates into simpler forms during the initial phase of digestion in the oral cavity.

Nonkeratinized stratified squamous epithelium. This epithelium provides protection against mechanical stress from food passage while allowing flexibility in the esophagus.

Prevents gastric reflux into the esophagus. The LES acts as a valve to maintain unidirectional flow of food and prevent acidic stomach contents from damaging the esophageal lining.

Secrete hydrochloric acid and intrinsic factor. Parietal cells contribute to the acidic environment necessary for pepsin activation and facilitate vitamin $B_{12}$ absorption.

Secrete pepsinogen. Chief cells release the inactive precursor pepsinogen, which is converted to pepsin for protein digestion in the acidic stomach environment.

Pepsin. Pepsinogen is activated by the low pH in the stomach to form pepsin, which cleaves peptide bonds in proteins.

Protects gastric epithelium from acid and pepsin. Gastric mucus forms a protective barrier that neutralizes acid and prevents enzymatic degradation of the stomach lining.

Enables vitamin $B_{12}$ absorption in the terminal ileum. Intrinsic factor binds to vitamin $B_{12}$, protecting it from degradation and facilitating its uptake via specific receptors in the ileum.

Peptides/amino acids in the stomach (and gastric distension). Gastrin release is stimulated by the presence of protein breakdown products and mechanical stretching to promote digestive processes.

Increases gastric acid secretion and gastric motility. Gastrin enhances the stomach's digestive capacity by stimulating parietal and chief cells and promoting peristalsis.

Increases pancreatic $HCO_{3}^{-}$ secretion. Secretin promotes bicarbonate release to neutralize acidic chyme, creating an optimal pH for pancreatic enzyme activity in the duodenum.

Acidic chyme (low pH) entering the duodenum. Low duodenal pH from gastric acid triggers secretin to regulate pancreatic bicarbonate secretion for acid neutralization.

Stimulates pancreatic enzymes and gallbladder contraction. CCK coordinates the release of digestive enzymes and bile to emulsify and break down fats in the small intestine.

Fatty acids and amino acids in the small intestine. The presence of fats and proteins in chyme signals CCK release to enhance lipid and protein digestion via pancreatic and biliary responses.

Emulsify fats and form micelles to aid absorption. Bile salts reduce surface tension of fat droplets and stabilize micelles, facilitating lipase action and lipid uptake by enterocytes.

Small intestine (primarily the jejunum). The jejunum's extensive surface area and vascular supply optimize the uptake of digested nutrients into the bloodstream.

Plicae circulares, villi, and microvilli (brush border). These adaptations exponentially expand the mucosal surface, enhancing nutrient contact and absorption efficiency in the small intestine.

A villus. The villus integrates absorptive enterocytes, blood capillaries, and a central lacteal for efficient nutrient transport, especially lipids.

Chylomicrons entering lymphatic lacteals. Chylomicrons package triglycerides and are transported via lymphatics to bypass initial hepatic processing.

SGLT1 (Na$^{+}$-glucose cotransport; secondary active transport). SGLT1 utilizes the sodium gradient established by the Na$^{+}$-K$^{+}$ ATPase to drive glucose uptake against its concentration gradient.

GLUT2 (facilitated diffusion). GLUT2 allows passive exit of accumulated glucose down its concentration gradient into the bloodstream for systemic distribution.

GLUT5 (facilitated diffusion). GLUT5 enables fructose to enter enterocytes passively along its concentration gradient, independent of sodium.

Large intestine (colon), especially proximal colon. The proximal colon maximizes water and electrolyte recovery from fecal matter to maintain fluid balance and form solid stool.

Parasympathetic (vagus and pelvic splanchnic nerves). Parasympathetic stimulation enhances digestive functions by promoting smooth muscle contraction and glandular secretion throughout the GI tract.