Gastrointestinal (GI) Physiology

• Gastrointestinal (GI) physiology encompasses the mechanical, chemical, and enzymatic processes involved in the digestion and absorption of nutrients, as well as the elimination of waste products

Overview of GI Processes

• Motility refers to the contractions of the GI tract's smooth muscles, which mix and propel its contents.
• Secretion involves the release of fluids and enzymes into the GI lumen to aid digestion.
• Digestion is the breakdown of complex food molecules into smaller, absorbable units.
• Absorption is the movement of digested nutrients, water, and electrolytes from the GI lumen into the bloodstream or lymphatic system.
• Elimination is the process of removing undigested and unabsorbed materials from the body via defecation.

Regulation of GI Function

• The GI system is regulated by a complex interplay of neural, hormonal, and paracrine mechanisms.
• The enteric nervous system (ENS) is a network of neurons within the GI tract that can function independently of the central nervous system (CNS) to control motility, secretion, and other functions.
• Hormones such as gastrin, cholecystokinin (CCK), secretin, and gastric inhibitory peptide (GIP) are released by enteroendocrine cells in response to specific stimuli and regulate various GI processes.
• Paracrine factors like histamine and somatostatin act locally within the GI tract to influence neighboring cells.

Mouth and Esophagus

• Saliva contains amylase, which begins the digestion of carbohydrates, and lingual lipase, which starts the digestion of fats.
• Swallowing involves the coordinated action of the tongue, pharynx, and esophagus to move food bolus from the mouth to the stomach.
• The upper esophageal sphincter (UES) prevents air from entering the esophagus during breathing, while the lower esophageal sphincter (LES) prevents gastric acid from refluxing into the esophagus.

Stomach

• The stomach stores food, mixes it with gastric secretions to form chyme, and regulates its emptying into the small intestine.
• Parietal cells secrete hydrochloric acid (HCl), which denatures proteins, kills bacteria, and activates pepsinogen.
• Chief cells secrete pepsinogen, which is converted to pepsin in the presence of HCl and begins the digestion of proteins.
• Mucous cells secrete mucus, which protects the stomach lining from the corrosive effects of HCl and pepsin.
• G cells secrete gastrin, which stimulates HCl secretion by parietal cells.

Small Intestine

• The small intestine is the primary site of nutrient digestion and absorption.
• The duodenum receives chyme from the stomach and secretions from the pancreas and gallbladder.
• The jejunum is the major site of nutrient absorption.
• The ileum absorbs vitamin B12 and bile salts and delivers the remaining undigested material to the large intestine.
• Pancreatic enzymes, including amylase, lipase, and proteases, digest carbohydrates, fats, and proteins, respectively.
• Bile, produced by the liver and stored in the gallbladder, emulsifies fats to facilitate their digestion and absorption.
• The small intestine's lining is highly folded and contains villi and microvilli, which increase its surface area for absorption.

Large Intestine

• The large intestine absorbs water and electrolytes from undigested material, forming feces.
• The colon harbors a diverse community of gut microbiota that ferment undigested carbohydrates and produce short-chain fatty acids (SCFAs), which are absorbed and provide energy.
• The rectum stores feces until defecation.
• Defecation is the process of eliminating feces from the body via the anus, involving relaxation of the internal and external anal sphincters.

Carbohydrate Digestion and Absorption

• Amylase breaks down starch and glycogen into smaller oligosaccharides.
• Brush border enzymes such as sucrase, lactase, and maltase further digest oligosaccharides into monosaccharides (glucose, fructose, and galactose).
• Glucose and galactose are absorbed into enterocytes via sodium-glucose cotransporter 1 (SGLT1), while fructose is absorbed via facilitated diffusion by GLUT5.
• All three monosaccharides are transported from enterocytes into the bloodstream via GLUT2.

Protein Digestion and Absorption

• Pepsin in the stomach begins the digestion of proteins into smaller peptides.
• Pancreatic proteases, including trypsin, chymotrypsin, and carboxypeptidase, further digest peptides in the small intestine.
• Brush border peptidases break down peptides into individual amino acids.
• Amino acids are absorbed into enterocytes via sodium-dependent amino acid transporters.
• Small peptides (dipeptides and tripeptides) are absorbed into enterocytes via the PepT1 transporter.
• Amino acids are transported from enterocytes into the bloodstream via various amino acid transporters.

Lipid Digestion and Absorption

• Lipase digests triglycerides into monoglycerides and fatty acids.
• Bile salts emulsify fats, forming micelles that transport lipids to the enterocyte surface.
• Monoglycerides and fatty acids are absorbed into enterocytes, where they are re-esterified into triglycerides.
• Triglycerides, cholesterol, and apolipoproteins are assembled into chylomicrons, which are secreted into the lymphatic system.
• Chylomicrons enter the bloodstream via the thoracic duct and deliver triglycerides to tissues throughout the body.