Gastrointestinal Physiology

Gastrointestinal Physiology

• The alimentary tract provides the body with a continual supply of water, electrolytes, vitamins, and nutrients through:
  • Movement of food through the GIT
  • Secretion of digestive juices and digestion of food
  • Absorption of water, electrolytes, vitamins, and digestive products
  • Circulation of blood through the GI organs to carry away absorbed substances
  • Control of all these functions by local, nervous, and hormonal systems

Physiologic Anatomy of the GI Wall

• The GI wall consists of five layers from outside to lumen:
  • Serosa
  • Longitudinal muscle layer
  • Circular muscle layer
  • Submucosa
  • Mucosa

Electrical Activity of Gastrointestinal Smooth Muscle

• Slow waves:
  • Are slow, undulating changes in resting membrane potential
  • Intensity varies between 5 and 15 millivolts
• Spike potentials:
  • Are true action potentials
  • Occur automatically when resting membrane potential becomes more positive than about −40 millivolts
  • Normal resting membrane potential in smooth muscle fibers is between −50 and −60 millivolts
• Factors that depolarize the membrane:
  • Stretching of the muscle
  • Stimulation by acetylcholine
  • Stimulation by several GI hormones
• Factors that hyperpolarize the membrane:
  • Effect of NE or EP on the fiber membrane
  • Stimulation of the sympathetic nerves that secrete mainly NE at their endings

Neural Control of Gastrointestinal Function—Enteric Nervous System

• The GI has its own nervous system called the enteric nervous system (ENS)
• The ENS lies entirely in the wall of the gut, beginning in the esophagus and extending to the anus
• The ENS is composed of two plexuses:
  • Myenteric plexus (outer plexus) lying between the longitudinal and circular muscle layers
  • Submucosal plexus (inner plexus) lying in the submucosa
• The myenteric plexus controls mainly GI movements
• The submucosal plexus controls mainly GI secretion and local blood flow
• The ENS can function independently, but stimulation by the parasympathetic and sympathetic systems can greatly enhance or inhibit GI functions

Autonomic Control of the Gastrointestinal Tract

• The parasympathetic supply to the gut is divided into cranial and sacral divisions
• Powerful factors that inhibit stomach emptying (duodenal factors):
  • Degree of distention of the duodenum
  • Presence of any degree of irritation of the duodenal mucosa
  • Degree of acidity of the chyme
  • Degree of osmolality of the chyme
  • Presence of certain breakdown products in the chyme

Small Intestine Function

• Digestion: enzymes are intracellular (disaccharidases and aminopeptidases)
• Absorption: completed in the small intestine, large surface area (as large as a tennis court area)
• Secretion: secretion of digestive enzymes by the brush border of the villi
• Motility: movement of food through the small intestine

Secretion of Digestive Enzymes

• The enterocytes of the mucosa, especially those that cover the villi, contain digestive enzymes that digest specific food substances while they are being absorbed through the epithelium
• These enzymes include:
  • Peptidases for the splitting of small polypeptides into amino acids
  • Disaccharidases that break down disaccharides into monosaccharides
  • Small amounts of lipases for the digestion of fat

Defecation Reflexes

• There are two types of defecation reflexes:
  • Intrinsic reflex mediated by the local enteric nervous system in the rectal wall
  • Parasympathetic reflex that involves the sacral segments of the spinal cord

Secretory Functions of the GIT

• Secretory glands have two primary functions:
  • Secrete digestive enzymes in most areas of the GIT
  • Provide mucus for lubrication and protection of all parts of the GIT

Basic Mechanisms of Stimulation of the GIT Glands

• Contact of food with the epithelium stimulates secretion (direct contact stimulation of the surface glandular cells by the food)
• Parasympathetic stimulation increases the GIT glandular secretion rate
• Sympathetic stimulation has a dual effect (sympathetic stimulation alone decreases secretion, but parasympathetic stimulation increases the secretion)

Esophageal Secretions

• Esophageal secretions are entirely mucous and provide lubrication for swallowing
• The main body of the esophagus is lined with many simple mucous glands

Pancreatic Secretion

• The pancreatic digestive enzymes are secreted by pancreatic acini and large volumes of sodium bicarbonate solution are secreted by the small ductules and larger ducts leading from the acini
• The combined product (enzymes + sodium bicarbonate) flows through a long pancreatic duct that joins the common bile duct
• Pancreatic enzymes for digesting proteins:
  • Trypsin
  • Chymotrypsin
  • Carboxypolypeptidase
• Pancreatic enzymes for digesting carbohydrates:
  • Pancreatic amylase
• Pancreatic enzymes for digesting fat:
  • Pancreatic lipase
  • Cholesterol esterase
  • Phospholipase

Secretion of Bile by the Liver

• Bile serves two important functions:
  • Aids in fat digestion and absorption by emulsifying fat particles and aiding in absorption of digested fat end products
  • Serves as a means for excretion of several important waste products from the blood