Digestive System

Gastrointestinal System

• The body absorbs all nutrients ingested, regardless of whether they are needed or not.
• Control mechanisms of the gastrointestinal system are governed by the volume and composition of the luminal contents.

Stomach

• Gastric motility is increased by the concentration of gastrin and distension of the stomach.
• The force of initial stomach contractions is greater after a large meal, resulting in a greater emptying per contraction.
• Gastric emptying is inhibited by distension, presence of fat, high acidity, and hypertonic solutions in the duodenum.

Small Intestine

• 90% of absorption occurs in the small intestine.
• The small intestine is divided into three segments: duodenum, jejunum, and ileum.
• It is composed of fingerlike villi and microvilli structures, which increase the surface area for absorption and digestion.

Absorption in the Small Intestine

• The small intestine is anatomically arranged for a large surface area, enhancing absorption of nutrients.
• Villi increase surface area and contain blood vessels and lacteal, which play a role in absorption.
• Microvilli increase surface area and form the brush border.

Pancreas

• The pancreas has both exocrine and endocrine functions.
• The exocrine portion secretes pancreatic juice into the small intestine, which is rich in bicarbonate and digesting enzymes.
• Pancreatic secretions break down triglycerides into monoglycerides and 2 fatty acids.

Regulation of HCl Production

• Parietal cells in the stomach secrete about 2L of HCl per day.
• The acid secretion is regulated by four chemical messengers: gastrin, acetylcholine, histamine, and somatostatin.
• Somatostatin inhibits acid secretion, while the other three stimulate it.

Pathophysiology of the Gastrointestinal Tract

• Ulcers affect approximately 10% of the population in the USA.
• Ulcers are an erosion of the lining of the GI wall, usually due to pepsin and acid.
• Symptoms include a chronic rhythmic and periodic gnawing or burning pain.

Liver and Gall Bladder

• The liver serves as a secretory organ, secreting bile.
• Bile is a yellow-brownish liquid that contains bile salts, bicarbonate, lecithin, cholesterol, bilirubin, and other ions.
• Functions of bile include emulsifying large lipid molecules and neutralizing acidic chyme.

Hepatic Portal System

• The Hepatic Portal System is a specialized vasculature that delivers absorbed nutrients to the liver for processing.
• It is responsible for the synthesis of plasma proteins, processing and storing nutrients, and removal of old red blood cells.

Large Intestine

• The large intestine consists of the cecum, colon, and rectum.
• Functions of the large intestine include absorption of salt and water, mixing and propulsion of contents, storage and concentration of undigested matter, and defecation.

Motility of the Large Intestine and Defecation

• Contractions of the circular smooth muscle in the large intestine produce a segmentation motion.
• Three to four times a day, a wave of intense contraction known as a mass movement spreads rapidly over the transverse segment of the large intestine toward the rectum.
• The sudden distension of the walls of the rectum initiates the defecation reflex.

Learn about the breakdown process of triglycerides by lipase, the movement of food through the pharynx and esophagus, and the key functions of the stomach in the digestive system.