Gastrointestinal System Overview

Gastrointestinal System

• The gastrointestinal (GI) system is responsible for turning food into energy.
• The GI system consists of primary organs and accessory organs.
• Primary organs include the mouth, esophagus, stomach, small intestine, and large intestine.
• Accessory organs include the liver, gallbladder, and pancreas.

Anatomic Review

• The GI tract is divided into upper and lower portions.
• The upper GI tract consists of the mouth, esophagus, stomach, and part of the small intestine.
• The lower GI tract consists of the remaining portion of the small intestine, the large intestine, rectum, and anus.
• The mouth is the site of ingestion and initial digestion where salivary glands produce saliva.
• Saliva contains amylase and lysozyme and is mostly composed of water.
• The esophagus is a muscular tube that carries food from the mouth to the stomach through peristalsis which is a process of muscle contractions.
• The stomach is a hollow organ that receives food from the esophagus. It is divided into four regions: fundus, cardia, body, and pylorus.
• The stomach has four layers: mucosa, submucosa, muscular, and serosa.
• The small intestine is approximately 22 ft long and divided into three sections: duodenum, jejunum, and ileum. It plays a key role in digestion and absorption.
• The large intestine begins at the cecum, includes the appendix, and is responsible for reabsorbing nutrients, water, and forming solid feces.
• The large intestine is followed by the rectum, where feces are stored, and the anus, containing internal and external anal sphincters.
• Accessory organs: Liver, gallbladder, and pancreas are connected to the GI tract through ducts for transferring substances to the small intestine.

GI Normal Physiology: Carbohydrate Digestion

• Salivary amylase converts starch and glycogen into disaccharides.
• Chyme is a mixture of food, saliva, and gastric juices.
• Pancreatic amylase is added to the chyme in the duodenum.
• Intestinal villi are responsible for absorbing most carbohydrates in the small intestine with the help of digestive enzymes.
• Monosaccharides are absorbed by the gut and transported to the liver by blood.
• Glucose is the only carbohydrate that can be directly used for energy or stored as glycogen.
• The liver maintains normal blood glucose levels by storing excess glucose as glycogen, converting glycogen to glucose, and converting other organic molecules into glucose when necessary.

GI Normal Physiology: Protein Digestion

• Proteins are broken down into peptides and amino acids.
• Digestion begins in the stomach with the secretion of pepsinogen, which is converted to pepsin by hydrochloric acid.
• Digestion is completed in the duodenum.
• Villi absorb amino acids.
• The liver plays a role in protein digestion by deaminating amino acids, forming and excreting urea nitrogen, and synthesizing proteins.

GI Normal Physiology: Lipid Digestion

• Lipids are not water-soluble.
• Cholecystokinin released from the small intestine slows down motility and enhances fat digestion.
• Bile is produced by the liver and stored in the gallbladder. It emulsifies fat into micelles in the duodenum.
• Pancreatic and intestinal lipases breakdown triacylglycerols into monoacylglycerols and fatty acids.
• The liver plays a role in lipid digestion by converting excess carbohydrates and proteins into fatty acids and triglycerides.

GI Diseases

• A peptic ulcer is an erosion of the stomach or duodenal lining exceeding 3 mm in diameter.
• Duodenal and gastric ulcers can be caused by Helicobacter pylori (H. pylori).
• H. pylori infection is diagnosed through a breath test, serology tests, and biopsies.

Diverticulosis and Diverticulitis

• Diverticula are small, bulging pouches that develop in the colon.
• When one or more of these pouches become inflamed or infected, the condition is called diverticulitis.
• Diverticulosis and diverticulitis are referred to collectively as diverticular disease.
• Diverticulitis is diagnosed with a CBC to detect inflammation and a CT scan to visualize diverticula.

Steatorrhea

• Excess fat in stools known as steatorrhea, characterized by greasy or oily stool appearance.
• Steatorrhea can be caused by issues with pancreatic enzymes, cystic fibrosis, malabsorption, celiac disease, bacterial overgrowth in the bowel, and a decrease in bile acids.
• Fecal fat tests are used to diagnose steatorrhea.