Gastrointestinal tract
Overview
The gastrointestinal tract (GI tract) is a complex system within the human body responsible for the ingestion, digestion, absorption, and excretion of food. It consists of a series of hollow organs joined in a long, twisting tube from the mouth to the anus. The primary organs involved include the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. Accessory organs such as the liver, pancreas, and gallbladder play crucial roles in digestion by producing enzymes and other substances necessary for breaking down food.
Anatomy
Mouth
The mouth, or oral cavity, is the entry point for food and is involved in the initial phase of digestion. Saliva produced by the salivary glands contains enzymes like amylase that begin breaking down carbohydrates. The teeth and tongue work together to mechanically process food into a bolus, which is then swallowed.
Esophagus
The esophagus is a muscular tube that connects the throat (pharynx) to the stomach. It transports the bolus through peristaltic movements, which are rhythmic contractions of the muscles in the esophageal wall. The lower esophageal sphincter (LES) at the junction of the esophagus and stomach prevents the backflow of stomach contents.
Stomach
The stomach is a sac-like organ where food is mixed with gastric juices, including hydrochloric acid and pepsin, to form a semi-liquid substance called chyme. The stomach's muscular walls churn the food, aiding in mechanical digestion. The pyloric sphincter regulates the passage of chyme into the small intestine.
Small Intestine
The small intestine is divided into three sections: the duodenum, jejunum, and ileum. It is the primary site for nutrient absorption. The duodenum receives chyme from the stomach and digestive enzymes from the pancreas and bile from the liver. The jejunum and ileum continue the process of nutrient absorption through their highly folded surfaces lined with villi and microvilli.
Large Intestine
The large intestine, or colon, absorbs water and electrolytes from the remaining indigestible food matter and compacts it into feces. It consists of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. The rectum stores feces until defecation.
Rectum and Anus
The rectum is the final section of the large intestine, where feces are stored before being expelled through the anus. The anus is equipped with internal and external sphincters that control the release of feces during defecation.
Physiology
Ingestion
Ingestion is the process of taking in food through the mouth. It involves chewing (mastication) and the mixing of food with saliva to form a bolus that can be easily swallowed.
Digestion
Digestion involves both mechanical and chemical processes. Mechanical digestion includes chewing and the churning actions of the stomach and intestines. Chemical digestion involves enzymes and other substances breaking down complex molecules into simpler ones that can be absorbed.
Absorption
Absorption primarily occurs in the small intestine, where nutrients pass through the intestinal wall into the bloodstream. The large surface area provided by villi and microvilli facilitates efficient absorption of nutrients.
Excretion
Excretion is the process of eliminating indigestible substances and waste products from the body. The large intestine compacts waste into feces, which are stored in the rectum until expelled through the anus.
Accessory Organs
Liver
The liver produces bile, which is stored in the gallbladder and released into the small intestine to emulsify fats, aiding in their digestion and absorption. The liver also processes nutrients absorbed from the small intestine and detoxifies harmful substances.
Pancreas
The pancreas produces digestive enzymes and bicarbonate, which are released into the small intestine. These enzymes break down carbohydrates, proteins, and fats, while bicarbonate neutralizes stomach acid.
Gallbladder
The gallbladder stores and concentrates bile produced by the liver. During digestion, it releases bile into the small intestine to assist in the emulsification of fats.
Disorders of the Gastrointestinal Tract
Gastroesophageal Reflux Disease (GERD)
GERD is a chronic condition where stomach acid frequently flows back into the esophagus, causing irritation and symptoms like heartburn. It is often due to a weakened lower esophageal sphincter.
Peptic Ulcers
Peptic ulcers are sores that develop on the lining of the stomach, small intestine, or esophagus. They are commonly caused by Helicobacter pylori infection or long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs).
Inflammatory Bowel Disease (IBD)
IBD encompasses disorders like Crohn's disease and ulcerative colitis, which involve chronic inflammation of the gastrointestinal tract. Symptoms include abdominal pain, diarrhea, and weight loss.
Irritable Bowel Syndrome (IBS)
IBS is a functional disorder characterized by symptoms such as abdominal pain, bloating, and altered bowel habits. The exact cause is unknown, but it is believed to involve a combination of gut-brain axis disturbances, gut motility issues, and hypersensitivity.
Colorectal Cancer
Colorectal cancer originates in the colon or rectum and is often detected through screening methods like colonoscopy. Risk factors include age, family history, and lifestyle factors such as diet and smoking.