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MBBS Basic Science: Gastrointestinal System Overview

Structure of the Gastrointestinal System

The gastrointestinal (GI) system consists of the alimentary canal, which includes the mouth, esophagus, stomach, small intestine, and large intestine, along with associated glands such as the liver, pancreas, and gallbladder. Its primary function is to digest food, absorb nutrients, and eliminate waste. The system is organized to ensure the breakdown of food into smaller components and the absorption of nutrients into the bloodstream.

Digestion and Absorption

Digestion begins in the mouth, where food is broken down mechanically by chewing and chemically by enzymes in saliva. The food bolus is then transported through the esophagus to the stomach via peristalsis. In the stomach, gastric juices containing hydrochloric acid and pepsin further break down proteins. The partially digested food, now called chyme, moves into the small intestine, where most digestion and absorption occur.

The small intestine is divided into three sections: the duodenum, jejunum, and ileum. Enzymes from the pancreas and bile from the liver aid in digesting fats, proteins, and carbohydrates. The absorbed nutrients pass through the intestinal walls into the bloodstream, while undigested material moves into the large intestine for water reabsorption and eventual elimination.

Functions of the Liver, Gallbladder, and Pancreas

The liver is a vital organ in digestion, producing bile which helps emulsify fats in the small intestine. It also detoxifies substances and metabolizes nutrients. The gallbladder stores bile and releases it into the duodenum as needed. The pancreas plays a dual role: producing digestive enzymes to break down carbohydrates, proteins, and fats, and secreting insulin and glucagon to regulate blood sugar levels.

Regulation of the GI System

The gastrointestinal system is regulated by both the nervous and hormonal systems. The enteric nervous system, sometimes referred to as the "second brain," coordinates motility and secretions within the GI tract. The autonomic nervous system, including the sympathetic and parasympathetic divisions, also modulates GI activity. Hormones such as gastrin, secretin, and cholecystokinin (CCK) control various digestive processes, including acid secretion, enzyme release, and bile production.

Clinical Significance

Knowledge of the GI system is critical for diagnosing and managing disorders such as gastroesophageal reflux disease (GERD), peptic ulcers, inflammatory bowel disease (IBD), and liver diseases. The MBBS curriculum emphasizes the integration of gastrointestinal physiology with clinical practice, enabling students to effectively diagnose and treat various GI conditions.

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