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DrX Whiz Niraj
Digestive System: Anatomy & Physiology
Comprehensive notes on the GI Tract, Stomach acid regulation, Accessory organs (Liver & Pancreas), and Digestion processes.
The Gastrointestinal (GI) tract is a continuous tube that extends from the mouth to the anus. The Stomach is a J-shaped enlargement of the GI tract located between the esophagus and the duodenum. It acts as a mixing chamber and holding reservoir.
- 4 Main Regions: Cardia, Fundus, Body (central part), and Pylorus (connects to duodenum).
- Gastric Glands: The mucosa contains deep narrow channels called gastric pits, leading to gastric glands containing specialized secretory cells.
Parietal Cells (Oxyntic)
Secrete Hydrochloric Acid (HCl) which kills microbes and denatures proteins. Also secrete Intrinsic Factor, which is essential for the absorption of Vitamin B12.
Chief Cells (Zymogenic)
Secrete Pepsinogen (the inactive precursor to pepsin) and Gastric Lipase. HCl converts pepsinogen into the active enzyme Pepsin, which breaks down proteins into peptides.
Regulation of Acid Production
Gastric acid (HCl) secretion by parietal cells is highly regulated. It is stimulated by three main chemicals:
- Acetylcholine (ACh): Released by the Parasympathetic Nervous System (Vagus Nerve). It binds to M3 (Muscarinic) receptors on parietal cells.
- Gastrin: A hormone secreted by G-cells in the stomach. Binds to CCK2 receptors.
- Histamine: Released by ECL cells. Binds to H2 receptors. (These three act synergistically to activate the H+/K+ ATPase pump, also known as the Proton Pump).
Pharmacy Alert: Antacids & PPIs
In cases of hyperacidity or Peptic Ulcer Disease (PUD), pharmacists dispense two main classes of drugs:
1. H2 Blockers (e.g., Ranitidine): Block the Histamine receptors on the parietal cell.
2. Proton Pump Inhibitors - PPIs (e.g., Omeprazole, Pantoprazole): Directly shut down the H+/K+ ATPase pump. This is the most effective way to stop acid production, as it blocks the final step regardless of which receptor was activated!
The Liver & Gallbladder
The liver is the heaviest gland in the body. Its primary digestive function is to produce Bile (which emulsifies fats). The Gallbladder stores and concentrates the bile produced by the liver until it is needed in the small intestine.
The Pancreas
Acts as both an endocrine (insulin/glucagon) and exocrine gland. It secretes Pancreatic Juice into the duodenum, which contains enzymes (Amylase, Trypsin, Lipase) and sodium bicarbonate (to neutralize acidic chyme from the stomach).
Small Intestine: Divided into Duodenum, Jejunum, and Ileum. It is the major site of digestion and absorption. The presence of Villi and Microvilli drastically increases the surface area for absorption.
Large Intestine: Consists of the Cecum, Colon, Rectum, and Anal canal. Its main function is the completion of absorption (mostly water), production of certain vitamins (by gut bacteria), and formation of feces.
Movements of the GIT:
- Peristalsis: Successive muscular contractions along the wall of a hollow muscular structure. Its main job is Propulsion (pushing food forward).
- Segmentation: Localized, mixing contractions that occur in portions of the intestine distended by a large volume of chyme. It mixes food with digestive juices (No forward movement).