Digestive System (Unit 2) - Handwritten Notes
Download handwritten notes for Human Anatomy and Physiology 2 (Semester 2), covering Unit 2: The Digestive System. These notes provide a detailed look at the anatomy of the GI tract, with specific focus on the stomach (acid production and regulation, pepsin's role), small intestine, and large intestine. The notes also cover the anatomy and function of accessory organs like the salivary glands, pancreas, and liver. You'll find information on the movements of the GI tract, the processes of digestion and absorption of nutrients, and common disorders of the digestive system. Available in PDF format.
Keywords: Digestive System, GI Tract, Stomach, Acid Production, Pepsin, Small Intestine, Large Intestine, Salivary Glands, Pancreas, Liver, Digestion, Absorption, Nutrients, Disorders, Human Anatomy and Physiology, Handwritten Notes, PDF, Download, Semester 2, B.Pharm.
Digestive System (Unit 2) - Detailed Explanation
Anatomy of the GI Tract
The gastrointestinal (GI) tract, also known as the alimentary canal, is a long, continuous tube that extends from the mouth to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The primary function of the GI tract is to break down food into smaller molecules that can be absorbed into the bloodstream.
Stomach
The stomach is a J-shaped organ located in the upper left quadrant of the abdomen. It has four main regions: the cardia, fundus, body, and pylorus. The stomach lining contains specialized cells:
- Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor (essential for vitamin B12 absorption).
- Chief cells: Secrete pepsinogen, the inactive precursor to pepsin.
- Mucous cells: Secrete mucus to protect the stomach lining from the acidic environment.
- G cells: Secrete gastrin, a hormone that stimulates acid secretion.
Acid Production in the Stomach: Parietal cells actively pump hydrogen ions (H+) into the stomach lumen, creating a highly acidic environment (pH 1.5-3.5). This process involves the enzyme H+/K+ ATPase (the proton pump). Carbonic anhydrase within parietal cells generates H+ and bicarbonate ions (HCO3-). The H+ is pumped out, while HCO3- is exchanged for chloride ions (Cl-) from the blood. The Cl- then enters the stomach lumen, combining with H+ to form HCl.
Regulation of Acid Production: Acid production is regulated by the parasympathetic nervous system (via the vagus nerve), gastrin, and histamine. The vagus nerve stimulates parietal cells directly and also stimulates G cells to release gastrin. Gastrin stimulates parietal cells and enterochromaffin-like (ECL) cells to release histamine. Histamine then acts on H2 receptors on parietal cells, potently stimulating acid secretion. Somatostatin inhibits acid secretion.
Pepsin's Role in Protein Digestion: Pepsinogen is converted to its active form, pepsin, by the low pH of the stomach. Pepsin is a protease, an enzyme that breaks down proteins into smaller peptides. It works optimally in an acidic environment.
Small Intestine
The small intestine is the primary site of nutrient absorption. It's divided into three sections: the duodenum, jejunum, and ileum. The inner lining of the small intestine has folds called plicae circulares, which are covered in finger-like projections called villi. Each villus contains blood capillaries and a lacteal (lymphatic capillary) for nutrient absorption. The surface of the villi is further covered in microvilli, forming the "brush border," which greatly increases the surface area for absorption.
Large Intestine
The large intestine absorbs water and electrolytes from the remaining indigestible material, forming feces. It includes the cecum, appendix, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal. The large intestine also houses a large population of bacteria (gut microbiota) that play a role in vitamin synthesis (vitamin K and some B vitamins) and fermentation of undigested carbohydrates.
Accessory Organs
- Salivary Glands: Produce saliva, which contains salivary amylase (begins carbohydrate digestion), mucus (lubrication), and lysozyme (antibacterial). There are three pairs of major salivary glands: parotid, submandibular, and sublingual.
- Pancreas: Has both exocrine and endocrine functions. The exocrine pancreas secretes pancreatic juice into the duodenum. Pancreatic juice contains enzymes for digesting carbohydrates (pancreatic amylase), proteins (trypsin, chymotrypsin, carboxypeptidase), and fats (pancreatic lipase). It also contains bicarbonate ions to neutralize the acidic chyme entering the duodenum.
- Liver: Produces bile, which is stored in the gallbladder and released into the duodenum. Bile emulsifies fats, breaking them into smaller droplets to aid in digestion by lipase. The liver also plays a crucial role in metabolism, detoxification, and storage of nutrients.
Movements of the GI Tract
The GI tract exhibits various movements to propel food and mix it with digestive secretions. Peristalsis is a wave-like contraction that moves food along the tract. Segmentation involves localized contractions that mix the contents and enhance absorption. The stomach undergoes churning movements and the migrating motor complex helps sweep undigested material.
Digestion and Absorption
Carbohydrates: Digestion begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase. Brush border enzymes (e.g., maltase, sucrase, lactase) break down disaccharides into monosaccharides (glucose, fructose, galactose), which are absorbed into the bloodstream.
Proteins: Digestion begins in the stomach with pepsin and continues in the small intestine with pancreatic proteases (trypsin, chymotrypsin, etc.). Brush border peptidases further break down peptides into amino acids, which are absorbed.
Fats: Bile emulsifies fats, and pancreatic lipase breaks them down into fatty acids and monoglycerides. These form micelles, which are absorbed by enterocytes (intestinal cells). Inside the enterocytes, they are reassembled into triglycerides and packaged into chylomicrons, which enter the lymphatic system via lacteals.
Disorders of the GI Tract
Common disorders include:
- Gastroesophageal reflux disease (GERD): Stomach acid flows back into the esophagus.
- Peptic ulcers: Sores in the lining of the stomach or duodenum.
- Inflammatory bowel disease (IBD): Chronic inflammation of the GI tract (Crohn's disease, ulcerative colitis).
- Irritable bowel syndrome (IBS): Functional disorder with abdominal pain and altered bowel habits.
- Gallstones: Hardened deposits in the gallbladder.
- Celiac disease- gluten related problems.
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