Digestive System | Parts Of Digestive System | Human Anatomy And Physiology | B Pharma 2nd Semester

The lecture begins by establishing the fundamental purpose of the digestive system: breaking down complex food molecules (carbohydrates, proteins, fats) into simple, absorbable forms so cells can produce energy. The instructor explains that carbohydrates break into monosaccharides, proteins into amino acids, and fats into fatty acids and glycerol — collectively referred to as digestion, performed by the gastrointestinal (GI) tract spanning from mouth to anus.

The stages of digestion are outlined as: ingestion (receiving food), propulsion (movement of food), digestion (mechanical and chemical breakdown), absorption, and excretion. The GI tract's major organs include the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus, while accessory digestive glands include salivary glands, liver, gallbladder, and pancreas.

The mouth (buccal/oral cavity) contains lips, gums, teeth, salivary glands, and the tongue. Teeth perform mechanical digestion while salivary glands secrete saliva that mixes with food to form a bolus. The tongue mixes food with saliva and contains taste receptors. The pharynx is a common pathway for both digestive and respiratory tracts, and the epiglottis prevents food from entering the trachea during swallowing. The esophagus (food pipe) carries the bolus from the pharynx to the stomach; no digestion occurs here due to absence of digestive enzymes, though minor starch digestion begins in the mouth via salivary amylase.

The stomach is a J-shaped structure located in the left side of the abdominal cavity with a capacity of 30ml to 1.5 liters. It has four layers (serosa, muscularis, submucosa, mucosa) and four regions: cardiac, fundus, body, and pylorus. The stomach secretes gastric juice containing HCl, pepsin, and mucus. The cardiac region contains a cardiac sphincter preventing backflow. The fundus contains chief cells (secreting pepsinogen, rennin, lipase) and parietal cells (secreting HCl). The body is the largest part where protein digestion begins. The pylorus connects to the duodenum and contains G-cells that secrete gastrin hormone. Goblet cells in the stomach lining secrete mucus to protect the stomach from HCl.

The small intestine (6-7 meters long) is the site of final digestion and major absorption, divided into duodenum, jejunum, and ileum. The duodenum receives bile juice from the liver and pancreatic juice from the pancreas, converting acidic chyme to alkaline form. The jejunum (4-8 feet) is the major site for absorption of sugars, amino acids, and fatty acids, and final food breakdown is completed here. The ileum (10-11 feet) is the major site for vitamin B12 absorption. Absorption occurs via villi and microvilli structures on the intestinal wall.

The large intestine (1.5 meters) absorbs water from digested food; excess water absorption causes constipation while insufficient absorption causes diarrhea. It is divided into cecum, ascending colon, transverse colon, descending colon, and sigmoid colon. The rectum stores waste products (feces) and expels them through the anus in a voluntary process controlled by internal (involuntary) and external (voluntary) sphincter muscles.

Regarding digestive glands: salivary glands (parotid — largest but secretes only 25% serous saliva; submandibular — secretes 70% mixed saliva; sublingual — smallest, secretes 5%) produce saliva containing salivary amylase. The liver is the largest internal organ (1.5 kg), with highest regeneration power, performing 100+ functions including bile juice secretion, glycogen synthesis, urea synthesis, vitamin/mineral storage, heparin synthesis, blood sugar regulation, and detoxification. The gallbladder stores and concentrates bile. The pancreas (mixed gland) secretes pancreatic juice (lipase, amylase, trypsin) and hormones from islets of Langerhans: alpha cells secrete glucagon, beta cells secrete insulin, and delta cells secrete somatostatin.

The HCl production mechanism involves CO2 combining with H2O to form carbonic acid (H2CO3), which splits into H+ and HCO3-. The H+/K+ ATPase pump moves H+ into the stomach lumen while a chloride/bicarbonate exchanger brings Cl- in, allowing H+ and Cl- to combine as HCl. Acid secretion regulation involves three phases: cephalic (40%, triggered by sight/smell of food), gastric (60%, when food enters stomach), and intestinal phases — regulated through the parasympathetic nervous system and gastrin hormone.