Human Digestive System

Stomach secretes intrinsic factor required for absorption of vitamin B12, which is a water-soluble vitamin.

Chief cells (also called peptic cells or zymogenic cells) in the gastric glands of the stomach secrete pepsinogen, which converts to active pepsin for protein digestion. Alpha and beta cells are found in pancreatic islets of Langerhans and secrete glucagon and insulin respectively. Option A correctly identifies chief cells. Islets of Langerhans (C) is the pancreatic region containing alpha and beta cells, not a gastric cell type.

Pepsin is a protease (protein-digesting enzyme) that functions optimally in acidic conditions (pH 1.5-2). The stomach's hydrochloric acid provides this acidic environment and also converts pepsinogen to active pepsin. In alkaline, neutral, or isotonic conditions, pepsin becomes denatured or inactive. Option A correctly states pepsin is active in acid. Pepsin would be inactive in alkaline (B), neutral (C), or isotonic (D) environments.

Gastric juice contains pepsin (protease), gastric lipase (fat-digesting), and rennin/chymosin (milk-clotting, particularly in infants). Trypsin is a pancreatic enzyme, not a gastric enzyme - it's secreted by the pancreas into the small intestine. Option A correctly lists gastric juice components. Options B, C, and D incorrectly include trypsin, which is pancreatic, not gastric.

The stomach maintains an acidic pH primarily through hydrochloric acid (HCl) secretion from parietal cells in the gastric glands. This acidic environment (pH 1.5-2) is essential for activating digestive enzymes and killing ingested pathogens. Mucus secretion (option A) protects the stomach lining, while bile (B) and pancreatic juice (C) are secreted into the small intestine, not the stomach.

Correct Answer: stomach

Gastric epithelial cells are protected by a thick layer of mucus secreted by goblet cells and surface epithelial cells. This mucus forms a physical barrier and contains bicarbonate that neutralizes acid at the cell surface. The HCl is NOT too dilute (it's very potent), epithelial cells are NOT inherently acid-resistant, and HCl is NOT neutralized by alkaline gastric juice (the stomach is acidic).

The parasympathetic nervous system (rest and digest) stimulates digestive activities via the vagus nerve. It increases GI secretions and enhances peristalsis after food ingestion. The sympathetic nervous system (fight or flight) would inhibit digestion, while the central nervous system and thyroid membrane are not directly responsible for digestive regulation.

Pepsin functions in the stomach's acidic environment (pH 1.5-2), where it begins protein digestion. Trypsin operates in the alkaline duodenum (pH 7.5-8) and requires different conditions. Option A incorrectly states pepsin works in alkaline medium, while options B and D place these enzymes in the wrong locations.

Bilirubin and biliverdin are breakdown products of hemoglobin from red blood cells. These pigments are excreted by the liver into bile, giving bile its characteristic greenish-yellow color. They are not found in blood, pancreatic juice, saliva, or lymph in significant quantities.

The liver produces bile, which contains bile salts, cholesterol, and bilirubin, but NO digestive enzymes. Bile mechanically emulsifies fats to increase surface area for lipase action. The actual digestive enzymes that break down carbohydrates, proteins, and fats come from the pancreas and small intestine.

The inner surface of the large intestine, like the entire digestive tract, is lined by epithelial tissue. This is specifically simple columnar epithelium with numerous goblet cells that secrete mucus to protect the intestinal wall and lubricate passage of contents. The muscular tissue lies beneath this epithelial layer in the submucosa and muscularis externa.

Succus entericus refers to the intestinal juice secreted by the glands of the small intestine. This clear to pale yellow fluid contains digestive enzymes (maltase, sucrase, lactase, peptidases, lipase), mucus, and electrolytes that continue the chemical digestion of food in the small intestine.

Pancreatic juice secretion is regulated by two key hormones. Secretin, produced by S-cells in the duodenum when acidic chyme enters, stimulates bicarbonate secretion to neutralize acid. Cholecystokinin-pancreozymin (CCK-PZ), produced by I-cells when fats/proteins are present, stimulates digestive enzyme secretion. Gastrin primarily stimulates HCl secretion, while enterogastrone inhibits gastric motility.

Bile is produced in the liver but stored and concentrated in the gallbladder. When fatty food enters the duodenum, bile is released into the duodenum (first part of small intestine) where it emulsifies fats - breaking large fat globules into smaller droplets to increase surface area for lipase action. Emulsification does not occur in liver, stomach, or pancreas.