Human Anatomy: The Stomach

Stomach

• The stomach is a dilated portion of the digestive tract, capable of holding 1.5-2 liters of food.
• Its walls contain three layers of muscle.
• The stomach has four main parts: cardia, fundus, body, and pylorus (including antrum, canal, and sphincter).
• Rugae are folds in the stomach mucosa.
• The stomach contains glands with specialized cells:
  • Surface mucous cells secrete mucus.
  • Mucous neck cells secrete mucus.
  • Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor.
  • Chief cells secrete pepsinogen and gastric lipase.
  • G cells secrete the hormone gastrin.
• Stomach mixing waves mix food with gastric juice, turning it into a liquid called chyme.
• Stomach acid inactivates salivary amylase but activates lingual lipase.
• Proton pumps use ATP to move hydrogen ions into the stomach.
• Potassium and chloride ions move into the stomach lumen.
• Carbonic anhydrase catalyzes the reaction of water and carbon dioxide to form carbonic acid.

Regulation of Gastric Secretion and Motility

• Gastric secretion and motility are controlled by neural and hormonal mechanisms.
• Three overlapping phases: cephalic, gastric, and intestinal.
• Cephalic phase: initiated by stimuli like smell, sight or thought of food. This triggers responses. Medulla oblongata is involved as well.
• Gastric phase: initiated by the presence of food in the stomach. Stretch receptors and pH receptors control motility and acid secretion.
• Intestinal phase: initiated by the entry of chyme into the small intestine. Reflexes slow down stomach emptying to avoid overloading the duodenum.

Intestinal Phase

• Chyme entering the small intestine triggers intestinal reflexes to inhibit gastric emptying and slow down stomach emptying to prevent overloading the duodenum.
• The small intestine secretes secretin and cholecystokinin (CCK) to regulate stomach emptying further.
• Secretin inhibits gastric secretion; CCK inhibits stomach emptying.

Small Intestine

• The small intestine is approximately 3-3.5 meters long, responsible for nutrient absorption.
• It consists of three parts: duodenum, jejunum, and ileum.
• Its mucosa is highly folded, including circular folds (plicae circulares), villi, and microvilli to maximize surface area for absorption.
• It possesses unique glands and cells (absorptive, goblet, enteroendocrine, and Paneth cells).
• Small intestine contains fluids (intestinal juice) and enzymes (e.g., alpha-dextrinase, maltase, sucrase, lactase, aminopeptidase, dipeptidase, nucleosidase, phosphatase).

Digestion of Carbohydrates, Proteins, and Lipids

• Salivary amylase begins carbohydrate digestion; pancreatic amylase continues in the small intestine.
• Pepsin begins protein digestion in the stomach; pancreatic enzymes continue in the small intestine.
• Lingual and gastric lipase begin lipid digestion; pancreatic lipase continues in the small intestine.

Absorption in Small Intestine

• Absorption of nutrients from the small intestine lumen into the blood and lymphatic vessels.
• Monosaccharides (glucose, fructose, and galactose) are absorbed via facilitated diffusion or active transport.
• Amino acids, dipeptides, and tripeptides are absorbed by active transport.
• Fatty acids, glycerol, and monoglycerides are absorbed by simple diffusion but form micelles to be transported.
• Electrolytes, vitamins, and water are also absorbed.

Large Intestine

• The large intestine is approximately 1.5 meters long.
• It includes the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal, and anal opening.
• The main function is the absorption of water, electrolytes, and some vitamins
• Bacteria in the large intestine ferment undigested material and produce vitamins (e.g., vitamin K).
• Feces are formed and eliminated via the defecation reflex.