Stomach Anatomy and Function

Stomach

• Has three layers of muscle, unlike the rest of the<small>Gastrointestinal tract</small>which has two.
• Churns and mixes food with gastric juice.
• Has two sphincters: cardiac and pyloric.
• Pyloric sphincter is a ring of muscle that normally maintains constriction of a body passage.

Neural Feedback

• Food distends the stomach, stimulating stretch receptors in its walls.
• Chemoreceptors monitor pH changes.
• Activates submucosal plexus, causing waves of peristalsis and gastric juice flow.

Stomach Secretions

• Secretes 2-3L of highly acidic (pH 2-3) gastric juice and mucus per day.
• Mucus provides a barrier against acidity within the stomach.
• The stomach lining constantly renews itself through apoptosis, replacing old cells with new to maintain a healthy gastrointestinal tract and efficient digestion, happening every three days.

Exocrine Cells

• Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor (IF) for vitamin B12 absorption.
• Chief cells secrete pepsinogen, which is converted to the active enzyme pepsin for protein digestion.
• Goblet cells secrete mucus for protection against acid.

Stomach Functions

• Mixing chamber: churns up food.
• Holding reservoir: storage.
• Defence: non-specific defence against gastric acidity.
• Absorption (limited): water, alcohol, drugs (e.g., aspirin).
• Digestion: mechanical and chemical digestion of proteins and lipids.
• Iron: made more soluble with stomach acid.
• Satiation: tells you to stop eating.

Hormones

• Ghrelin: stimulates hunger.
• Gastrin: responds to stomach distension, stimulating gastric juice secretion and gastric motility.

Pancreas

• An accessory digestive organ with both exocrine and endocrine functions.
• Exocrine function: secretes pancreatic juice (1.2-1.5L/day) containing sodium bicarbonate, water, and protease enzymes.
• Endocrine function: secretes hormones insulin, glucagon, and somatostatin into the blood.

Pancreatic Enzymes

• Secreted into the duodenum in their inactive form.
• Enzymes include pancreatic amylase (breaks down starches into sugars), pancreatic lipase (lipid/fat digestion), trypsin and chymotrypsin (protein digestion), ribonuclease (digests RNA), and deoxyribonuclease (digests DNA).

Small Intestine: Absorption

• Vitamin B12 is absorbed in the terminal ileum (with the help of intrinsic factor).
• Vitamins, mineral salts, and water enter blood capillaries and are sent to the liver (hepatic first pass).
• Bile salts are reabsorbed and recycled from the ileum.

Fluid Movement

• All water absorption in the GI tract occurs via osmosis from the lumen into absorptive cells and into blood capillaries.
• Osmosis is bi-directional, so absorption of water depends on electrolyte, monosaccharide, and amino acid levels to maintain an osmotic balance/concentration gradient with the blood.

Large Intestine

• The final stage of digestion, absorption, some vitamin synthesis (through bacterial activity), and stool formation occurs.
• The ileocaecal valve allows one-way flow of materials from the small to large intestine.
• Mucus is produced, but no enzymes are secreted.
• Absorption of water, minerals, vitamins, and some drugs occurs.
• Four regions: caecum, colon, rectum, and anal canal.