Histology and Functions of the Stomach

Histology of the Stomach

• The serosa is the outermost layer of the stomach.
• Muscularis layer comprises three layers: outer longitudinal, middle circular, and inner oblique.
• Submucosa and mucosa layers follow the muscularis; the stomach is lined with simple columnar epithelium.
• Mucosal surface features gastric pits leading to gastric glands.
• Four main types of epithelial cells:
  • Surface mucous cells produce mucus.
  • Parietal (oxyntic) cells generate hydrochloric acid and intrinsic factor.
  • Chief (zymogenic) cells secrete pepsinogen.
  • Endocrine cells release the hormone gastrin into the bloodstream.

Functions of the Stomach

Mechanical Digestion

• Stomach walls enable mechanical digestion through mixing waves produced by smooth muscles.
• Mixing waves combine food boluses with gastric juices, creating chyme.

Chemical Digestion

• Stomach secretions: mucus, hydrochloric acid, gastrin, intrinsic factor, and pepsinogen:
  • Mucous cells supply viscous alkaline mucus for lubrication and protection.
  • Parietal cells secrete hydrochloric acid and intrinsic factor to enhance vitamin B12 absorption.

Enzymes Secreted by the Stomach

• Major stomach enzymes include pancreatic lipase, trypsinogen, and chymotrypsinogen.
• Pancreatic amylase hydrolyzes polysaccharides to maltose and glucose.
• Pancreatic lipase hydrolyzes fats into mono and diglycerides, glycerol, and fatty acids.

Enzyme Precursors

• Trypsinogen and chymotrypsinogen are inactive enzyme precursors found in pancreatic juice.
• Enterokinase activates trypsinogen, which then activates more trypsin to convert proteins into polypeptides.
• Trypsin also converts chymotrypsinogen into its active form, chymotrypsin.

Bile and Fat Digestion

• Bile, produced in the liver and stored in the gallbladder, emulsifies fats for easier digestion.
• Emulsification increases the surface area for lipase action.

Jejunum and Ileum

• Jejunum length: about 2.5 meters; Ileum length: approximately 3.5 meters.
• Final digestion of proteins, carbohydrates, and fats occurs here.
• Enzymes processed:
  • Aminopeptidase splits polypeptides into dipeptides.
  • Erepsin converts peptides into amino acids.
  • Lactase converts lactose to glucose and galactose.
  • Maltase converts maltose to glucose.
  • Sucrase converts sucrose to glucose and fructose.

Chyle

• Chyme is transformed into a watery emulsion called chyle through enzymatic action.

Absorption of Nutrients

• Ileum serves as the primary site for nutrient absorption with villi that increase surface area.
• Each villus contains a blood capillary network and lymphatic capillary (lacteal).
• Nutrients must pass through epithelial cells to reach blood or lymphatic vessels.

Large Intestine Structure

• The ileocecal junction connects the ileum to the large intestine, guarded by the ileocecal sphincter.
• Caecum is the beginning of the large intestine, with the vermiform appendix attached.
• Colon measures around 1.5 meters, consisting of ascending, transverse, descending, and sigmoid sections.

Functions of the Large Intestine

• Absorbs water and electrolytes, converting content into solid feces.
• Absorbs vitamins produced by bacteria, especially Vitamin K and Biotin.
• Secretes bicarbonates to neutralize acidity and acts as a mucosal barrier against pathogens.

Defecation Reflex

In Adults

• Initiated by deep breathing and contracting abdominal muscles to increase pressure.
• Feces move into the rectum, stimulating peristaltic waves and sphincter relaxation for elimination.
• Reflex is brief, and a person can voluntarily inhibit defecation.

In Infants

• Defecation reflex results in involuntary bowel emptying due to lack of external sphincter control.