Histology and Functions of the Stomach

Study Notes

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.

Description

Explore the intricate histology and critical functions of the stomach in this quiz. Learn about the layers of the stomach, its cellular composition, and the processes involved in mechanical and chemical digestion. Test your knowledge on gastric secretions and their roles in digestion.