Digestive System Overview and Stomach Structure

Digestive System Overview

• The digestive tract (alimentary tract) includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anal canal.

• The functions of the gastrointestinal tract (GIT) are: the passage (transport) of food; digestion; absorption; and elimination of residues, and absorption of water.

Structure of the Stomach

• The stomach is the most dilated part of the GIT, and is divided anatomically into cardia, fundus, body, and pyloric regions.
• The fundus and body have similar structures.
• The stomach acts as a food reservoir.
• Food is acidified and converted into chyme by mechanical and chemical action.
• The cardia of the stomach contains mucous-secreting glands.

Stomach Mucosa

• Gastric pits are small depressions in the stomach lining where the gastric glands open.
• The epithelium is simple columnar, mucus-secreting.
• The lamina propria contains the fundic glands.
• The muscularis mucosa is present.

Stomach Muscularis

• The stomach muscularis has inner oblique, middle circular, and outer longitudinal layers.
• The mechanical action of the muscles is in all directions.

Pylorus of Stomach

• The pylorus is the part of the stomach that connects to the duodenum.
• The pyloric gland characters include: not crowded, highly convoluted, and wider and longer pits.
• The pyloric sphincter (thick inner circular layer) and the outer longitudinal layer are present.

Small Intestine

• The small intestine is the site of terminal food digestion, absorption, and endocrine secretion.
• It has three parts: duodenum, jejunum, and ileum.
• The duodenum is the first and shortest part, connecting the stomach to the jejunum and surrounding the head of the pancreas.
• The jejunum makes up the proximal 2/5 after the duodenum.
• The ileum is the distal 3/5 of the small intestine.

Small Intestine Adaptations

• The small intestine is 6 meters long.
• Adaptations for absorption include plicae circulares (increasing surface area 3-fold), villi (increasing surface area 10-fold) and microvilli (increasing surface area 20-fold).

Small Intestine Tissue Layers

• The mucosa has villi and crypts.
• The epithelium has simple columnar, goblet, Paneth, enteroendocrine, and caveolate cells.
• The lamina propria has connective tissue.
• The muscularis mucosa contains the inner circular and outer longitudinal layers.
• The submucosa contains Brunner's glands, and lymphatic nodules (Payer's patches).
• There’s a muscularis externa (inner circular and outer longitudinal layers); and a serosa.

Large Intestine

• The large intestine absorbs water and produces mucus for lubrication.
• Parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.

Large Intestine Structure

• The mucosa has crypts, but no villi.
• The epithelium has absorptive columnar cells, many goblet cells, enteroendocrine cells, and stem cells.
• The lamina propria contains lymphatic nodules.
• The submucosa has no glands.
• The muscularis externa has an inner circular and an outer longitudinal layer, with only three bundles of the smooth muscle (tenia coli).
• The serosa is rich in fat cells.

Glands Associated with the Digestive Tract

• Salivary glands
• Pancreas
• Liver

Pancreas

• The pancreas is a mixed gland with exocrine and endocrine parts.
• The exocrine part secretes pancreatic juice containing enzymes like lipase, trypsin, chymotrypsin, and amylase crucial for digesting fats, proteins, and carbohydrates.
• The endocrine part secretes hormones like insulin and glucagon regulating blood sugar.

Liver

• The liver is a large and important metabolic organ.
• It receives nutrient-rich blood from the small intestine via the portal vein.
• It also receives oxygenated blood from the hepatic artery.
• Functions include exocrine secretion of bile and endocrine secretion of glucose and plasma proteins.
• Classic liver lobules are hexagonal masses of liver cells (hepatocytes) drained by a central vein, and demarcated by connective tissue.

Liver Hepatic Structures

• The hepatocytes are arranged in branching cords or plates (radiating from the central vein).
• The cords are separated by blood sinusoids.
• Each cord is formed of two rows of cells enclosing bile canaliculus.
• The endothelial lining of the blood sinusoids is separated from the hepatocytes by the space of Disse.

Hepatic Blood Sinusoids

• Hepatic sinusoids are present between cords of liver cells.
• They are lined with fenestrated endothelial cells and von Kupffer cells (macrophages) that clean and destroy old or abnormal red blood cells (senile RBCs).