Small Intestine Anatomy and Function

Small Intestine Structure and Function

• The small intestine is approximately 20 feet long and its primary function is absorption and digestion.
• It consists of four parts: duodenum, jejunum, ileum, and ileocecal valve.
• The mucosal lining of the small intestine is covered with villi, which are tiny fingerlike projections.
• Each villus contains goblet cells that release digestive enzymes, secrete mucus, and absorb nutrients.
• The villi are also covered with microvilli, which are located on the epithelial cells, forming a double set known as the brush border.
• The brush border increases the surface area of the small intestine, significantly increasing its absorptive capacity.

Digestive Process in the Small Intestine

• The intestinal phase of digestion involves neural and hormonal responses.
• The neural response, the enterogastric reflex, is responsible for opening the pyloric sphincter and decreasing gastric motility and acid production.
• In the duodenum, bile from the liver, enzymes from the pancreas, and chyme from the stomach come together to complete the digestive process.
• The duodenum and jejunum contain receptors that sense acidity, osmotic pressure, and products of digestion such as fats and peptides.

Functions of Small Intestine Segments

• The primary function of the duodenum is to complete the digestive process.
• The primary function of the jejunum is absorption of nutrients such as amino acids, glucose, iron, calcium, and fat-soluble vitamins.
• The primary function of the ileum is the reabsorption of vitamin B12 and the return of bile acids to the liver through the enterohepatic circulation process.
• The gastroileal reflex stimulates the opening of the ileocecal valve, which controls the release of digestive contents into the large intestine.