Most Chemical Digestion Occurs in the Small Intestine

How does the structure of the small intestine affect its function?

 The surface of the small intestine is highly folded and each fold has many projections (villi). Brush border cells at the surface of a villus have tiny projections (microvilli) at their surface. The many folds and projections greatly increase the surface area for the two functions of the small intestine—digestion and absorption.

Dudenum :It’s a tube of about 30 cm long :

–        Receives bile via bile duct from the liver

–        Receives pancreatic juice from the pancreas, through the pancreatic duct

–        Releases digestive juice from its walls. This intestinal juice contains protease and the enzyme lipase for fat digestion

The Small Intestine

Duodenum: Controls the release of food into the small intestine

Most of the chemical breakdown (Digestion) and absorption of Food takes place here. Bile salts break down fats in the chyme while pancreatic fluid, composed of bicarbonate, neutralizes the acid. The small intestine also contains a host of enzymes that help digest various food molecules. The enzymes present in the small intestine and their functions are outline in the table below.

Pancreatic amylase, maltase, sucrase, lactase ------Starch

Trypsin, chymotrypsin, aminopeptidase,carboxypeptidase ------Protein 

Nucleases------ Nucleic acids 

Bile salts, lipase ------Fats

What are the roles of the small intestine?

Chemical digestion is completed in the small intestine. Enzymes from the pancreas and enzymes embedded in the membrane of brush border cells break large molecules into smaller absorbable subunits. Small subunits (monosaccharides, amino acids, fatty acids, and monoglycerides) enter the internal environment when they are absorbed into the intersitial fluid in a villus. Most fluid that enters the gut is also absorbed across the wall of the small intestine.

The Small Intestine

• The small intestine is split into three sections: the duodenum (30 Cm), jejunum (2.5m), and the ileum(2-4m).

• Chemical digestion takes place in the duodenum and the jejunum.

• The ileum, the final section of the small intestine, is where most absorption of the nutrients takes place.

• Small projections called villi cover the ileum walls.

• Cells lining the villi are covered in folds of plasma membrane that form even smaller projections called microvilli.

• Villi and microvilli increase the ileum’s surface area, providing more surface across which nutrients can be absorbed.

• Nutrients pass through capillaries in the lining of the villi and into the bloodstream, where they circulate first to the liver, then throughout the rest of the body.

• Villi and microvilli projections also contain digestive enzymes to further digest food.