Human Digestion Small Intestine and Large Intestine Small

Human Digestion Small Intestine and Large Intestine

Small Intestine • More than 6 m in humans • The longest section of the alimentary canal. • Its name refers to the diameter which is smaller than the large intestine. • Most of the enzymatic hydrolysis and absorption of nutrients occurs in the small intestine.

• The first 25 cm of the small intestine is called the duodenum. • In the duodenum, chyme from the stomach is mixed with digestive juices from the pancreas, the liver and the gallbladder. (accessory glands)

Duodenum

Pancreas • The pancreas produces enzymes and an alkaline solution rich in bicarbonate. • The bicarbonate acts as a buffer offsetting the acidity of the chyme from the stomach. • Enzymes include proteases like trypsin and aminopeptidase, pancreatic amylases, pancreatic nucleases and lipase.

Liver • Produces bile, a mixture of salts that act as detergents that break down fats. • Bile salts surround fat droplets aiding lipase to hydrolyze fat molecules into glycerol and fatty acids. • Bile is stored in the gallbladder until needed.

Small Intestine • Most digestion is completed in the duodenum. • Peristalsis moves the mixture of chyme and digestive juices along the small intestine. • The jejunum and the ileum areas in the small intestine that function in absorption of nutrients and water.

Small Intestine • To enter the body, nutrients must cross the lining of the digestive tract. • Most absorption takes place in the small intestine. • The surface area of the small intestines is 300 m 2.

Small Intestine

Small Intestine • Large circular folds in the lining make fingerlike projections called villi. • Each cell of a villus has many structures called microvilli. • Penetrating the core of each villus is a group of microscopic blood vessels called lacteal.

Absorption • Nutrients are absorbed across two layers of epithelial cells. – Epithelium of the villus – Epithelium of the lacteal

Absorption • Transport may be passive by diffusion or active by pumping against a gradient. – Passive - fructose moves by facilitated diffusion down its concentration gradient. – Active - amino acids, glucose, and vitamins are pumped across the epithelial cell membrane. Fats move out of the epithelial cells by exocytosis.

Absorption • Absorbed amino acids and sugars enter capillaries. • The nutrient rich blood from the small intestine is carried directly to the liver by the hepatic portal vein. • The liver regulates the nutrient content of blood.

Absorption • From the liver, blood travels to the heart, which pumps the blood and the nutrients it contains to all parts of the body.

Large Intestine • The small intestine leads into the large intestine. (colon) • There are three areas of the large intestine – Ascending – Transverse – Descending

Large Intestine Cecum & appendix Sigmoid Colon

• The colon finishes the reabsorption process of water. • Absorbs vitamin K and B • Hardens stool • Ferments undigested organic material

E. coli • Escherichia coli and other mostly harmless bacteria live on unabsorbed organic material in the large intestine. • Some of these bacteria produce vitamin B and vitamin K. • Feces contain cellulose, other undigested materials and bacteria

• Feces is stored in the rectum to be released.