Small and Large Intestine: Digestion and Absorption

The Small Intestine

The small intestine is about 6 meters long. In the duodenum, two liquids are added to continue digestion: pancreatic juice and bile. Pancreatic juice contains enzymes such as lipases, proteases, and carbohydrase (e.g., amylase).

Enzyme Breakdown

Each enzyme breaks down food in the following way:

• Protein -> protease -> amino acids
• Lipid -> lipase -> fatty acids and glycerol
• Carbohydrate -> carbohydrase -> sugar

Bile's Role in Digestion

Bile is made in the liver, stored in the gallbladder, and delivered to the small intestine via the bile duct. Bile neutralizes the acid which was added to the food in the stomach because it is alkaline. This provides the optimal pH for enzymes in the small intestine to work. Bile salts emulsify fats (i.e., break them up into small droplets) so they have a large surface area for lipase enzymes to act upon them. The digestible material in the food is then changed into small, soluble compounds which can pass through the wall of the intestine into the blood easily.

Absorption in the Small Intestine

The small intestine is an excellent absorption surface because:

• It is very long
• It has thousands of villi which increase the surface area greatly
• It has a very good blood supply
• The walls are very thin

Small molecules of digested food, such as glucose and amino acids, pass through the walls of the villi into the blood. They are then carried to the liver via the hepatic portal vein for use in metabolism or storage. Most of the fatty acids and glycerol do not enter the blood in the villi. Instead, they enter the lacteal, a part of the villus specialized to carry fats. The fluid in the lacteal is then transported in the lymph system around the body and may enter the blood in a vessel of the neck. It can then be taken to where it is needed.

The Large Intestine

When the food gets to the large intestine, there is not much useful left. The large intestine mainly contains fiber, dead cells, bacteria, and water. As the material passes along the large intestine, water is absorbed into the blood. The solid waste, or feces, are stored in the rectum. The feces are then egested through the anus. It takes between 24 and 48 hours for food to pass along the whole length of your digestive system.