Digestion and Absorption in the Intestines

Digestion in the Intestines

• Most chemical digestion takes place in the small intestine
• Despite its narrow diameter of 2.5 cm, the small intestine is quite long at 7 meters
• The first 25-30 cm of the small intestine is where most digestion occurs, and is called the duodenum.
• Sections of the small intestine:
  • Jejunum
  • Ileum
• The small intestine secretes digestive enzymes, and moves contents through peristalsis

Absorption in the Intestines

• Most absorption takes place in the small intestine
• Villi are projections that increase the surface area for absorption
• Each villus has microvilli that further increase surface area
• Each villus contains:
  • Capillaries
  • Lymph vessels known as lacteals
  • Monosaccharides and amino acids are transported via capillaries
  • Fats are absorbed into lacteals

Pancreas and pH Regulation

• Stomach contents entering the small intestine have a low pH
• The small intestine lacks the mucous lining that the stomach has
• To prevent damage to the small intestine:
  • Prosecretin is converted to secretin when acids enter the small intestine
  • Secretin is released into the bloodstream
  • Secretin signals the pancreas to release bicarbonate ions into the small intestine
  • Bicarbonate ions buffer the acid from the stomach, raising the pH of the small intestine to 8-9
  • The change in pH denatures pepsin

Pancreatic Enzymes

• The pancreas releases digestive enzymes, including trypsinogen
• Enterokinase in the small intestine converts trypsinogen to trypsin
• Trypsin continues the breakdown of proteins started by the stomach
• Long-chain polypeptides are converted into short-chain polypeptides
• Erepsins, are released from the pancreas and small intesine
• Erepsins work on short-chain polypeptides, converting them into amino acids
• The pancreas releases amylase, which continues the breakdown of carbohydrates started in the mouth, starches are broken down into disaccharides
• Disaccharidases from the small intestine complete the breakdown of carbohydrates

Lipases

• Lipases, released from the pancreas, break down lipids (fats)
• Pancreatic lipase breaks down fats into fatty acids and glycerol
• Phospholipase breaks down phospholipids

Liver and Gallbladder

• The liver produces bile, which contains bile salts that aids in fat digestion
• Bile is stored in the gallbladder
• When fats enter the small intestine, cholecystokinin (CCK) is released
• CCK is carried to the gallbladder via the bloodstream to trigger the release of bile salts
• Bile salts travel to the small intestine via a duct, where they emulsify large fat globules
• Emulsification increases the surface area for enzymes like pancreatic lipase to work on
• Bile pigments cause the brown color of feces

Liver Functions

• The liver has several functions in addition to the production of bile
• The liver stores:
  • Glycogen
  • Vitamins A, B12, and D
• The liver detoxifies harmful substances in the body (e.g., alcohol)

Gallstones and Liver Disease

• Gallstones result from the combination of cholesterol and salt crystals from bile and can block the bile duct, causing pain
• A block of the bile duct can cause Jaundice
• Jaundice causes skin and the whites of the eyes to become yellow
• Chronic alcohol abuse can lead to cirrhosis of the liver

Large Intestine

• Chemical digestion is complete by the time food reaches the large intestine
• The colon, the largest part of the large intestine, stores waste and absorbs water, minerals, and vitamins
• The large intestine contains bacteria, such as E. coli, that helps manufacture vitamins B and K
• Cellulose from plant sources makes up fiber, which acts as "steel wool" for the colon
• Colon cancer is connected to diets high in processed food and lower in fiber

Control of Digestion

• The nervous and endocrine systems control the rate of digestion
• Secretin is a hormone that signals the release of bicarbonate ions
• Gastrin is produced by the stomach when the walls stretch with partially broken down food
• Gastrin travels to stomach cells to release HCL.
• The nerve cells in the stomach wall activate when food is consumed, causing the muscles to contract and gastric juices to be released
• Meals with high fat stimulate the small intestine to secrete enterogastrone
• Enterogastrone slows down peristalsis to promote complete breakdown and absorption of lipids