Digestion

Structures Continued

  1. Pyloric Sphincter - Band of muscle which closes off the lower part of the stomach and only allows small amounts (~1 teaspoon) of chyme to enter the small intestine.
  2. Duodenum - First 10 inches of the small intestine. Enzymes from both the pancreas and the small intestine digest all 3 food groups here. (chemical digestions) Bile, which is made in the liver and stored in the gall bladder, is also secreted into the small intestine to emulsifies fat.
  3. Pancreas - Produces enzymes and sodium bicarbonate (the sodium bicarbonate is required to neutralizes acid chyme (pH of about 2) and bring it to a pH of about 8.5 so that the enzymes in the small intestine are not denatured.
  4. Gall Bladder - Stores Bile which is secreted into the small intestine from here.
  5. Liver - Produces bile to emulsify fats which is then stored in the gall bladder.liver

    pancreas





















cck

6. Small Intestine - Final digestion and absorption of nutrients through the villi.

CONTROL OF INTESTINAL SECRETIONS:

The duodenal wall produces hormones, the most important of which are secretin and CCK (Cholocytokinin) in response to the presence of acidic chyme. Secretin stimulates release of Pancreatic Juices from the pancreas. CCK stimulates the release of Bile from the gall bladder.

Acid, especially HCl, stimulates the release of secretin, while partially digested protein and fat stimulate the release of CCK.
The Small Intestine is about 7m long - first 25 cm (10ยจ) is the duodenum

a) Duodenum: Sodium bicarbonate is secreted from the pancreas which turns acid chyme entering the small intestine basic (from a pH of about 2 to a pH of about 8.5)

Other Pancreatic Juices:
i) Pancreatic amylase breaks down starch to maltose.
ii) Trypsin breaks down protein to peptides.
iii) Lypase breaks down fat droplets, after bile has emulsified the fat, to fatty acids and glycerol.

b) Remainder of Small Intestine is responsible for the final digestion of protein and carbohydrates from intestinal juices made from millions of digestive glands along the intestinal wall.

i) Peptidase breaks down peptides to amino acids
ii) Maltase breaks down maltose to 2 glucose molecules.
Main function of the small intestine though, is the absorption of nutrient molecules. (A.A., glucose, fatty acids, and glycerol)
Other dissacharides (similar to maltose) are digested in the small intestine by their own enzymes. (e.g., lactase aids in lactose digestion, which is sugar in milk)

Absorption in the Small Intestine

The small intestine is specialized for absorption in that:
-- The small intestine is long with convoluted wall (folded walls)
-- The walls of the small intestine have villi (finger-like projections along the walls). The villi themselves have tiny microvilli on columnar epithelial cells.

Within each villus are blood vessels and small lymph vessel called a lacteal which absorbs fluids and returns it to the veins later on.

Absorption occurs across the walls of each villus by active transport (uses energy). Glucose and Amino acids enter the blood vessels and travel to the liver. Glycerol and fatty acids enter the lacteals, which will go back into the bloodstream later at the subclavian veins. 

sm intest
**Be sure you understand how the structure of the small intestine facilitates its function**Appendix - Found at junction of small intestine 
  1. digestive systemand large intestine.
    --unknown function in humans
  2. Large Intestine (colon) absorption of water from undigested food.
    -- The large intestine usually contains a large population of E. Coli bacteria. These organisms live off any substances that were not digested earlier. When they break down these substances, they give off odorous molecules that cause the characteristic odor of feces.

    Some vitamins, Amino Acids, and other growth substances required by the body ( Growth factors), are produced by these bacteria. The growth factors spill out into this gut and are absorbed by the gut lining. E. Coli helps us metabolize what our bodies were unable to thereby providing us with a vital service.

  3. Rectum - Enlarged portion of the colon. Undigested food is concentrated and stored here temporarily.

  4. Anal Sphincter (anus) - Bands of muscle which allow undigested wastes to exit the body.