Digestion - Part 3 (Large intestine -> End)

Intestinal enzymes

• Villi in the small intestine secrete many enzymes to complete digestion:

  • maltase: maltose → glucose + glucose

  • sucrase: sucrose → glucose + fructose

  • lactase: lactose → glucose + galactose

  • peptidases: di/tri-peptides → amino acids

• Intestinal juice includes enzymes and mucus to protect and lubricate the lumen.

Large intestine

• Shorter than the small intestine, but has a larger diameter.

• Has 3 parts: ascending colon, transverse colon, and descending colon.

• The small intestine joins the large intestine ~7 cm from the bottom of the large intestine.

• Forms a “blind end.”

• Appendix: a vestigial organ that is easily infected because food can get stuck.

Absorption in large intestine

• As chyme passes through the large intestine, water and Na⁺ ions are absorbed back into the blood, along with other ions.

• Bacteria (especially E. coli) further break down food and release vitamin K (used in blood clotting) and some B vitamins, as well as some amino acids and various growth factors which are absorbed and used by the body.
  → Symbiotic with humans

Elimination

Chyme becomes more dense and is moved into lower part of the descending colon (by peristalsis) called the rectum.

• Stretch receptors in the rectum signal the brain when the rectum is full.

• Brain causes peristalsis and feces are eliminated through the anal sphincter (anus).

Constipation

Too much water has been removed from the feces.

• Occurs when food moves too slowly through the large intestine.

• Can be caused by a low fibre (cellulose) diet.
  → Fibre “holds on” to water, preventing its absorption.

• Colon cancer can result from prolonged contact of carcinogens with the lining of the large intestine.

Diarrhea

Too much water in feces.

• Causes: too much fibre, anxiety, laxatives, parasites (irritate intestinal lining), toxins, antibiotics (kill off E. coli and reduce solid mass in intestine)

• Effects: dehydration, loss of salts, hemorrhoids, loss of vitamins produced by E. coli.