Cikgu Anisa gave us a research to do, and here is what I have to do for my group: How food is absorbed in the villi of the small intestine. I looked it up in Google.

The small intestine is the site where most of the nutrients from ingested food are absorbed and is covered in wrinkles or folds called plicae circulares.

These are considered permanent features in the wall of the organ.They are distinct from rugae which are considered non-permanent or temporary allowing for distention and contraction.

From the plicae circulares are projected microscopic finger-like pieces of tissue called villi (The latin word meaning "shaggy hair").

The small intestine is lined with simple columnar epithelial tissue. The epithelial cells also have finger-like projections known as microvilli.

The function of the plicae circulares, the villi and the microvilli is to increase the amount of surface area available for secretion of enzymes and absorption of nutrients.

The purpose of these wrinkles and projections is to increase surface area for absorption of nutrients.

Each villus is covered in microvilli, which increase the surface area manyfold.

(The above are not important. However, they are relevant so I just put them up.)

ABSORBTION

Each villus contains a lacteal and capillaries. The lacteal absorbs the digested fat into the lymphatic system which will eventually drain into the circulatory system. The capillaries absorb all other digested nutrients.

The surface of the cells on the microvilli are covered with a water layer, which has a number of functions in absorption of nutrients.

• Water and lipids are absorbed by passive diffusion throughout. (Passive Diffusion not taught yet)
  
  
• Sodium is absorbed by active transport. (See bioT/B pg 46)
  
  
• Fructose is absorbed by facilitated diffusion. (See bioT/B pg 45)
  Lacteal
  (The cross, white thing in the very middle) (Picture: Transverse section of a villus, from the human intestine.)

  A lacteal is a lymphatic capillary that absorbs dietary fats in the villi of the small intestine.

  The combination of fat and lymph in the lacteals is milky in appearance and called chyle. Individual lacteals merge to form larger lymphatic vessels that transport the fats to the thoracic duct which empties into the left subclavian vein.

  At this point, the fats are in the bloodstream. They can be converted to lipoproteins (HDL, LDL or VLDL) in the liver or be transported to tissues throughout the body and stored in adipose cells as triglycerides.

  
  
  Villus (Plural: Villi)
  Intestinal villi are tiny, finger-like structures that protrude from the wall of the intestine and have additional extensions called microvilli (singular: microvillus) which protrude from epithelial cells lining villi.
  They increase the absorptive area of the intestinal wall.
  It is important that the food is absorbed at a considerably fast rate so as to allow more food to be absorbed.
  (If the process is too slow, the concentration of the blood in the blood vessels and the food will be equal, thus, diffusion will not occur. Remember, need a region of high concentration and low concentration. If both sides have the same concentration, diffusion will not occur.)
  Digested nutrients (including sugars and amino acids) pass into the villi through diffusion. Circulating blood then carries these nutrients away.
  Function of villus:

  In all humans, the villi and microvilli together increase intestinal absorptive surface area 30-fold and 600-fold, respectively, providing exceptionally efficient absorption of nutrients in the lumen.

  This increases the surface area of the intestine to around the area of a small parking lot or a tennis court.

  There are also enzymes on the surface for digestion. Villus capillaries collect amino acids and simple sugars taken up by the villi into the blood stream. Villus lacteals (See above) collect absorbed fatty acids.