Absorption

Absorption of nutrients is a complex process. Food must be broken down into very small particles before it can be absorbed and utilized by the body. Most absorption takes place in the small intestine.

We think of the small intestine as being a long tube that leads from the stomach to the large intestine. In reality, it is more like a long tube covered with shag carpeting. This increases the surface area of the small intestine, since the surface now includes not just the tube but all the sides of each strand of shag carpeting.

These strands are called villi. They are tiny finger-like projections that cover the walls of the small intestine. Each villi is covered with microvilli, a second set of even smaller finger-like projections. Absorption takes place along the microvilli.

The villi are located in the small intestine, on the intestinal folds.
The microvilli which cover the villi.
 Fats enter via the lymph system (yellow); water-solubles enter via capilaries in the blood stream (where red meets blue) and go to the liver via the portal vein.

Simple sugars, amino acids, water-soluble vitamins, some minerals and water all enter the blood stream through the sides of the microvilli. From here, they are carried to the liver via the portal vein. The liver decides how the body will handle the incoming nutrients, based on what is needed by the body at that time.

Fats are not water soluble so they don't enter the blood stream directly. Triglycerides are broken down into monoglycerides and fatty acids and enter the lymphatic system through the tops of the villi, as do the fat-soluble vitamins. After being reassembled as triglycerides, fats circulate through the lymphatic system. The lymphatic system mixes with the bloodstream near the heart. Fats circulate through the bloodstream and are taken to the parts of the body where they are needed.

There are three ways for nutrients to be absorbed, depending on their size and and molecular structure:

  • Simple diffusion: absorbed directly into the body. Water and some small lipids are absorbed this way.
  • Facilitated diffusion: These nutrients need a carrier to transport them across the cell membrane, like a ferry taking passengers downstream across a river. Water-soluble vitamins are absorbed this way.
  • Active transport: Glucose and amino acids are not readily transported and energy is needed to complete the process, like a motorized ferry taking passengers upstream across a river.
   
 

This process takes place along the entire small intestine. By the time the remnants of digestion reach the large intestine, fiber, some minerals, and water should be all that's left. The large intestine does not contain villi. Minerals and water are absorbed through the large intestine and fiber passes on to the rectum where it waits to be discharged through the anus.