d. Water. Each day about nine quarts of water enter the small intestine. This
fluid is composed of liquid intake and various gastrointestinal secretions. The small
intestine absorbs roughly eight quarts of this water. The remainder of the water passes
into the large intestine where most of the water is absorbed. The small intestine
absorbs water by osmosis through epithelial cells and into the blood capillaries of the
villi (small hair-like projections from the surface of mucous membranes). Water is
normally absorbed at the rate of 200 to 400 ml/hour.
e. Electrolytes. The small intestine absorbs electrolytes which are parts of
gastrointestinal secretions. This organ also absorbs electrolytes from ingested foods
and liquids. Absorption again takes place through the villi in the small intestine.
f. Salts. Salts are absorbed by the villi in the large intestine.
g. Vitamins. The small intestine absorbs fat-soluble vitamins such as vitamins
A, D, E, and K. The majority of water-soluble vitamins are absorbed by diffusion.
Certain parts of food resist digestion and are eliminated from the intestines in the
feces. These residues of digestion include cellulose from carbohydrates, undigested
connective tissue, and toxins from meat proteins and undigested fats. Additionally,
feces contain bacteria, pigments, water, and mucus.
The feeding of each tissue is critical to growth and maintenance of the body.
When this system is disrupted, the body cannot maintain itself. The understanding of
the anatomy and physiology of this system will help you develop the concepts
necessary to anticipate the problems disease or trauma may cause.