a. Surface Area. The amount of absorption is proportional to the surface area
of the walls which contact the contents. Two anatomical specializations serve to
increase this surface area:
(1) There are permanent circular folds (plicae circulares) in the mucosal
lining of the small intestines.
(2) The entire inner surface of the mucosa is covered with villi. Villi are
minute, fingerlike processes that extend into the lumen (cavity) of the small intestines.
b. Capillaries. The simple sugars and amino acids are absorbed into the blood
capillaries. Most of the fatty acids and glycerol are absorbed into the lymphatic
6-20. HEPATIC VENOUS PORTAL SYSTEM
All of the blood capillaries in the absorptive areas of the digestive tract join to
form the hepatic portal venous system. A venous portal system is a system that begins
in capillaries, which join to form veins, which in turn end in another group of capillaries.
The hepatic portal vein carries the blood from the absorptive areas of the digestive
system to the liver.
6-21. THE LIVER
In the liver, a number of actions are performed on the blood. Excess materials
are removed and stored. For example, some glucose is stored as glycogen. Toxic
materials are degraded, microorganisms are removed, and so forth. The "treated"
blood is then routed from the liver to the heart and then throughout the body.
6-22. UTILIZATION OF THE LIPIDS
The lipid materials, such as fatty acids and glycerol, are carried to the venous
system beyond the liver.
a. Lipid materials are a high-energy item. They are stored as fat throughout the
body so that they will be available when needed for energy.
b. Body fat also serves as insulation in the subcutaneous tissues. It gives
buoyancy to the body in water.
c. Cholesterol is a very important substance in the body. It participates in the
functioning of the liver and in other activities of the body.
d. However, there are certain medical conditions in which physicians prescribe a
low-cholesterol and/or low-fat diet.