c. Capillary Structure. Capillary walls have only one layer--the intima.
Capillary networks (beds) are the exchange areas for the cardiovascular system. This
includes the internal exchange areas between the blood and the individual cells of the
body. Since the capillary wall consists of flat single cells, substances can move readily
between the body cells and the blood.
10-20. SPECIAL SITUATIONS
This paragraph describes several special situations associated with the blood
a. Nutrient Versus Functional Blood Supplies. The lungs, liver, and heart
actually have two blood supplies. The functional blood supply provides blood to be
worked upon by the organ. The nutrient blood supply provides blood for the usual
exchange of materials between body cells and the blood.
b. Collateral Circulation. A collateral circulation is a special organization of
blood vessels around a major joint of other area of the body. Its purpose is to provide a
continuing supply of blood even if one of the vessels is damaged. Several blood
vessels are included so that there will be an alternate route when needed.
c. End Arteries. There are other areas of the body where a single artery is the
sole supply of blood. Such an artery is called an end artery. When an end artery is
damaged and can no longer supply blood to an area, the tissues of the areas will die.
End arteries are most common in the brain and the heart.
d. Portal Veins. A portal vein is a venous blood vessel that begins with
capillaries in one area and ends in capillaries of another area. The most important
portal vein in the human body is the hepatic portal vein. The hepatic portal vein extends
from the capillaries of the digestive system to the capillaries/sinusoids of the liver.
10-21. LOCATIONS OF BLOOD VESSELS TYPES
In the human body, blood vessels are located differently according to their types.
a. Arteries. If an artery is injured, the threat to life is greater than with other
types of blood vessels. For protection, arteries tend to be located deep within the
structures of the body. Only the very smallest of arteries, especially the cutaneous
arteries, come close to the surface of the body.
b. Veins. There are both deep veins and cutaneous veins. The deep veins
accompany the arteries side by side. The cutaneous veins are found in the
subcutaneous layer of the body. The cutaneous veins drain into the deep veins at
specific locations (especially the inguinal region and the axillary region).