There are some special situations associated with the blood vascular system.

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 through the usual exchange of materials between body cells and the blood.

Collateral Circulation
A collateral circulation is a special organization of blood vessels around a major joint or 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.

End Arteries
There are 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.

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.