Blood vessels require oxygen and nutrients, so larger ones have blood vessels in their walls.
The walls of arteries and veins have three layers. The inner layer is composed largely of endothelium, with a basement membrane that has elastic fibers; the middle layer is smooth muscle tissue; the outer layer is connective tissue (largely collagen fibers). Arteries (on left) have a thicker wall than veins because they have a larger middle layer than veins. Capillary walls (center) are one-cell-thick endothelium. Veins (on right) are larger in diameter than arteries, so that collectively veins have a larger holding capacity than arteries.
The inner layer of an artery wall is a simple squamous epithelium called endothelium with a connective tissue basement membrane with elastic fibers. The outer layer is fibrous connective tissue near the middle layer, but it becomes loose connective tissue at its periphery.
Capillaries have one-cell-thick walls composed only of endothelium with a basement membrane. Capillaries form vast networks with a total surface area of 6,000 square meters in humans.
A capillary bed forms a maze of capillary vessels that lies between an arteriole and a venule. When sphincter muscles are relaxed, the capillary bed is open, and blood flows through the capillaries. When sphincter muscles are contracted, blood flows through a shunt that carries blood directly from an arteriole to a venule. As blood passes through a capillary in the tissues, it gives up its oxygen (O 2 ). Therefore, blood goes from being O 2 -rich in the arteriole (red color) to being O 2 -poor in the vein (blue color).
The coronary arteries and cardiac veins pervade cardiac muscle. The coronary arteries bring oxygen and nutrients to cardiac cells, which derive no benefit from blood coursing through the heart.
The blue-colored vessels carry O 2 -poor blood, and the red-colored vessels carry O 2 -rich blood; the arrows indicate the flow of blood. Capillaries are present in all parts of the body, so no cell is located far from a capillary.
A realistic representation of the major blood vessels of the systemic circuit shows how the systemic arteries and veins are arranged in the body.
Although the blood pressure in the brachial artery is typically about 120/80, blood pressure actually varies throughout the body. Blood pressure is highest in the aorta and lowest in the venae cavae.
Blood pressure and blood velocity drop off in capillaries because capillaries have a greater cross-sectional area than arteries.
Osmotic pressure in the blood is created by the presence of salts and the plasma proteins.
At the arterial end of a capillary, the blood pressure is higher than the osmotic pressure; therefore, water tends to leave the bloodstream. In the midsection, oxygen and carbon dioxide follow their concentration gradients. At the venous end of a capillary, the osmotic pressure is higher than the blood pressure; therefore, water tends to enter the bloodstream.
Thromboembolism is a clot that has moved and is now stationary in a new blood vessel where it can cause damage.
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