The veins

Veins
Veins carry blood back to the heart from microvasculature all over the body. Blood entering veins is under very low pressure and moves toward the heart by contraction of smooth muscle fibers in the media and by external compressions from surrounding muscles and other organs. Valves project from the tunica intima to prevent backflow of blood. Most veins are small or medium veins , with diameters of 10
mm or less . Such veins are usually located close and parallel to corresponding muscular arteries.
The intima usually has a thin subendothelial layer, and the media consists of small bundles of smooth muscle cells intermixed with reticular fibers and a delicate network of elastic fibers.
The collagenous adventitial layer is well developed. The big venous trunks, paired with elastic arteries close to the heart, are the large veins Large veins have a well-developed intima.
The media: is relatively thin, with alternating layers of smooth muscle and connective tissue.
The adventitial layer: is thicker than the media in large veins and frequently contains longitudinal bundles of smooth muscle. Both the media and adventitia contain elastic fibers, but internal and external elastic laminae like those of arteries are not present.
Medium and large veins have valves consisting of paired folds of the intima projecting across the lumen . They are rich in elastic fibers and are lined on both sides by endothelium. The valves, which are especially numerous in veins of the legs, help keep the flow of venous blood directed toward the heart.
Capillary Beds
Capillaries permit and regulate metabolic exchange between blood and surrounding tissues. These smallest blood vessels always function in groups called capillary beds, whose size and overall shape conforms to that of the structure supplied . The richness of the capillary network is
related to the metabolic activity of the tissues. Tissues with high metabolic rates, such as the kidney, liver, and cardiac and skeletal muscle, have an abundant capillary network; the opposite is true of tissues with low metabolic rates, such as smooth muscle and dense connective tissue.
Capillary beds are supplied preferentially by one or more terminal arteriole branches called metarterioles, which are continuous with thoroughfare channels connected with the postcapillary venules .True capillaries branch from the metarterioles, which are encircled by scattered smooth muscle cells, and converge into the thoroughfare channels, which lack muscle. At the beginning of each true capillary, muscle fibers act as precapillary sphincters that contract or relax to
control the entry of blood. These sphincters contract and relax
cyclically, with 5 to 10 cycles per minute, causing blood to pass
through capillaries in a pulsatile manner. When the sphincters
are closed, blood flows directly from the metarterioles and
thoroughfare channels into postcapillary venules.
Capillaries are composed of a single layer of endothelial cells rolled up as a tube . The average diameter of capillaries varies from 4 to 10 µm, which allows transit