Functions of Blood Vessel
Each vessel type has important functions in addition to being a conduit for blood.
The branching system of elastic and muscular arteries progressively reduces the pulsations in blood pressure and flow imposed by the intermittent ventricular contractions.
The smallest arteries and arterioles have a crucial role in regulating the amount of blood flowing to the tissues by dilating or constricting. This function is regulated by the sympathetic nervous system, and factors generated locally in tissues. These vessels are referred to as resistance arteries, because their constriction resists the flow of blood.
Capillaries and small venules are the exchange vessels. Through their walls, gases, fluids and molecules are transferred between blood and tissues. White blood cells can also pass through the venule walls to fight infection in the tissues.
Venules can constrict to offer resistance to the blood flow, and the ratio of arteriolar and venular resistance exerts an important influence on the movement of fluid between capillaries and tissues, thereby affecting blood volume.
The veins are thin walled and very distensible, and therefore contain about 70% of all blood in the cardiovascular system. The arteries contain just 17% of total blood volume. Veins and venules thus serve as volume reservoirs, which can shift blood from the peripheral circulation into the heart and arteries by constricting. In doing so, they can help to increase the cardiac output (volume of blood pumped by the heart per unit time), and they are also able to maintain the blood pressure and tissue perfusion in essential organs if haemorrhage (blood loss) occurs.