Human body
Blood vessels carry blood throughout the body, which moves because of the beating of the heart. Venules and veins collect blood low in oxygen from tissues throughout the body. These collect in progressively larger veins until they reach the body's two largest veins, the superior and inferior vena cava, which drain blood into the right side of the heart. From here, the blood is pumped into the lungs where it receives oxygen and drains back into the left side of the heart. From here, it is pumped into the body's largest artery, the aorta, and then progressively smaller arteries and arterioles until it reaches tissue. Here blood passes from small arteries into capillaries, then small veins and the process begins again. Blood carries oxygen, waste products, and hormones from one place in the body to another. Blood is filtered at the kidneys and liver.
Mesenteric arteries
The term mesenteric artery is also used to describe smaller branches of these vessels which, particularly in smaller animals, provide a significant source of vascular resistance. These branches have a dense innervation by sympathetic nerves, allowing the brain to control their diameter and hence the resistance to blood flow to the gut. This is of particular importance when blood flow is required elsewhere, such as in exercise or shock, where a large proportion of the blood is required to supply the needs of skeletal muscle; by constricting the smaller mesenteric vessels, the resistance of blood flow to the gut increases, and hence blood flows more readily to other organs.