The very general term “Circulatory System” is used to express the travelling of fluids around the body, surprisingly undefined until Harvey pointed out in 1628 that what goes out must have a way of getting back in. However, there are a number of discrete elements in the overall plan, just as there may be in an urban system of transport, within which there are several discrete routes.
In the pregnant female, there is a separate circulation associated with the placenta and fetus.
The thick walled aorta branches to give off named arteries to the limbs and internal structures of the body. In turn these branch into smaller, unnamed, vessels designated as arterioles, and from them is developed a branching network of fine vessels, the capillaries. Although it is the purpose of capillaries to be slightly permeable, so fluids, electrolytes and metabolic substances can pass from the capillaries to the tissue of the body, they do form a continuing networks and do not come to a terminal.
The capillaries as they collect the tissue fluid, and as the blood becomes relatively de-oxygenated, begin to join together and at some point are designated venules and then veins as they become larger. For the smaller named arteries there is a vein each side, two veins to each artery, but as they become larger there is one main vein for the lower body and one for the upper. The driving force of the heart is expended by the time the blood is in the capillaries, return depends on gravity and muscular action, With the legs hanging down, as when seated in an airplane, gravity works against return. The veins are equipped with valves, like a lock system in a canal, so when muscular action squeezes the vein the blood can flow only in one direction. Some persons are deficient in valves, the veins engorge and become varicose.
The right side of the heart receives the de-oxygenated blood, and passes it through the lungs. The lung tissues are supplied with vessels derived from the aorta, so there are two systems, one nourishing the tissues of the lung, the other a fine capillary network over the air cells in the lung (alveoli) where the respiratory exchange of gases takes place. From the lungs the oxygenated blood passes to the left side of tie heart.
Although the kidneys are the source of numerous vital hormones, their most obvious function is akin to the municipal sewerage works. A large volume of blood passes through them, and the extensive network of little knots of capillaries known as glomeruli. Fluid passes through the walls of these capillaries, together with electrolytes and metabolites, and is selectively reabsorbed and taken back into the capillaries according to the body’s needs. The excess water, products of metabolism and generally undesirable substances, are excreted as urine.
Veins from the intestine, bearing the nutritious elements absorbed from the gut, coalesce and form a large thin walled portal vein that branches to left and right lobes of the liver. The liver, like the lung, has two circulatory stems, one feeds the tissue, just like any other part of the body, and the other is related to its function as the factory of the body. The two venous systems have small points where they meet, around the umbilicus and at the base of the esophagus. In cirrhosis when the liver scars up, the pressure on the thin walled portal system increases, veins may engorge around the umbilicus forming the caput medusae, like the snake head of the goddess, or engorge and rupture, commonly terminally, in the esophagus.