It is often assumed that the heart determines how muchtravels to the other organs. To some degree this is true. In a heart failure, for instance, the heart muscle is unable to pump sufficient through the arteries. But under normal circumstances the heart pumps all the blood, which is received by the right ventricle, into the arteries. Depending on a particular organ’s need for oxygen, the arteries open wider or contract, and it is this mechanism that regulates the flow of blood. Thus, under normal circumstances, the organs themselves determine how much is pumped by the heart.
The need for blood in differentvaries according to their size, importance and the work they have to do. The brain is a very important organ and takes 15 per cent of the heart’s output. It requires a constant flow of blood because its energy and oxygen requirements vary very little, even during sleep. Blood flow to the brain is very rigorously controlled and the
Blood pressure depends on uWi blood pressure and levels of oxygen and carbon dioxide in the blood are regulated by feedback mechanisms which constantly adjust the various regulatory systems. Blood flow to the brain is maintained by this means in nearly all circumstances. The kidneys are also important. They receive 20 per cent of the heart’s total output. Their vital role is in excreting waste products and toxins from the body which would otherwise build up and poison other. Blood flow to the kidneys is also maintained, but selectively, so that if blood pressure does drop drastically excretion of urine decreases and toxic products build up in the blood.
The muscles receive varying amounts of blood according to how much work, or exercise, they are doing. The vessels supplying them are capable of rapid adjustments in response to localized demand for oxygen or build-up of waste products. The skin is interesting because changes of blood flow to it can actually be seen. When people blush or go pale in response to emotion, these visible differences are caused by alterations in the diameter of small skin vessels which cause changes in skin blood flow. Similarly, should the body be acutely short of blood because of heavy bleeding, the skin becomes very pale as blood is shunted away from it in favour of more important organs. Skin blood vessels also respond to changes in temperature because heat loss from the blood travelling close to the surface has an important role in body temperature regulation. Hence, people look flushed and pink when hot as the body sends blood to the skin surface in order to radiate heat away. They become pale when cold as the surface vessels restrict the amount of warm blood that might lose heat near the surface.
The liver receives a supply of oxygenated blood, for its own needs, from the hepatic artery. However, it also receives a second blood supply from the portal vein. This carries blood which comes from the stomach and intestines. It is deoxygenated, venous blood and contains all the important foodstuffs in their digested form. If these were simply transported around the body in this form they would be largely wasted; the supply of nutrients would be excessive after meals and at other times totally inadequate. To avoid this problem, the nutrient-rich blood passes to the liver – a complicated chemical factory in which the chemicals are stored until needed or reprocessed for storage or use elsewhere in the body. In this way allare assured of a ready supply via the blood of nutrients and energy either for regular use or whenever the demand arises.