The circulatory system has two separate blood flows – one brings blood into the central pump, the heart, and the other drains from the heart. In the shorter of these two flows (which is called pulmonary circulation or small blood flow) “worn out” blood is pumped into the lungs in which it takes oxygen and gives carbon dioxide. After that, it returns to the heart from where the oxygenated blood is pumped through the body (this is called large bloodstream or systemic circulation) to supply all the tissues with nutrients and to pick up the waste material before returning to the heart where it is oxygenated again in the lung circulation.
Arteries, which drain blood from the heart, have thick, muscular walls that can withstand “amortizing” the maximum blood pressure generated by heartbeat. The main artery (aorta) has an internal diameter of 30 mm. It is branched into smaller arteries, then into arterioles, and finally into microscopic small capillaries of thin and perforated walls that allow an easy exchange of nutrients and oxygen with waste materials between blood and tissue. The capillaries gradually collapse and pass into the venula, and the venules into the elastic soft veins of the soft walls, and the veins, with the aid of the larynx, drain the blood, liberated oxygen, back into the heart.
Blood does not run into all parts of the body at constant speed. This speed varies according to the amount of blood that is needed at certain times for certain tissues. For example, the uterus of pregnant women requires more blood than the uterus of a woman who is not pregnant. When you run, blood turns into the leg muscles at the expense of the organs in the abdominal cavity (which requires more blood to digest after eating). When it is cold, it means that less blood flows through the vessels with cold skin (which is why skin turns blue) and more in deeper veins, thus saving heat. This is reversible when it is too hot – more blood flows through the blood vessels on the surface of the skin, which is the reason why your skin turns red.
Circulatory system is a very complex system that can be disrupted not only when central pump operation is disturbed, but also because of the problems in the blood vessels itself. Thus, for example, the artery wall may weaken – and in some cases can even cover the layers of which it is composed. The artery can also be hardened, so it can not “amortizate” ahigh blood pressure, and blood clots that block the arteries can occur, too.