Activity of system of blood circulation organs - the heart and vessels - provides continuous movement of blood in the body. As a result of such movements blood are its diverse transport functions, in particular the delivery to the tissues of oxygen and nutrients, and the removal of tissue substances formed as a result of metabolic processes.
At the movement in the body the blood is the hard way - the big and the small circle of blood circulation. A large circle starts from the left ventricle of the heart, includes the aorta, radiating artery with all their branches, arterioles, capillaries and veins throughout the body and ends with two hollow veins flowing into the right atrium. The pulmonary circulation starts from the right ventricle, includes the pulmonary artery with all its branches, pulmonary arterioles, capillaries and veins and ends pulmonary veins flowing into the left atrium (Fig. 9).
The movement of blood in the vessels due to the work of the heart. With the reduction of ventricular myocardium blood pressure is banished from the heart to the aorta and the pulmonary artery. Further movement of blood through the vessels and returning it to the heart, because the blood pressure in the large arteries much higher than the pressure of blood in the small arteries, and blood pressure in these last higher than in the capillaries, in which the failure of all the pressure is higher than in the veins and the Atria. Thus, in the course of blood flow available pressure difference, which causes the movement of blood in the circulatory system, blood flowing from those vessels, where the pressure is higher in the vessel where the pressure is lower. The gradual drop in pressure during blood flow from the arteries to the capillaries and veins) due to the fact that produced by the heart energy blood pressure takes to overcome resistance, the movement of fluid in the blood vessels that depend on friction of fluid particles on the walls of the vessel and each other.