Two circles of blood circulation

How is the circular motion of the blood? There are two circulation - big and small, also called pulmonary. From the left ventricle into the aorta receives oxygen-rich blood is the blood that flows through the branches of the arteries to all organs and tissues of the body, where ukrupnenie gradually veins is returned, giving the cells of your body oxygen, in the form of venous blood and enters the right atrium. This ends the systemic circulation. So again it be done, the blood must be oxygenated. So before you get into the left ventricle and the aorta, it goes into the lungs. Going from the right atrium into the right ventricle, it's pulmonary artery flows in the lungs, which was full of oxygen, returning on the pulmonary veins to the left half of the heart - in the left atrium. The track is from the right ventricle through the lungs to the left atrium is called the small circle of blood circulation. From the left atrium the blood goes into the ventricle and flows again in a large circle. Thus, making one complete revolution" in the body, blood twice returned to the heart - once in the right half (for "pushing" the lungs), and another time left (for regular transport of oxygen to the body cells). All the structure of the circulatory system, as we see, is subject primarily to the implementation of the first and most important function of the blood - breathing.
When the artery is suited to any authority, it branches out in the thick of it on smaller vessels, which eventually split into a dense network of the finest hair vessels or capillaries (from lat. capillaris - hair). Although the capillaries called capillary vessels, they are much thinner than a hair. So, the hair is thick on average from 1/10 up to 1/20 mm; meanwhile, the thickness of the capillary just 1/100 - 1/200 mm, or 5-10 microns. Erythrocytes go through the capillaries one at a time, in file - two of the erythrocyte be placed near can't.
Bronisaw their networks whole body, capillaries are gathering in small veins, which, together, form an increasingly large venous trunks. Thus, between an artery and a vein is always capillary network. Very interesting that in the times of Harvey in the hands of the physiologist has not yet been microscope, and therefore the assumption of a mass finest vessels that connect the arteries to the veins, remained a hypothesis for another fifty years. Only the opening of capillaries finally proved the correctness of the teachings of Harvey about the movement of blood.
Above it was stressed that between an artery and a vein is always capillary network. However, not everywhere the situation is so simple. Especially for vessels of the liver. The blood that came from an artery in the intestines, is going out of its capillary network in big, thick vein called the portal vein. And so instead, like all other veins to go to the Vena cava and next to the heart, this Vienna is included in the liver. The region of the liver, which includes vessels, as if the front gate, called the gate of the liver. Hence the name of Vienna. Once in the liver, Vienna again splits into richest capillary network, surrounding liver cells. Only from this network are formed hepatic vein through which blood flows to the heart. Thus, the portal vein is "sandwiched" between the two capillary networks. As a result, the blood, flowing from the intestines, necessarily passes through the liver, close contact with its cells. About the physiological value of this feature circulation in this area of the body we find in the conversation about the work of the digestive system.