Vessels and circulation
In humans, as in all mammals – a closed circulatory system, blood circulates in the body through the vessels.
Heart blood flowing through the arteries. The pressure in the arteries is relatively large, and they have a dense three-layer walls. Outer layer – of connective tissue, middle layer – smooth muscle, the inner layer is formed by a single layer of cells called the endothelium. The largest artery of the body – the aorta – begins in the left ventricle. With the aorta begins circulation. From the aorta leaves several major arteries: carotid supplying blood to the brain; subclavian carrying blood to the upper extremities; iliac feeding the lower part of the body, and so on. d. As a rule, large arteries are well protected, lying deep in the striated muscles. Moving away from the aorta and two coronary arteries that provide blood flow to the heart muscle. Major arteries branch off into smaller ones, and those, in turn, on the arterioles. Branching capillaries to arterioles move – finest vessels having walls of a single layer of cells. Within these walls there is an exchange of substances between blood and tissues. The diameter of the capillaries up to 5 microns, the length of the capillary is 0.5-1.0 mm, and its total length in the human body is approximately 100 000 km! The higher level requirements exchange in any tissue, the better the branched capillary network therein.
The arterial end of the capillaries of the dissolved substance in the blood is filtered in the surrounding tissue. The venous end of the capillary blood pressure drops and the osmotic pressure generated by the plasma proteins, causes the water with dissolved metabolic products enter from the surrounding tissue in venous capillaries separated. Of these capillary blood enters the venules which in turn pass into a vein. The pressure in the veins is much less than in the arteries. The walls of the veins have the same three layers as the arteries, but they muscle layer is much thinner. Large veins have internal valves ensure blood flow only toward the heart. From the upper part of the body, venous blood is drained into the superior vena cava and from the bottom – in the inferior vena cava. Hollow veins empty into the right atrium, where it ends circulation.
Pulmonary circulation begins in the right ventricle, from which the pulmonary artery. Since the vessel makes the blood from the heart, it is called artery, although it contains blood, oxygen-poor – venous. The pulmonary artery branches into the left and right pulmonary arteries, which the deoxygenated blood to the lungs, which is enriched with oxygen, turning into arterial, and pulmonary veins of the arterial blood enters the left atrium.
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