The movement of blood through the vessels is called blood circulation.
The circulatory system is made up of the heart and blood vessels.
Man, like all representatives of chordates, has a closed circulatory system. Blood moves in our body only through the blood vessels, which form two circles of blood circulation.
Blood vessels depart from the heart: arteries and veins.
The vessels from the heart are called arteries.
The vessels that enter the heart are called veins.
The largest artery from the left ventricle of the heart is the aorta.
A number of large arteries depart from the aorta: coronary (supplying blood to the heart muscle), sleepy (carrying blood to the brain), iliac (going to the lower body), subclavian (supplying blood to the upper limbs), etc.
Large arteries branch into smaller vessels – arteries, arterioles, which branch many times to the smallest vessels penetrating tissues – capillaries. In the tissues of various organs, the capillaries pass into thin venules. These vessels gradually merge into larger veins, the largest of which flow into the heart.
Arteries have three-layer dense, smooth and elastic walls. The outer layer of the walls consists of connective tissue, the middle layer is made up of smooth muscles, the inner layer is formed by one layer of cells and is called the endothelium. The structure of the walls allows the arteries to withstand the high pressure under which blood is expelled from the heart.
The walls of the capillaries are very thin: they consist of one layer of flat cells (through them there is an exchange of gases and substances between blood and tissues).
The walls of the veins consist of the same three layers as the arteries, the muscle layer is thinner. The large veins that carry blood from the lower body have pocket-like valves that prevent blood from flowing back.
Since the venous walls do not have the density and elasticity of the arteries (they are soft and thin), the contractions of the muscles surrounding the veins help blood to move. By contracting, the muscles squeeze the vessel and help push the blood towards the heart. The movement of blood in the opposite direction is impeded by pocket-shaped semilunar valves located inside the veins.
The speed of movement of blood in the veins as it approaches the heart gradually increases to 0.2 m / s. As a result, the same amount of blood flows to the heart through both vena cava per unit time as is thrown into the aorta.