Blood Capillaries

Capillaries are very small vessels with a diameter of a hair. They form dense networks between arteries and veins. Their walls are very thin and consist of only one layer of epithelial cells. It is in the capillaries that the exchange of oxygen and nutrients takes place between the blood and the cells: the arteries branch, their lumen narrows more and more, and finally they turn into arterial capillaries, in which red blood cells circulate one after another, ensuring the exchange of the oxygen they carry with carbon dioxide contained in tissues. At this point, arterial capillaries turn into venous ones, and then into veins with a large lumen, up to the vena cava. The pulse is formed when blood is pushed from the left ventricle into the aorta and spreads in the form of a wave through the arteries at a speed of 11 m/s, that is, 40 km/h. If all the blood vessels of the body were laid out in one row, then their length would be 96,000 km, that is, they could encircle the Earth 2.5 times.