English: blood cells of vertebrates (including humans) and the hemolymph of some invertebrates (sipunculida, in which the red blood cells are floating in the cavity of the extant, and some bivalves). They are saturated with oxygen in the lungs or gills and then spread it (the oxygen) to the body of the animal. The cytoplasm of erythrocytes is rich in hemoglobin — the red pigment containing iron atom, which can bind oxygen and gives red blood cells their red color. Human erythrocytes is a very small elastic cells are biconcave discoid shape with a diameter of 7 to 10 microns. The size and elasticity contribute to them when moving through the capillaries, their shape increases the surface area and facilitates gas exchange. They lack cell nucleus and most organelles, which increases the hemoglobin content. About 2.4 million new red blood cells formed in the bone marrow every second. They circulate in the blood for about 100-120 days and then absorbed by macrophages. About a quarter of all cells in the human body — red blood cells.