Due to the lack of oxygen, more and more red blood cells are formed. If there is an increased need for red blood cells, the formation in the bone marrow can be increased by eight to ten times the normal production. In this way, the organism succeeds naturally in compensating for this existing deficiency. Unlike humans, horses have an extraordinary ability: During resting periods, they store about 40% of their red blood cells as a performance reserve in the spleen. These blood cells are released again under stress.