The work studies the content and fatty acid composition of phospholipids as well as the absorption spectra of lipid extracts from red blood cells of poikilothermal and homoiothermal animals at different evolutionary levels. The objects of study include two poikilothermal species, the river lamprey (Lampetra fluviatilis) that uses oxygen dissolved in water, and the common frog (Rana temporaria) that consumes oxygen both from water and from air. A homoithermal animal is the white rat (Rattus rattus) that inhabits the terrestrial-aerial environment. The animals are studied in winter and spring. The phospholipid content in lamprey blood plasma is found to be twice higher than that in its erythrocytes. In the frog and the rat, the ratio is reverse. Determination of the fatty acid lipid composition of red blood cell phospholipids suggests that membranes in the lamprey are denser than in the frog. As for the fatty acids in the erythrocyte fraction of rat blood, they appear to be less diverse, with a double prevalence of saturated acids over unsaturated ones and devoid of long chain (C22) ω3 fatty acids. All of this results in a lower degree of unsaturation and a denser packing of fatty acids in the membrane structures of rat erythrocytes. The mechanism of reversible binding of O2 molecules to hemoglobin in erythrocytes is discussed. Presumably, the mechanism of interaction between molecules of O2 and molecules of water prevents the exchange interaction of electrons of the hemoglobin iron atoms with an oxygen molecule. This is confirmed by our obtained absorption spectra, which show that in the lipid extract almost totally devoid of water the heme isolated from erythrocytes is converted to hemin.