Why is oxygenated blood and deoxygenated blood in poikilothermic animals mixed and not in homeothermic animals?

) Poikilothermic describes an organism with variable body temperature due to a lack of a thermoregulatory mechanism. Homeothermic describes an organism with the capacity to actively autoregulate its body temperature.

2) Homeotherms usually have much more complex and elaborate circulatory and respiratory systems than poikilotherms because of their higher metabolic rate. Their metabolic rate is higher because maintaining their own temperature is more energetically expensive despite the liberating nature of homeothermy. Poikilotherms also tend to be smaller in body size than their homeothermic counterparts, probably because it is more difficult to internally thermoregulate a larger body without mechanisms to do so.

3) The specific cell membrane composition of lipids and proteins is crucial for the survival of poikilothermic organisms. Because the specific types of lipids in the membrane are instrumental in determining the membrane's fluidity, the properties of these lipids can make or break a poikilotherm's survival. In the cold weather, membranes become less fluid, so more unsaturated lipids are necessary to maintain fluidity. In high heat conditions, however, defensive strategies involving heat shock proteins are more common.

4) Specifically, in mostly cold-weather poikilotherms, there is an increase in fatty acid unsaturation in order to increase membrane fluidity. Unsaturated fatty acid tails do not pack as tightly as saturated tails, so the membrane would remain more fluid despite cold conditions. Likewise, heat shock proteins function as molecular chaperones to prevent denaturation of protein due to high heat conditions.