In birds and mammals, blood pressures are different between the pulmonary and sy

In birds and mammals, the pulmonary and systemic circuits are completely separated. One advantage of this complete separation is that pressures can be different in each circuit. When blood flows to the lungs, the capillaries in the lungs must be very thin to permit gas exchange. However, if blood flows too forcefully through them, fluid will leak through the capillary walls. When this fluid accumulates, it increases the diffusion distance and reduces the efficiency of gas exchange. Therefore, a low-pressure pulmonary circuit is advantageous. The systemic circuit however, can be very long, especially in very large animals. High blood pressure is required to force blood all the way through it. Having two separate pressures allows the differing demands to be met.