Question

what if humans have only single circulation?​

Answer 1

I assume you are thinking of the "dual" system represented by the left and right heart chambers (to which you should probably add the venous vales & muscles of the legs adding a third pump).

Once blood has been through a capillary bed, the pressure is extremely low and to move it at a significant rate you need to raise the pressure again. Without doing so, flow through the lungs will be extremely slow and we would need either to seriously slow our metabolic rate or be much smaller with a distributed respiratory system.

Answer 2

• Assume you're referring to the "dual" system, which consists of the left and right heart chambers (to which you should probably add the venous vales & muscles of the legs adding a third pump).
• Once blood has passed through a capillary bed, the pressure is extremely low, and you must increase the pressure to transport it at a meaningful rate.
• Without it, airflow through the lungs will be exceedingly sluggish, and we'll have to either drastically reduce our metabolic rate or become significantly smaller with a dispersed respiratory system.