Single celled organisms can get all the required substances by diffusion.
Multi-cellular organisms have a low surface area to volume ratio.
Most multi-cellular organisms cells are often respiring very quickly needing a constant supply of oxygen and carbohydrates.
To overcome this the organisms have a transport system, in mammals this is the Circulatory System which carries everything the body needs.
Single Circulatory Systems:
Blood only passes through the heart once. In fish the heart pumps the blood to the gills to pick up oxygen and dump Carbon Dioxide and then moves off to the rest of the body to deliver oxygen and collect Carbon dioxide.
Double Circulatory Systems:
Blood passes through the heart twice. In mammals the heart is divided down the middle. The right side…