In large organisms, surface area to volume ratio is too small for diffusion alone to supply e.g oxygen, glucose and other molecules to cells requiring them.
What are common features of circulatory system?
Suitable medium: mammals transport medium is blood, water based so substances easily dissolve
Moving the medium: heart (pump) maintains pressure differences in blood
Control of flow: valves in veins prevent backflow
Closed system of vessels: circulatory system closed and branched to deliver substances all around body
What is a closed double circulatory system?
Heart has two pumps, one pumps blood to lungs to be oxygenated.
The other is thicker and stronger and pumps oxygenated blood around the body to supply vital organs and tissues
