1.3.6 Explain why many animals have a heart and circulation (mass transport to overcome limitations of diffusion in meeting the requirements of organisms).

in small organisms, simple diffusion is fast enough to supply the body as small organisms have a large surface area to volume ratio. however, big organisms need a mass transport system to move and exchange materials around the body as there’s not enough surface area to serve the needs of the large volume inside by diffusion (small surface area to volume ratio; diffusion distances are large so diffusion on its own is too slow).

in humans, the mass transport system is the circulatory system, it consists of the heart (two muscular pumps one sending blood to the lungs for removal of carbon dioxide and oxygenation and the other sending the blood round the body), valves (ensure that blood flows through the heart in one direction), and blood vessels (transport blood around the body).