The Circulatory System

Why do multicellular organisms need a transport system?

Single-celled organisms can get all the substances they need by diffusion across their outer membrane. Multicellular organisms find it harder to provide cells with what they need as they are relatively bigger and have a low surface area to volume ratio. They are also usually more active. This means alot of cells are respiring quickly so they need a constance, rapid supply of glucose and oxygen. 

To make sure every cell has a good enough supply, multicellular organisms need a transport system. In mammals, this is the circulatory system which uses blood to carry glucose and oxygen around the body. It also carries hormaones, anitbodies and watse. 

Single and Double Circulatory Systems

Single

Blood passes through the heart once for each complete circuit of the body.

e.g. Fish: heart pumps blood to the gills to pick up oxygen and then through the rest of the body to deliver it in once single circuit. 

Double

Blood passes through the heart…