1. The circulatory system is an organ system that permits blood and lymph circulation to transport nutrients, oxygen, carbon dioxide, hormones, blood cells, etc. to and from cells in the body.
2. The circulatory system in human and animals consists of blood, blood vessels and heart.
3. Blood is the medium of transport in the circulatory system in human and animal.
4. In SPM, you need to know
1. the composition of human blood
2. the function of blood and haemolymph in transport
3. the types of circulatory system
4. the circulatory system in humans
5. the regulation of blood pressure

Why the Transport System is Important for Multicellular Organism?

1. Multicellular organisms are bigger and hence have a small total surface area to volume (TSA/V) ratio. As a result, the rate of diffusion of substances is low. The larger the organism, the more difficult it is for the substances to diffuse in and out of the body cells.
2. Also, as the size of an organism increases, the amount of nutrients and oxygen gas needed also increases. So does the metabolic waste products. As such, diffusion alone is not sufficient to fulfill the needs of the body cells and to remove the waste products from the body.
3. Other than that, for a multicellular organism, the cells are located further in the body. They are not directly in contact with the environment.
4. Therefore, multicellular organisms need special transport systems to transport the nutrients to all the body cells and at the same time to remove the metabolic waste from the body cells.