Edexcel A-Level Biology

## Why large animals need a transport system Every cell needs a supply of oxygen and nutrients and a way to get rid of carbon dioxide and other waste. In a very small organism these substances simply diffuse in and out across the body surface. Diffusion is quick over short distances but its time rises steeply with distance, so it cannot supply the middle of anything more than a fraction of a millimetre thick. A second problem is the ratio of surface area to volume. When an organism gets bigger its volume grows faster than its surface area, so the surface available for exchange is small compared with the number of cells that depend on it. A cube with sides of $\pu{1 cm}$ has a surface area of $\pu{6 cm2}$ and a volume of $\pu{1 cm3}$, a ratio of $6:1$. A cube with sides of $\pu{2 cm}$ has a surface area of $\pu{24 cm2}$ and a volume of $\pu{8 cm3}$, a ratio of $3:1$. The larger cube has half the surface for each unit of volume. Large, active animals therefore have specialised exchange surfaces, such as lungs, and a **mass transport** system. Mass transport is the bulk movement of substances over long distances, in a fluid carried by a pump. In mammals the fluid is blood, the pump is the heart and the pipes are blood vessels. Because the blood is moved by pressure and not by diffusion, substances arrive at every tissue quickly, and diffusion is needed only over the final short distance from a capillary to a cell. Mammals have a **double circulation**: blood passes through the…