Transport in Animals
The need for a transport system
All living cells need an oxygen supply and nutrients to grow and survive. They also need to remove waste products from their systems e.g blood supply. Therefore the cells have a blood supply which can remove carbon dioxide and provide oxygen and nutrients such as glucose.
Small animals don’t need a separate transport system as all their cells are at comfortable diffusion distance from where the oxygen enters. Large animals however do require a transport system as the diffusion distance is too long and the diffusion alone is not enough to give all cells their requirements such as oxygen and glucose. The three factors that influence requiring a separate transport system are; level if metabolic activity, size and surface area to volume ratio.
Surface area to volume ratio
Small animals have a large surface area to volume ratio e.g the flatworm. This means that for each gram of body tissue there is a sufficient supply of oxygen and nutrients entering the body and there is sufficient surface where exchange can occur. However, larger animals have a smaller surface area to volume ratio and therefore for every gram of body tissue in their body there is an insufficient supply of nutrients and oxygen and is therefore an insufficient exchange surface. Therefore the larger animals require a separate transport system.
Cells in a larger animal are further away from the surface and therefore the pathway for diffusion is increased and therefore the diffusion distance is further. Outer layers of cells which could gain nutrients via diffusion would keep and use the supplies leaving none for the inner layers of cells
Level of metabolic activity
Releasing energy from food by aerobic respiration requires oxygen. Active animals need to have good supplies of nutrients and oxygen to stay active. Animals that have to keep themselves warm such as mammals need even more energy from food via aerobic respiration and therefore require a greater oxygen supply.
Single circulatory systems
Fish have a single circulatory system which means that blood flows via their gills and gains oxygen, the goes around the fish’s body and then goes back to the heart.
Double Circulatory systems
Mammals have a double circulatory system. Deoxygenated blood flows from the heart to the lungs to gain oxygen and then back to the heart. The blood then is pumped as oxygenated blood around the body in the arteries to capillaries and then to the veins.
Advantages of double circulation
-As fish do not need to keep warm their cells need less oxygen therefore the single system is sufficient, in mammals it is not sufficient as the cells need more energy
-After going to the lungs the heart can increase the pressure and send oxygenated blood around the body faster, in fish this option is not available.
-Single circulation cannot contain as high a pressure as it can damage the capillaries, in mammals this is not a problem.