Transport in animals

Size

once an animal has several layers of cells, any oxygen or nutients difusing in from outside will be used up by outser layers of cells. The oxygen and nutients will not reach the cells deeper within the body. 

Surface-area-to-voloume ratio

Small animals have a large surface area compared to their volume. This is known as their surface-area-to-voloume ratio. This ratio is affected by an animals shape. A flatworm has a very thin, flat body, which gives it a large surface-area-to-volume ratio. But such a body form limits the overall size that the ainimal can reach. 

To allow animals to grow to a large size, they need a range of tissues and structural support to give the body strength. Their volume increases as their body gets thicker. But the surface area does not increase as much. So the surface-area-to-volume ratio of a large animal is relatively small. In larger animals, the surface area is not large enough to supply all the oxygen and nutients needed by the internal cells. 

Animals need enerygy from food so that they can move around. releasing energy from food by respiration requires oxygen. if an animal is very active, its cells need good supplies of nutrients and oxygen to supply the energy for movemnt. Those animals, such as mammals, that keep themselves war need even more energy.

An effective transport system will include:

• a fluid or medium to carry nutients and oxygen around the body- this is the blood
• a pump to create pressure that will pushthe fluid around the body- this is the heart
• exchange surfaces that enable oxygen and nutrients to enter the blood and to leave it again where they are needed.

 An efficient transport system will also include:

• tubes or vessels to carry blood
• two circuits- one to pick up oxygen and another to deliver oxygen to the tissues 

Fish have a single circulatory system. The blood flows from heart to the gills then on to the body before returning to the heart. 

Heart -> gills -> body -> heart

Mammals have a circulation that involves two seperate circuits. This is known as a double circulatory system. One circuit carries blood to the lungs to pick up oxygen . This is the pulmonary circulation. The other circuit carries the oxygen and nutriets around the body to the tissues. This is the systemic circulation.

The mammalian heart is adapted to form two pumps- one for each circulation. Blood flows through the heart twice for each circulation of the body.

Heart -> body -> heart -> lungs -> heart