Features of a transport system and types of circulatory system
As covered in 2.1 Special Surfaces for Exchange, the size and surface-area-to-volume ratio have a big impact on the need for a transport system. Single-celled organisms do not need a specialised transport system because materials can be transported via simple diffusion, whereas larger animals require a more elaborate transport system.
Another factor affecting the need for a specialised system is the level of activity. Animals need energy from food so that they can move around, and releasing this energy from food requires oxygen. If an animal is very active, it is clearly going to need a decent supply of nutrients and oxygen to supply the energy to cater for its movement. Animals which keep themselves warm (e.g. mammals) need even more energy.
An effective transport system will include:
a fluid or medium to carry nutrients and oxygen around the body (blood)
a pump to create pressure that will push the fluid around the body (heart)
exchange surfaces that enable oxygen and nutrients to enter the blood and leave it again where they are needed
More efficient transport systems also have:
tubes or vessels to carry the blood
two circuits – one to pick up oxygen and another to deliver it to the places it is needed in the body
Fish are organisms which have a single circulatory system. In fish, the blood flows from the heart to the gills and then onto the body, and back to the heart. This is shown in the left-hand flow chart.
Mammals have a circulation which uses two circuits; this is a double circulatory system. One carries blood to the lungs to pick up oxygen (this is the pulmonary circulation) and the other carries the oxygenated blood around the body to the tissues where it’s needed (systemic circulation).