Very small animals (like flatworms) don't require a special transport system because they can get the substances they need and excrete the substances don't by diffusion alone.
However, for larger animals this is not sufficient. Larger animals require a variety of different tissues to support them. This means that they have a bigger size, so substances take longer to diffuse to an from their internal tissues; also they have a smaller surface-area-to-volume ratio, so substances can't diffuse fast enough.
In addition, animals that are more active use substances like food and Oxygen more quickly, and also produce waste products faster, hence there is again an increased need for a special transport system.
Single and Double Circulatory Systems
The animal transport system consists of:
A fluid medium to transport substances (the Blood)
A pump to push the fluid around the body
Specialised exchange surfaces
Most transport systems also contain tubes (Blood Vessels) to hold the blood.
This is the Circulatory System, which is a very efficient transport system.
There are two types of circulatory system:
A Single Circulatory System is a simple loop in which blood flows: Heart > Gills > Body > Heart
A Double Circulatory System is a double loop in which blood flows: Heart > Lungs > Heart > Body > Heart
Fish have a single circulatory system, while mammals have a double circulatory system.
In a double circulatory system, the loop that goes to the lungs is called the Pulmonary Circuit, while the loop that goes to the body is called the Systemic Circuit.
The double circulatory system is advantageous for active animals since, while in the single circulatory system the blood pressure is limited by the delicate nature of the tiny capillaries in the gills, in the double circulatory system the blood pressure can be high in the systemic circuit while remaining low and safe in the pulmonary circuit. This allows animals with a double circulatory system to be more active, since blood can reach their respiring tissues faster due to the higher pressure.
Fish are not as active as other animals, so their single circulatory system is sufficient for their needs, while more active animals like mammals need a double circulatory system.