All vertebrate animals possess a circulatory system to transport materials between the cells in the body

• This is a closed system consisting of a continuous network of vessels (arteries, capillaries and veins)
  

• The material is transported in the blood with a muscular heart pumping the fluid via rhythmical contractions  
  

Some animals have a single circulation system, while other animals possess a double circulation system

• Bony fish have a single circulation system, whereby blood flows through a two-chambered heart only once per cycle 
  

• Mammals have a double circulation system, whereby blood flows through a four-chambered heart twice per cycle
  

Double circulation systems are more efficient, as oxygenation at the lungs is uncoupled from deoxygenation at the tissues

• However double circulation systems require more energy and the blood has to flow at higher pressure