To survive an organism requires food, water and oxygen. These substances should be transported to all the parts of the body for its proper functioning. Similarly, the wastes need to be removed from the body. Hence all the living organisms require a circulatory and an excretory system which together form the transportation system.
Certain animals such as Hydra and Sponges do not possess a circulatory system. As these animals live in water, they get food and oxygen from the water as it enters their bodies. The waste materials such as the carbon dioxide is also removed by the water. Hence, they don't need a circulatory system.
The human circulatory system
The circulatory system of humans is a transport system which comprises of a network that circulates the food, water and oxygen to the various parts of the body.
The three main components that form the circulatory system are:
  1. Blood
  2. Blood vessels
  3. Heart