It contains a circulatory medium and a network of continuous tubes called vessels. In unicellular organisms such as Amoeba and Paramecium, the circulatory system is completely absent as in these organisms oxygen and other useful substances including food can easily diffuse to all parts of the cytoplasm from the environment and waste products such as carbon dioxide and ammonia can easily be eliminated from the body surface.
In Hydra and other coelenterates as well as in flat- worms the transport of oxygen and other substances to the cells and elimination of waste products is achieved by making the gastro vascular cavity a highly branched structure.
The food taken into such a cavity is circulated to all paits of the branching system by the movements of the body and the cells of the body can pick up required substances and remove unwanted substances.
These animals, thus, do not require any special sort of circulatory system. In higher animals, on the other hand, because of their complex body organization there is an imperative need of some sort of circulatory or transport system through which various substances may be transported to different parts of the body which are not in direct contact with the external environment.
William Harvey (1628) was the first who discovered the function of heart and the circulation of the blood.
He stated that the heart was a pumping organ provided with valves to maintain the flow of blood in one direction only, that blood was distributed to the organs by means of deep lying vessels which he called arteries and the blood was returned to the heart by more superficial vessels which he called veins.
He showed this by a simple experiment employing a bandage applied to the upper arm of one of his patients.
The bandage was tightened until the pulse at the wrist could not be felt. The patient’s arm after a lapse of few periods became cold.
On loosening the bandage a little to release the pressure on the deeper arteries, the patient felt a sensation of warmth returning to his arm while the veins below the bandage swelled.
Therefore, Harvey concluded that the blood was flowing down the arteries from the heart but could not return in the veins since the bandage was still compressing them.
Harvey did not know of the existence of capillaries as there was no microscope at that time.
The capillaries were discovered by Malpighi in 1661 after Harvey’s death. Later in 1732 Hales succeeded in to determine the blood pressure.