13 Jan 2019



In higher animals, like the mammals, the blood is made to circulate round the body by the pumping action of the heart through the blood vessels like the arteries, veins and capillaries.
Mammals exhibit double circulation, blood passes through the heart twice everytime it makes on complete movement round the body.

In other words, there are two different circulations. These are pulmonary and systemic circulation.
1. Pulmonary circulation: during the pulmonary circulation, blood is taken from the heart to the lungs through the pulmonary artery and taken back to the heart through the pulmonary vein. The role of this circulation is to oxygenate the blood and remove carbon dioxide from the blood in the lungs.
2. Systemic circulation: this circulation takes the blood (oxygenated blood) from the heart to all parts of the body through the arteries and sends the blood back to the heart through the veins. The role of the systemic circulation is to transport products of digestion, oxygen, water, hormones and other substances to all parts of the body and collect excretory products like water, urea, carbon dioxide and mineral salts from the cells of the body to organs that will excrete them.

The Contribution of the Mammalian Blood Circulatory System
Exchange of gases: the gases involved in this process are oxygen and carbon dioxide. The blood system has many capillaries in close contact with the air sacs (alveoli) of the lungs. The thin wall of capillaries and air sacs enable oxygen to be exchanged for carbon dioxide through the process of diffusion. Also, in the tissues, carbon dioxide is exchanged with oxygen. The continuous circulation of blood ensures diffusion of oxygen into the blood and carbon dioxide out of the blood in the lungs.

(b) Absorption of digested food: the digested food materials are absorbed by the villi of the small intestine. The mesenteric arteries take the food from the small intestine to the liver through the hepatic portal vein. The liver then regulates the amount of food going into the blood. The blood then transports these food materials to capillaries of tissues and organs. With the aid of the lymph, the food substances diffuse into the cells.
(c) Removal of excretory products: the blood is able to collect excretory products like urea, nitrogenous materials, carbon dioxide, and water from the cells by simple diffusion. The waste products are then transported to the various organs like skin, liver, lungs, and kidney from where they are removed from the body.
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