Because it prevents venous blood from mixing with arterial blood (which is rich in oxygen), in this way the circulation is more efficient.
In land vertebrates, the blood circulation is structured in two independent circuits: the pulmonary circulation, where oxygenation of the blood occurs and the elimination of the carbonic anhydride that it contains, returning back to the heart through its left atrium; and the systemic or major circulation, impelled from the left ventricle, transports the oxygenated blood and the nutrients that it assimilates as it passes through the digestive system, to the tissues of the animal, where it is charged again with anhydride carbonic and other waste substances, returning back to the heart, where it enters through the right atrium. These systems are independent and prevents venous blood (which is poor in oxygen) from mixing with arterial blood. These systems are independent and prevents venous blood (which is poor in oxygen) from mixing with arterial blood.
There are two types of circulation in mammals. The pulmonary circulation and the systemic circulation. As you might know, mammals use double circulation of blood.
The pulmonary circulation is the circulation of blood from the heart to the lungs.
The systemic circulation is the circulation of blood from the heart to other parts of the body.
I hope this helps.
This is because of the reduced blood pressure gradient of pulmonary circulation compare to systemic circulation.
The main function of pulmonary circulation is for exchange of gases( O2 ⇄ CO2 ) in the lungs. Therefore it is low pressure high flow circulation, for evenly exchange of theses gases.
In addition, lack of muscles in the pulmonary artery , which makes them very thin with high resistance compare to system arteries is another pressure reducing factor.
Furthermore, pulmonary arteries branched many times in the respiratory tracts to increase the surface area for perfusion.This further decreases the pressure gradient.
Gravity, effects of vasodilators, blood viscosity., are other factors which reduced the pulmonary circulation, therefore pressure gradient to pump blood into the whole body.(systemic circulation.)
Therefore for efficient distribution of gasses and nutrients for immediate needs of the cells and removal of waste, a higher pressure circulatory system(systemic Circulation) is needed in Land vertebrates, who need a high atmospheric pressure oxygen to be delivered to the cells promptly for their survival.
Vertebrates have different blood pressure in lungs than rest of the body.
Circulatory system provide the nutrients to the cells, collect the waste and also involved in transportation of gases. The circulatory system bring the transport liquid into close contact with cells to facilitate transportation. There are different types of circulatory systems present in different organism.
Land vertebrates poses double circulatory system. Pulmonary circuit circulate the blood between heart and lungs and systemic circulatory system circulates the blood between heart and other organs. It is because pulmonary circuits have lower blood pressure ( average 12 mmHg) than systemic circuits (80-100 mmHg). The blood pressure in pulmonary circulation is low but carry equal amount of deoxygenated blood to maintain cardiac output. The lower blood pressure in pulmonary circulation is because of lower vascular resistance to blood flow.