The Human Circulatory System - Systemic and Pulmonary circulations - Pulmonary circulation, Systemic circulation, Portal circulation

The Circulatory System

The multicellular organisation in animal world has resulted in the origin and evolution of circulatary system in animals. This arrangement facilitates internal transport of various substances to all organs and organ systems. Among majority of multicellular animals this system remains as a closed type. It has blood running inside closed blood vessels, the blood being pumped by heart. In man, as in all mammals there is a double circulation of blood. The primary circulation through pumping action of heart, supplies blood to all regions of the body. The blood later returns to the heart . It is called the systemic circulation or body circulation. A similar circulation carries blood to lungs for oxygenation and returns it back to the heart. It is called the pulmonary circulation.

Systemic and Pulmonary circulations

The most important component of this system is the heart. It is a large, muscular, valved structure having four chambers. The chambers are the right atrium, left atrium, right ventricle and left ventricle. Each atrium opens into a corresponding ventricle. The right and left chambers are separated bysepta.

Systemic circulation :- The left atrium receives oxygenated blood from the lungs, through the pulmonary vein. When the atria contract, blood from the left atrium is forced into the left ventricle. Later by a contraction of the ventricle, the blood leaves the heart through the aorta. The aorta is the single systemic artery emerging from the heart. By successive branchings, the aorta gives rise to hundreds of arteries taking blood to all regions of the body. As the branchings happen, the arteries divide into numerous (4 x 10 6) arterioles. In the target organs they produce four times as many capillaries. A similar number of venules converge into each other forming veins of increasingly larger size. Finally, only two veins, the superior and inferior vena cavae return the blood to the right atrium. Thus the course of blood from left ventricles through the body organs and back to the atrium forms the systemic circulation.

Pulmonary circulation :- The venous blood from right atrium is conducted to the right ventricle. The ventricle expels the blood via the pulmonary trunk to the lungs. The oxygenated blood later returns by the pulmonary veins to the left atrium. This circulation from right ventricle to the left atrium via the lungs is termed the pulmonary circulation.

Portal circulation :- In the systemic circulation the venous blood passing through spleen, pancreas, stomach and interstine is not carried back directly to the heart. It passes through the hepatic portal vein to the liver. This vein begins as capilaries from the visceral organs and ends in the liver again as capillaries. These capillaries converge to form the hepatic vein which joins the inferior vena cava, conveying blood to right atrium. This route is the portal circulation.