The course of the circulation is as follows: the veins pour their blood, coming from the head, trunk, limbs and abdominal organs, into the right atrium of the HEART. This contracts and drives the blood into the right ventricle, which then forces the blood into the LUNGS by way of the pulmonary artery. Here it is contained in thin-walled capillaries, over which the air plays freely, and through which gases pass readily out and in. The blood gives oﬀ carbon dioxide (CO2) and takes up oxygen (see RESPIRATION), and passes on by the pulmonary veins to the left atrium of the heart. The left atrium expels it into the left ventricle, which forces it on into the aorta, by which it is distributed all over the body. Passing through capillaries in the various tissues, it enters venules, then veins, which ultimately unite into two great veins, the superior and the inferior vena cava, these emptying into the right atrium. This complete circle is accomplished by any particular drop of blood in about half a minute.
In one part of the body there is a further complication. The veins coming from the bowels, charged with food material and other products, split up, and their blood undergoes a second capillary circulation through the liver. Here it is relieved of some food material and puriﬁed, and then passes into the inferior vena cava, and so to the right atrium. This is known as the portal circulation.
The circle is maintained always in one direction by four valves, situated one at the outlet from each cavity of the heart.
The blood in the arteries going to the body generally is bright red, that in the veins dull red in colour, owing to the former being charged with oxygen and the latter with carbon dioxide (see RESPIRATION). For the same reason the blood in the pulmonary artery is dark, that in the pulmonary veins is bright. There is no direct communication between the right and left sides of the heart, the blood passing from the right ventricle to the left atrium through the lungs.
In the embryo, before birth, the course of circulation is somewhat diﬀerent, owing to the fact that no nourishment comes from the bowels nor air into the lungs. Accordingly, two large arteries pass out of the navel, and convey blood to be changed by contact with maternal blood (see PLACENTA), while a large vein brings this blood back again. There are also communications between the right and left atria, and between pulmonary artery and aorta. The latter is known as the ductus arteriosus. At birth all these extra vessels and connections close and rapidly shrivel up.