The walls of the arteries are thicker than those of the veins. Two sets of arteries leave the heart, one from each side. The artery that starts from the right side of the heart goes to the lungs and carries venous blood, which has a very poor supply of oxygen and is full of impurities. We call this the pulmonary artery. The artery that leaves the left side of the heart goes to all parts of the body but the lungs and carries arterial blood, which has much more oxygen and is much more free from impurities.
Capillaries
If we follow the blood as it circulates we will see how it reaches all parts of the body. The big artery that leaves the left side of the heart divides into smaller and smaller arteries until there are branches going to every part of the body. These branches keep dividing until they are so small that we call them arterioles, and these little arterioles again divide and become so small that we call them capillaries.
Changes in the blood
While the blood is passing through the arterioles and the capillaries, something is happening to it. The little cells with which these small vessels come in contact have been taking the oxygen and the nourishing material out of the blood. At the same time they have been putting something into the blood. In place of the oxygen they have been putting in carbon dioxid and in place of the nourishing material they have been putting in the worn-out materials from the cells. As the blood passes through certain parts of the body, such as the kidneys, the worn-out materials from the cells are taken out of the blood and sent out of the body. The carbon dioxid is left in the blood until it goes to the lungs.
Fig. 83. The white vessels represent the circulation of the arterial blood; the gray, the circulation of the venous blood.
Venous blood
The blood has now been followed to the capillaries, where oxygen and nourishing material have been taken out of it, and where something else has been added to it. As we follow a little capillary, we find that instead of dividing again, it joins others and gradually grows larger. As these blood vessels grow larger the walls do not become so thick as the walls of the arteries of the same size. These larger vessels formed by the union of capillaries are called veins. In other words, the veins are simply continuations of the arteries that have divided into extremely small branches and have now come together again. The blood which has been changed is now called venous blood. It has much less oxygen in it, but has received a great deal of carbon dioxid. The veins continue to come together, until finally they form a single large vein which empties into the upper half of the right side of the heart. From there the blood is driven into the lower half of the right side of the heart and thence to the pulmonary artery, which goes to the lungs. The blood is not changed in the heart, so what goes into the pulmonary artery is still venous blood.