Breathing is the vital meeting of fresh air and blood, this gaseous exchange suddenly takes place. Carbon dioxide rapidly leaves the blood and diffuses through the microscopically thin membrane separating the blood cells from the oxygen-rich air in the air sacs. Simultaneously, oxygen seeps from the air spaces across the membrane and into the blood cells. The hemoglobin rapidly changes from its dark, oxygen lacking shade to become a brighter red, reoxygenated color. As the blood courses through these vital minute capillaries, there is just adequate time for this interchange to take place. More blood is following along, ready to achieve the same result, and the heart is continually pumping away, forcing the cells through the tracks.
As soon as this interesting phenomenon has been completed, the oxygen-rich blood gradually accumulates into progressively larger channels. These merge into still larger ones, until the total finally re-emerges from the lung structure through a massive tube that conveys the new blood back to the left atrium of the heart.
When it reaches the left side of the heart, a similar mechanical procedure takes place. Blood rushes into the left atrium, and with the next contraction, it flows through the valve into the left ventricle. A moment later and the massive, extremely powerful left ventricle contracts, and the blood is pushed out through the aortic valve into a huge vessel almost the size of a hosepipe, called the aorta.
The aorta is the key supply pipe that conveys the blood from the heart, and delivers it to the body. This is accomplished via a series of large branches that come off the aorta at strategic intervals. Large vessels (called arteries) take a huge volume of the blood to each major area of the body. Special vessels convey a large amount to the head; another branch takes a supply to the upper limbs. Other branches convey fresh blood to the various abdominal organs, and lower down, the aorta branches, letting the lower limbs gain an adequate supply.
These large arteries divide, redivide and keep on dividing as blood is directed to each vital part of the system. As the vessels become smaller and smaller, they penetrate into all parts. Finally the arteries give way to small pipes called arterioles, and finally these subdivide still further into capillaries.
Here, the reverse situation of the one already described as taking place in the lungs occurs. The vital end point of the circulation has been reached.
As the capillaries are traversed by individual blood cells, another important gaseous interchange occurs. Oxygen passes from the oxygen-rich hemoglobin into the tissue spaces. At the same time, carbon dioxide seeps back from the tissues into the blood. The red cells lose their luster, and become dull-looking again.