This is of supreme importance. For, once stimulation occurs at a given point, this immediately spreads with lightning rapidity over the entire organ. In this way electrical impulses, which originate at the pacemaker, produce an instant contraction. Or, in short, a heartbeat occurs. This continues on with regular rhythmical, unbroken continuity at the gentle rate described at the beginning—about 70 times a minute in most people.
But the heart, being a pump, contains a series of four separate chambers, and each has an important function. Not only is blood pumped around one exclusive reticulation system, but two entirely separate circulations are involved.
The larger of the two is the general circulation. This means the distribution of blood to all parts of the body. But after this has occurred, there is a back-up collection system bringing all the fluid back to the heart once more. But at this juncture the same fluid is then recirculated through the shorter system, and is sent to the lungs where major changes occur in the components of the blood.
Doctors talk about the left and the right side of the heart. Each side has two chambers: an upper one called the atrium, and a lower one termed the ventricle. Each side is hooked up to its own circulatory system. A series of simple but very efficient valves allows blood to circulate freely between the vessels and chambers of the individual sides.
This is how the entire system operates. Blood is delivered to the right atrium via a major vessel called a vein (the superior vela cava). This consists of the accumulated volume of blood that has been collected from all parts of the body at that moment, and it pours into the atrium.
At the succeeding heartbeat the heart contracts, and at this moment the blood in the right atrium suddenly pours into the right ventricle as the connecting valve is released and opens. A moment later as the heart relaxes between beats, the valve closes, and simultaneously the atrium fills with the next lot of blood being delivered to it in a similar manner.
With the next contraction, as the right ventricle squeezes down, its outlet valve is the artificial heart, unsuccessful as a long-term measure, is used in the interim for cardiac patients awaiting transplant surgery, suddenly released, and the blood surges out into a large vessel that conveys the blood to the lungs.
Here the blood is propelled through an intricate network of large then smaller and progressively smaller vessels until it reaches microscopic capillaries. When it reaches this level, the individual blood cells come into close contact with the air that has been inhaled into the lungs.
Very quickly the blood cells undergo an amazing change. The hemoglobin, the red material in the red blood cells, has an amazing affinity for oxygen. At this stage, most of the oxygen has been already taken up by the cells at far distant areas, and very little is left in the blood cells.
For this reason the hemoglobin is reduced to a dark color, and entirely lacks the normal, bright-red luster characterizing fresh, newly oxygenated blood. But apart from being deficient in life-sustaining oxygen, the blood contains a large amount of carbon dioxide, the waste produced by the body cellular structures in carrying out their normal working duties. This material is totally useless to the system, and there is an immediate need to get rid of it.