The heart is a hollow muscular organ about the size of a fist. The inner part of the heart is divided into four chambers, two on the right and two on the left side. The upper chambers are separated from the lower chambers by a partition each side of which contains a valve that opens downward. These valves open when the heart muscles relax, and close when contraction sets in, thus regulating the flow of blood to the body and to the lungs. This process continues without a stop throughout life.
Each side of the heart is completely separate from the other, and handles its own particular part of the circulation. Thus, the heart may be regarded as a dual pump, taking care of two separate but interrelated circulations.
The right side of the heart pumps the poison-laden and oxygen-deficient blood back from all over the body into the lungs, where carbon dioxide is given off and new oxygen is taken on. The left side of the heart receives the purified and oxygen-enriched blood from the lungs and then pumps it into the general circulation for distribution to all parts of the body.
Have you ever watched a pump at work, its piston moving up and down, up and down, at regular, rhythmic intervals ? The heart has no piston but, like the pumping engine, keeps pumping steadily, contracting and relaxing its muscular tissue at regular, rhythmic intervals, forcing the flow of blood ever onward and forward.
Here is a step-by-step description of how the heart performs its work.
When the poison-laden and oxygen-depleted blood is brought back to the heart, it is first emptied into the upper right chamber. From there it is pumped into the lower right chamber, and then carried by two blood vessels (the pulmonic arteries) into the lungs.
After giving up its carbon dioxide and taking on a new supply of oxygen, it is then carried through an entirely different set of blood vessels, this time four in humber, two from each lung, into the left upper chamber of the heart. From there it is pumped into the lower left chamber and then pumped into the general circulation for distribution to all parts of the body.
We have seen that each side of the heart is completely separated from the other. Occasionally, however, we are confronted with a case where an opening exists between the two sides of the heart. This is a mechanical abnormality of congenital origin and is dangerous to life since it permits the blood to seep through in the wrong direction, causing the blood of the two circulations to intermingle. Surgery is now being used in many of these cases as a corrective measure.
The upper chambers of the heart fill with blood during their moment of relaxation and empty their content into the lower chambers during their moment of contraction, both sides performing their work simultaneously.
This work goes on continuously, each pumping beat of the heart, each lub-dub, taking about nine-tenths of a second or about 72 times per minute. It is less when the body is at rest and more when the heart has more work to do.
However, while the heart maintains its work continuously, it also has its periods of rest. It rests after each beat, and each interval of rest is about twice as long as the beat itself.
With each contraction, the heart pumps about three ounces of venous or oxygen-depleted blood into the lungs, and an equal amount of arterial or oxygen-enriched blood into the general circulation.
The total volume of blood in our body is about six to eight quarts. Since three ounces of blood pass through the heart with each beat and since each beat of the heart takes about nine-tenths of a second, the total volume of blood passes through the heart and completes its cycle throughout the body in about one to one and a half minutes.
It is important to bear in mind that while each side of the heart handles its own specific part of the circulation, both sides work in unison. Both upper chambers of the heart fill with blood during their momentary state of expansion and empty their contents into the lower chambers during their moment of contraction.
The lower chambers of the heart also work in teamlike fashion. They fill during their momentary state of expansion, and force the blood onward during their moment of contraction.
The rapidity or intensity with which the heart works is determined to a great extent by the demand placed upon it. When the heart has less work to do, it is more at ease and usually works at a slower pace. When a greater demand is made, it is forced to work at a faster pace and with 1 Where this type of surgery is necessary, the importance of a carefully planned mode of living must be recognised since in addition to correcting the mechanical abnormality, the heart, as well as the rest of the body, must be strengthened so that the patient can come through the operation successfully and be protected against possible future breakdowns. greater intensity. When an organ is called upon to do more work, it must be supplied with more oxygen, and the heart must pump more blood to supply it. When we run or exercise, more oxygen is needed and the heart is forced to pump harder. The same is true when we eat. To digest food, the digestive organs require more oxygen, and the heart is called upon to do more work. More oxygen is required during illness, and this again increases the work of the heart. The same holds true in excitement, tension, overwork, or any type of emotional stress.
When the heart is in a healthy condition it possesses a great deal of power and strength that provide not only for the regular demands of the body, but also a great deal of reserve power to meet unforeseen or unexpected needs.
Whenever a demand for an increase in circulation arises anywhere in the body, this demand is transmitted with lightning speed to the pumping mechanism of the heart and when the heart is in good health, it unfailingly responds to this need.
It should be apparent, however, that whenever the heart is called upon to spend its reserve powers recklessly, it will ultimately become worn out, and its efficiency and power will become impaired.
These reserve powers should be husbanded carefully so that they may be at our disposal during periods of actual stress, as in the case of disease, accident, or shock, and it is the height of folly to squander them carelessly, thereby jeopardising our health and our life.
That the normal heart possesses amazing functional and recuperative powers is well known. Scientific writers often overwhelm us with their description of the amount of work the heart can do and marvel at the precision with which it performs its functions. However, it is the rare scientist who points out that even this powerful organ can ultimately become weakened and worn out and that, to protect ourselves against this possibility, our reserve powers must not be needlessly drawn upon.
Since the heart too is composed of living tissue, it too must receive nutrition and oxygen if it is to keep well and be able to do its work. However, the nutrient elements and oxygen that the heart receives are not obtained by it from the blood that passes through it during its pumping operation, but from the blood supplied to it through a special set of arteries: the coronary arteries.
The coronary arteries are the first two arteries that branch off from the main arterial trunk as it emerges from the lower left chamber of the heart. They divide in many smaller branches and carry blood to all parts of the heart.