On the eighteenth day after conception the human embryo is but a tiny bundle of cells. It is at that time that the heart starts beating regularly and continues to do so without stopping until death. The heart is probably the only organ which does not shirk its work and keeps funcĀ­tioning at a good rate, even if it belongs to the most inveterate lazybones. In a tiny three-week-old human embryo, that has no real blood as yet, the heart beats once every second. Later on, when the child is born, the pulse becomes more rapid, approaching 140 beats a minute.

Fortunately, this is the peak and the pulse rate then gradually drops. In an adult the heart beats at a rate of some 76 times a minute while a person is resting, but may increase by as much as 150 per cent during hard work. This means that in a hundred-year lifetime a man’s heart beats about five thousand milĀ­lion times.

When one considers this figure, it is surprising that the heart never grows tired and, as long as it is healthy, copes easily with its task, literally without stopping for a second.

Man’s metabolism is far from perfect and considerably inferior to that of small warm-blooded animals. The thing is that the smaller the size of a body, the less the area in which it decreases. For this reason smaller organisms have to produce much more warmth per gram of body weight than larger ones. Their metabolism is more intensive and thus the heart has to beat more energetically than in man. Indeed, the smaller the animal, the quicker is its heart beat. For instance, the heart of a whale whose body weighs 150 tons beats seven times per minute, that of an elephant weighing three tons 46 times, that of a cat weighing 1.3 kilograms 240 times, while the heart of a coal tit weighing as little as eight grams beats 1200 times per minute.