A Heart was Beating… - April 24, 2010 by admin

Owing to the automatism of the heart of the vertebrate, it can continue working even when removed from the body. The latest cardiac drugs were first tested on a heart taken from a frog which, under proper experimental conditions, goes on beating for many hours.

It is a popular misconception that when death occurs the heart automatically stops beating. In reality, this is not always the case. The Russian physician Andreev succeeded in making the heart of a newborn baby beat again four days after its death.

Several centuries ago people did not even suspect that this was possible. The famous physician Andreas Vesalius, who treated the Emperor Charles V, was among the few scientists granted the right to dissect bodies. He was sentenced to death by the Holy Inquisition on a charge of dissecting the body of a woman who was still alive. It was only thanks to the kindness of Philip II, the heir to the throne, that this dreadful and unjust death penalty was commuted to a penitent pilgrimage to the holy places on Mount Sinai and in Jerusalem. Vesalius did, incidentally, perish during this pilgrimage.

This accusation against the extremely popular scientist and famous physician of that epoch was motivated by the fact that the cardiac muscle of the woman who had been undoubtedly dead continued to contract. The reason why her heart continued to function for many hours after death cannot be established. None of the many astonished spectators who witnessed this dramatic event had a shadow of a doubt that the woman was alive. As for Vesalius, he was sure that accident was due to his own negligence and thought that the sentence proclaimed was just.

Cardiac Muscle in Animals - April 24, 2010 by admin

Although even in the adult animals the fundamental modifications in the basic rate of the heart beat are brought about by the brain, the heart can dispense with these com­mands and set its rhythm independently. Figuratively speaking, our heart works on its own initiative, a peculiarity which we somehow do not appreciate. If the fibres of an embryonic cardiac muscle are grown in a tissue culture on a special nutrient medium, they will contract rhythmically in a vial too, without waiting for any orders. They just cannot live without contracting.

Nonetheless, work cannot be well co-ordinated without a headquarters. If every muscle fibre contracted of its own accord, the common contraction could take place only by pure chance. This is what really happens at the earliest stages of embryonic life. In the rat’s embryo individual sections of the heart contract quite independently until the headquarters is set up and starts to operate. In birds and mammals it is located in a special region of the heart known as sino-auricular node.

The cardiac muscle has no nerves, and commands are conducted over the muscle fibres at the rate of one metre per second. This rate is quite adequate for the auricles to contract normally. The ventricles of the heart, which are larger than the auricles and which require commands to be communi­cated more rapidly, have a system, known as Purkinje fibres, over which excitation spreads five or six times more quickly.

In the heart of every self-respecting animal there is only one headquarters known as the pacemaker. More pace­makers would certainly cause a mess. Strange things, however, are not uncommon. The ascidians and some tunicates have two pacemakers, one at each end of the pulsating vessel. In such animals the blood flow periodically changes its direction.

About Heart Cycle - April 21, 2010 by admin

Why is the heart able to work at such a high rate? First of all, it is not absolutely correct to think that the heart works without rest. The cardiac muscle quite often rests, but the periods of rest are very brief. A heart beat lasts for about-0.49 of a second and, if a man is resting, a 0.31 second interval follows each beat. The period of rest is actually longer since not all parts of the heart work simultaneously.

The heart cycle starts with the contraction of the auricles, whilst the ventricles rest, and the ventricles contract while the auricles relax. The auricles take about 0.11-0.14 of a second to contract and this is followed by a 0.66 second rest. In other words, every day they work for no more than 3.5-4 hours and rest for about 20 hours. The ventricles take somewhat longer to contract, about 0.27-0.35 second, and rest for 0.45-0.53 second. Consequently, every twenty-four hours the heart’s ventricles work for 8.5-10.5 hours and rest for 13.5-15.5 hours.

In little birds the heart also rests, but their hearts contract and rest more frequently. The heart of a willow tit contracts 1000 times per minute; a single contraction of the auricles lasts 0.014 second with an ensuing rest of 0.046 second. The ventricles contract for 0.024 and rest for 0.036 second. Thus, the auricles work for only 5 hours 40 minutes and rest for 18 hours 20 minutes, whilst the ventricles work for 9 hours 36 minutes and rest for 14 hours 24 minutes. This differs very little from man’s.

Nevertheless, man is quite able to considerably improve the way in which his heart works by prolonging the period of its rest. According to medical research, in a well-trained sportsman the heart, when at rest, contracts less frequently than the heart of other people, the frequency being as low as 40 and even 28 beats per minute.

To cope with such a tremendous task as is the lot of the heart, rest alone is not enough. The heart must also be well nourished and have a good supply of oxygen. This explains why the heart in higher animals has its own, very powerful circulation system.

Heart Beat Dependant on Body Weight - April 20, 2010 by admin

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 func­tioning 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 mil­lion 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.