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.

High Blood Pressure - April 23, 2010 by admin

Normally, the larger the animal, the higher is its blood pressure. This can clearly be seen in eels, sharks and other fish whose sizes vary considerably. The longer the eel or shark, the higher is its blood pressure. There are, however, many exceptions to this rule, one of which is a cock whose blood pressure is the same as that of a horse.

There is no doubt that the heart of a great blue whale weighing 600 to 700 kilograms, even if it does not function normally, will do much more work than the heart of a coal tit weighing about 5 thousand million times less, i. e. only 0.15 gram. For a correct estimation, a comparison is made between the work done by one gram of cardiac muscle. In this case man also has nothing to boast about. Each gram of our heart does work equal to 4000 gram-centimetres per minute, about the same as the heart of a snail. A frog’s heart works three times as hard, a rabbit’s five times as hard, whilst that of a white mouse works twelve times as hard.

Most of the earth-dwelling animals are horizontal. Their brain and heart, the two most important organs, are on the same level. This is very convenient since no additional effort is required on the part of the animal’s heart to supply the brain with blood. It is quite different for man whose brain is on a much higher level than-his heart. The same applies to a six-metre giraffe whose heart is situated 2 to 3 metres lower than his brain. All the creatures, following the same general plan (man, the cock, the giraffe), have high blood pressure.

The heart of typically horizontal animals is unable to supply the brain with blood when they take up an unnatural position. If a rabbit or a snake is placed in a vertical position, they will soon ‘faint’ because of brain anaemia. Nor are such animals very comfortable when placed with their head much lower than the heart since the supply of blood to the brain is confused owing to a disrupted outflow. However, the animal kingdom abounds with virtuosi acrobats. An obvious example are bats who do not care very much in what position their body is.

Supplying Heart with Blood - April 21, 2010 by admin

Lower animals have sought their own means of supplying the heart with blood. Nature proved to be thousands of millions of years ahead of Napoleon when he said that the way to a soldier’s heart is through his stomach. In creating lamellibranch (bivalve) mollusks nature decided to pierce their heart through.

However, it did not use Cupid’s arrow for the purpose but merely the end-gut. No one knows why an intestine should go through the heart ventricles of a mollusk. This is, no doubt, the simplest way to supply the blood with nutrients, and perhaps the supply of nutrients to the cardiac muscle itself is most improved.

The main function of the cardiovascular system is to transport all the necessary materials to all parts of the body. Some substances move in the blood by themselves, but others, mainly gases, travel on the back of the red blood corpuscles (erythrocytes). Every cubic millimetre of blood contains 4.5-5 million carriers, making a total of 35000000000000, the world’s largest caravan.

The size of the erythrocytes is negligible, only eight microns each, but if arranged in a chain, like camels in a caravan, they would encircle the Earth seven times around the equator. The red corpuscles of a whale, the largest living creature on the Earth, would form several caravans and each would stretch as far as the Sun.

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.