A Heart was Beating…
April 24, 2010 by
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
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.
There is an essential contradiction in the activity of the cardiovascular system. On the one hand, to maintain an adequate supply of blood, high pressure is necessary. On the other hand, higher pressure spells hazards since it may disrupt the system at any time. If a major blood vessel is captured, death will follow quickly and unavoidably owing to a heavy loss of blood.
To maintain normal pressure, the system is provided with special controlling mechanisms known as baroreceptors. In mammals the most important receptors are located in the arch of the aorta, the sinuses of the carotid arteries transporting the blood to the brain, in the auricles and in the pain-sensitive nerve endings. Should any change in the pressure occur, the receptors will immediately send a signal to the medulla oblongata. The pressure is brought back to normal partly by the heart, but primarily by the blood vessels. The walls of the small vessels, the arterioles, have muscles and can easily constrict or dilate. When constricting, they create certain obstacles to the blood flow and cause higher pressure. Dilation, on the other hand, may reduce the pressure to a critical level and disrupt the circulation of the blood.
The heart beats continuously throughout life, one contraction following another, day and night, whether it is hot or cold. By the twenty-ninth hour something is already pulsating in the tiny ball of cells which makes up a chicken embryo, and the fluid is already being transported by some route. What makes the heart contract? From where does the order come for the chicken embryo to begin working? As yet there is no indication of the brain which governs the organism in the future.
High Blood Pressure
April 23, 2010 by
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.
One might think that a completely closed system would facilitate the work of the heart, but this is not so. A great deal of force is required to pump the blood through the capillaries and tiniest arterioles. As the arteries become more and more ramified, their total cross-section increases and finally becomes 800 times that of the aorta along which the blood flows from the heart, and this leads to an increase in resistance. The thing is that we have from 100 to 160 thousand million capillaries with a total length of 60 to 80 thousand kilometres. I. F. Cyon, a well-known Russian physiologist, calculated that the work performed by the heart in a man’s lifetime is equal to the effort which would be required to move a goods train to the top of the highest mountain in Europe. Mont Blanc, 4810 metres high.
Even in man in a resting state the heart pumps 6 litres of blood per minute, i. e. not less than 6 to 10 tons a day. During a lifetime our hearts pump 150 to 250 thousand tons of blood. But, in spite of all this, a man cannot boast of the work done by his heart.
Since it is difficult directly to compare the work done by the hearts of large and small animals, scientists usually calculate how much blood the heart pumps per minute per 100 grams of body weight. Even in a slow-moving snail the heart works under about the same strain as in man, while the hearts of most animals work more intensively. A dog’s heart, for instance, pumps about twice as much blood as a man’s, and a cat’s ten times that of a man’s heart.
While the heart is working, quite high pressure is maintained in the arteries. Even in such a small animal as the larva of a dragon-fly or in a frog, the pressure reaches 30 and even 38 millimetres of mercury. In most cases the pressure is even higher: in an octopus it is 60, in a rat 75, in a man 160-180 and in a horse it is as high as 200 millimetres of mercury.
Facts About Blood Pressure
April 22, 2010 by
Higher animals found it expedient to separate themselves not only from the external but also from the internal ocean by providing themselves with a closed circulatory system. However, this problem has as yet not been completely solved. The main channel of the internal river, i. e. the cardiovascular system in mammals, is a closed one, but it takes in many streamlets, lymphatic vessels, through which the fluids from the interstitial and intercellular spaces flow.
This means that the tissues and organs completely blocked themselves off from the waters of the internal ocean, but reserved the right to pour their waters into this mobile reservoir. Of course, the isolation of this internal ocean is only relative. In the arterial part of the capillaries, the walls of which are fairly thin, but the blood pressure is still high, a certain amount of liquid seeps into the intercellular spaces. This leakage would be still greater since the banks cannot withhold it sufficiently, if it were not the high oncotic pressure of the blood (caused by the proteins dissolved in it), which prevents the water from leaving the dissolved proteins.
In a resting state a small amount of water percolates into the tissues, but it all returns to the venous section of the capillary where the blood pressure is lower than the oncotic pressure of the plasma; the liquid starts to be actively attracted into the plasma by the proteins dissolved in it. The force which acts inside the venous section of the capillary and makes the liquid return to the blood stream is about twice that in the arterial section which forces the liquid into the interstitial spaces. This is why it is all returned.
However, during periods of work it is quite another matter. In this case the blood pressure in the arterial section of the capillary will be so high that its walls will be able to retain neither water nor proteins. In the venous section of the capillary the blood pressure will remain fairly high, while the oncotic pressure will drop owing to loss of proteins; the liquid will have neither the stimulus nor the opportunity to return to the blood stream. The only alternative left to it will be to enter the lymphatic system. Thus, in the body the lymphatic system acts in a way similar to the system of drains in towns which prevents the streets and squares from becoming flooded during heavy rainfall.
It’s not a secret that the smaller the aquarium, the more intensively it is used and the more rapid the currents in it have to be so that the same liquid can be used over and over again. It is small wonder that insects can afford the luxury of having very slow currents in their aquaria, taking 30-35 minutes to make one complete cycle. Man cannot afford this. The blood in our internal aquarium completes a cycle in as little as 23 seconds and performs over 3700 cycles per day. This is, however, not the maximum. In a dog a complete cycle takes 16 seconds, in a rabbit only 7.5 seconds, and in the smaller animals even less.
In vertebrates the matter is complicated since the aquarium itself is very large, but has little water in it. Not can it be filled up. The total length of all man’s blood vessels is about 100 thousand kilometres. Most of them are usually empty since 7-10 litres of blood are far from enough to till them and only the most hard-working organs are supplied intensively. For this reason heavy-duty functions cannot be performed by many systems simultaneously. After a good meal the digestive organs are the most energetic. They receive a considerable amount of blood, while the brain is not adequately supplied to function normally. Hence, we experience drowsiness.
To set the waters of the internal aquarium in motion, it was necessary to have devices very different from the cilia of sponges. Muscle pumps proved much more dependable. The earliest pumps were nothing more than a pulsating vessel, i. e. a very simple heart, which drove hemolymph into the smaller vessels and thence into the interstitial and intercellular spaces. Having watered them, the hemolymph returned to the pulsating vessel. Such an open system could not provide proper circulation, and this is why insects, the highest representatives of the invertebrates, developed pumps which not only force out, but also suck in. For this purpose their hearts are freely attached to special muscles, known as the pterygoid muscles, that stretch the heart, thus creating a negative pressure that sucks in the liquid passing through the tissues.
A pulsating vessel is a low-capacity unit, and lower animals usually have many pumping devices. In the earthworm the main pulsating vessel, that extends throughout its entire body, drives the blood from the rear to the front end. On its way, the blood flows into side vessels which themselves act as hearts pushing the blood into even finer arteries. All these numerous hearts function independently, co-ordinating, at best, their work with the partner in the segment. And this is the extent of the organization.
The system of transportation in animals developed gradually. When the particles of a live matter first joined together to form an independent unicellular organism and separated themselves from the ocean by means of an envelope, nature had to think of a way of organizing transportation within a unicellular body. A solution was soon found, and nature built the cell in the form of a microscopic ocean and provided it with its own currents. Thus, the simplest intracellular transportation system has been retained in multicellular animals and in man. The protoplasm of any cell in our body is mobile and protoplasmatic currents exist even in the nerve cells.
Multicellular animals had to develop a more complex system. The most primitive of them, for instance sponges, use the water where they live for this purpose. The ocean currents proved to be unreliable, so instead they use cilia to make the sea water flow through the ducts and pores of their body, thus supplying all parts with nutrients and oxygen.
Higher animals separated themselves completely from the ocean and provided themselves with their own ‘aquaria’ for transportation purposes. Nowadays the largest aquaria belong to the gastropod (univalve) mollusks, whose blood occupies 90 per cent of their body volume. This is evidently too excessive and the larvae of insects have an aquarium not exceeding 40 per cent of the weight of their body, whilst that of adult insects takes up 25 per cent. Birds and mammals have even smaller aquaria, only 7-10 per cent of their body weight, the tiniest reservoir being found in fishes where it is only 1.5-3 per cent of the body weight.
Supplying Heart with Blood
April 21, 2010 by
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
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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.