Cardiovascular System Contradiction - April 23, 2010 by admin

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 contracĀ­tion 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.

Pumping Blood with Pressure - April 22, 2010 by admin

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 milĀ­limetres of mercury.