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 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 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.

Facts About Blood Pressure - April 22, 2010 by admin

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.