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.