How does the organism solve the problem of adjusting the pressure and distribution of arterial blood to the constantly changing demands of its organs? In regulating blood pressure nature resorts to a principle of modern technology—and most likely has done so for millions of years—known as "regulator with feedback."
The walls of the large arteries, particularly the aorta and the kidney arteries, are lined with pressure-sensitive cells, something like tiny gauges. These "pressure receptors," to give them their scientific name, are connected by nerve channels with the circulation center of the brain, to which they transmit information about the pressure relationships in the different sectors of the arterial system. The circulation collates these with other vital data and reports it receives, such as metabolic information from the blood and brain impulses. The circulation center is linked to the muscular arteries and heart by the "sympathetic" channels of the autonomic nervous system. If an increase in blood pressure is necessary, the center sends appropriate signals to the bulblike terminals of the sympathetic nerve fibers in the target organs. The nerve fibers act somewhat like specialized laboratories with enormous production and storage capacity, and immediately release quantities of noradrenalin. This is a hormone that constricts the peripheral arteries and stimulates heart activitiy. The hormone thus augments arterial resistance to the flow of blood and the heart per minute volume. The result is a rise in blood pressure. If the brain center data analysis indicates that the reported blood pressure exceeds the demand, the signals abate, that is, the stimulation of the sympathetic channels diminishes and with it the noradrenalin production. To sum up, the controlling mechanism of blood pressure works as follows.
The circulation center is stimulated—sympathetic activity increases—vessels constrict—blood pressure increases—pressure receptors are stimulated— information goes to the circulation center—reversal— sympathetic activity decreases—blood vessels dilate— blood pressure decreases.
This closes the circle. If the system is activated by a sudden decrease in blood pressure, the cycle is reversed.
The functional unit of sympathetic nerve fibers and their noradrenalin-producing terminals in the heart and arterial walls are called the sympathetic-adrenergic system. This will be referred to repeatedly in the subsequent chapters because it plays a crucial role in the development of high blood pressure as well as in its treatment.