Understanding How The Nervous System Regulates Blood Pressure

Our blood pressure is regulated from minute to minute by nerves. At specific locations in the walls of the large arteries, special sensors "measure" blood

pressure by responding to the amount of stretch in the walls of the arteries. An important location of these sensors is the carotid sinus, which is in the arteries that run up the neck to supply the head with blood.

When blood pressure increases for any reason, these sensors send nerve signals to the blood pressure regulating center located in the lower portion of the brain. In response to the nerve signals, the blood pressure regulating center sends out nerve signals that slow the heart and dilate the arterioles. The lower output of blood by the heart and the lower peripheral resistance to blood flow both result in lowering the arterial blood pressure back toward normal. By rubbing the carotid sinus area on the side of your neck near your voice box, you can stimulate these receptors and cause a quick (but temporary) reduction of your blood pressure.

Another example of this reflex occurs when you suddenly sit or stand up after lying down. Gravity pulls the blood downward, lowering the blood pressure in the carotid sinus in your neck. If the carotid sinus reflex didn't act promptly, sending out nerve impulses (over sympathetic nerves) to increase heart output and constrict the arterioles, you would faint from the decreased flow of blood to your head.

The sympathetic nervous system is the portion of the autonomic (involuntary) nervous system that has as its main function the preparation of our bodies for emergency situations. The sympathetic (adrenergic) nervous system sends nerve signals to the blood pressure regulating center, telling it to raise the blood pressure, which the center accomplishes by sending signals over sympathetic nerves that go to the heart and blood vessels. This system becomes active when we are frightened, preparing us to run away or to fight by increasing our heart rate and reducing the blood flow to the stomach, intestines, and skin.

Some cases of primary hypertension appear to be associated with increased sympathetic nervous system activity. This could be partly the result of a decrease in the voltage across the surface membrane of the sympathetic nerve cells caused by accumulation of sodium and depletion of potassium inside the cell, which could be helped by dietary changes. The decreased membrane voltage of the sympathetic nerve cells would cause them to fire off nerve signals more often, thus raising blood pressure. Increased sympathetic nervous system activity can also result from the psychological stress caused by our reaction to unpleasant situations. In obese people, the stimulation of sympathetic nerves by elevated insulin levels may be a compensatory mechanism to limit weight gain.