Control Centers

4. Control Centers

Because the cardiovascular system is the major environmental control center in the body, it utilizes a battery of sensors and monitoring devices to trigger corrective action or relay information to other body systems. Thus monitors within the cardiovascular system measure blood pressure and send this data to the brain, which then determines necessary adjustments.

4.1 Nervous System

The primary regulator of blood pressure is the brain stem, where groups of specialized nerve cells control the activity of the heart (cardiac-activating and cardiac-inhibitory centers) and another nerve-cell group regulates the diameter of blood vessels (vasomotor center). Blood pressure is augmented when the sympathetic nervous system carries commands from the cardiac-activating center to increase the heart’s activity and from the vasomotor center to constrict arterioles and veins. Blood pressure is decreased when the vagus nerves convey commands from the cardiac-inhibitory center to the sinoatrial node, slowing heart activity.

When blood pressure rises, sensors in the aorta and carotid arteries send impulses to the brain which inhibit the cardiac-activating center and the vasomotor center and simultaneously stimulate the cardiac-inhibitory center.

Sensors in the heart and lungs monitor the volume of blood in the vascular system. Messages sent by these receptors lead to adjustments in the circulatory system via sympathetic nerves. Other sensors monitor the oxygen content of blood and lead to changes in the respiratory system. These various monitoring cells supply the brain with the essential data for adjusting the circulation so that it can respond to any stress.

4.2 Hormonal Control

The action of the nervous system on the circulation is augmented when necessary by hormones secreted into the bloodstream from endocrine glands. Among the many hormones that can affect the function of the heart and blood vessels, the most important are adrenal catecholamines and angiotensin II.

4.2.1 Adrenal Catecholamines

There is an adrenal gland located just above each kidney. These glands consist of a central group of cells, the medulla, surrounded by a layer of different cells, the cortex. The medulla produces two substances called catecholamines—norepinephrine and epinephrine. The norepinephrine secreted by the medulla increases the rate of the heartbeat and the strength of heart muscle contraction and at the same time causes constriction of the muscle cells in the blood vessel walls. The epinephrine causes relaxation of blood vessels in the skeletal muscles, which augments the blood flow to the muscles during exercise.

The cells of the adrenal medulla are controlled by the sympathetic nerves; each increase in the number of impulses from these nerves increases the secretion of the catecholamines.

4.2.2 Angiotensin II

Cells located in the walls of kidney arterioles produce a chemical substance, renin, which is converted into angiotensin II. This very active product increases the formation of aldosterone (one of the hormones produced by the adrenal cortex), which in turn causes greater reabsorption of sodium and water from the kidneys and a subsequent increase in blood volume. Angiotensin II also acts more directly to cause muscle cells in the walls of the arterioles to contract; the result of all this activity is an increase in arteriolar resistance.