Give an account of the regulation of arterial blood pressure in man.

Outline:

·        Short and long term regulation

·        Neural control through vasomotor center:

- afferent: baroreceptors

- heart: vagal control

- blood vessels: sympathetic control

·        Long-term control by kidneys: renin-angiotensin-aldosterone axis

Essay:

            The normal arterial blood pressure in man is between 90 and 100 mmHg. Too low a blood pressure decreases tissue perfusion while too high a pressure imposes excessive work load upon the heart. Therefore it is essential to regulate blood pressure within narrow limits, minimizing any major fluctuations in it. There are two systems of regulation: a short term system mediated by the autonomic nervous system which buffers any changes in blood pressure in daily life (due to exercise, changing of position of body, etc.); and a long term system of control by the kidneys which returns a previously hypotensive or hypertensive state back to normal (renal compensation in cardiac failure, etc).

            The autonomic nervous system exerts its control over regulation of arterial pressure through the sympathetic and parasympathetic fibers and reflex mechanisms of the baroreceptors and to a lesser extent, the chemoreceptors. The activities of the autonomic nervous system and reflex receptors are under direct control of the vasomotor center which may be regulated by the higher brain centers.

            The vasomotor center can be divided functionally into a vasoconstrictor, vasodilator and sensory area. The vasoconstrictor area is located in the anterolateral portions of the upper medulla. The neurons in this area are known as the C1 group of cells. Their fibers are distributed throughout the spinal cord, where they excite the vasoconstrictor neurons of the sympathetic nervous system.

            The vasodilator area is located in the anterolateral portions of the lower half of the medulla. The neurons in this area are known as the A1 group of cells. Their fibers project upward to the vasoconstrictor area and mediate inhibition of vasomotor discharge, thus causing vasodilation.

            The sensory area contains the A2 groups of cells. It is located in the tractus solitarius in the posterolateral portion of the medulla and lower pons. The neurons of this area receive afferent nerve signals from the vagus and glossopharyngeal nerves. The output signals from this sensory area then helps to control the activities of both the vasoconstrictor and vasodilator areas, thus providing reflex control of many circulatory functions.

            Under normal conditions, the vasoconstrictor area of the vasomotor center transmits signals continuously to the sympathetic nerves over the entire body. These impulses maintain a partial state of contraction in the blood vessels called vasomotor tone which plays an important role in maintaining the blood pressure and cardiac output. The medial portion of the vasomotor center transmit impulses continuously through the vagus nerve to the heart to decrease heart rate (vagal tone).

            The baroreceptors are stretch receptors in the walls of the heart and blood vessels. The carotid sinus and aortic arch receptors monitor the arterial circulation. Receptors are also located in the walls of the right and left atria, the pulmonary veins, in the wall of the left ventricle, and in the pulmonary circulation. These receptors are known collectively as the cardiopulmonary receptors. The baroreceptors are stimulated by distention of the structures in which they are located, and so they discharge at an increased rate whenthe pressure in these structures rises. When blood pressure drops, due to postural change from supine to standing position, the stretch of the arterial wall decrease. This is sensed by the baroreceptors which decrease their inhibitory discharge to the vasoconstrictor area of the vasomotor center. The resultant increase in sympathetic discharge raises blood pressure back to normal.

            Baroreceptors are responsive to mean pressures between 50-170 mmHg. The primary purpose of the arterial baroreceptor system is to reduce the daily variation in arterial pressure. The baroreceptors reset themselves to the prevailing pressure after 1-2 days. Therefore, they function primarily to stabilize acute variations in blood pressure from a day to day basis but are ineffective for long term regulation of blood pressure. The chemoreceptors are located in the carotid and aortic bodies. Strictly speaking, they are not directly involved in the regulation of arterial blood pressure as they respond primarily to levels of carbon dioxide, oxygen and hydrogen ions concentration in the blood rather than a change in blood pressure. Their predominant role is the control of respiration. They are stimulated only when arterial pressure falls below 80 mmHg.

            The kidneys is the chief regulator of long term blood pressure by regulating blood volume in response to its changes. The smooth muscle cells in the afferent and efferent arterioles are the site of synthesis, storage and release of renin. A decrease in effective circulating blood volume and perfusion pressure activates the sympathetic nerve fibers innervating the afferent and efferent arterioles as well as the afferent arteriole itself, which acts as a high-pressure baroreceptor, detecting any changes in the perfusion pressure to the kidneys. The end result is an increase in renin secretion. Renin functions solely as a proteolytic enzyme, cleaving angiotensinogen, which is produced by the liver, to yield a 10-amino acid peptide, angiotensin I. Angiotensin I is further cleaved to an 8-amino acid peptide, angiotensin II, by a converting enzyme found on the surface of vascular endothelial cells. Angiotensin II stimulates aldosterone secretion by the adrenal cortex, and arteriolar vasoconstriction, which increases blood pressure. It enhances NaCI reabsorption by the proximal tubule and stimulates ADH secretion and thirst. All these results in the conservation of sodium, and replenishment of water to raise the ECF volume and therefore the blood pressure back to normal. The kidneys play an important role in the restoration of cardiac output in the aftermath of cardiac failure.