The mechanisms which control the blood pressure within normal limits and how drugs can be used to correct abnormalities of these mechanisms.

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 The mechanisms which control the blood pressure within normal limits and how drugs can be used to correct abnormalities of these mechanisms.

Blood pressure is a measure of how hard the blood presses against the artery walls. When blood pressure is recorded, it is recorded as two numbers. The high number provides the systolic pressure and the low number provides the diastolic pressure. The systolic pressure correspond to the pressure of the arteries when the ventricles contracts. The diastolic pressure corresponds to the pressures when the arteries are at rest (after the left ventricular contraction and while the heart chambers are being refilled with blood). The Blood pressure is measured using a sphygmomanometer. There is no such thing as average (or normal) blood pressure as blood pressure differs between individuals depending on many factors. Some of the factors are: 1) age 2) Ethnicity 3) sex 4) Family history etc.

There are a number of physiological mechanisms which help regulate the blood pressure within normal limits. These include Autonomic nervous system responses (Baroreceptors and chemoreceptors), Capillary shift mechanisms, vascular stress relaxation, Hormonal responses and kidney and fluid balance mechanisms.
Baroreceptors
(pressure sensitive nerve endings) found in the arch of the aorta and the carotid bodies detect a drop in blood pressure. The Baroreceptors send a signal to the cardiovascular centre in the brain (medulla oblongata) which causes an increase in sympathetic activity. This increase in the sympathetic activity causes an increase in heart rate and promotes vasoconstriction of both the arterial and venous side of the circulation. As the heart rate is increased the cardiac output (volume of blood ejected from the heart per minute) will also increase. Together with the vasoconstriction of the blood vessels, the blood pressure is lowered.
When there is an increase in blood pressure, the increase is also detected by the baroreceptors due to the stretch of the blood vessels. The baroreceptors send a signal to the cardiovascular centre in the brain (medulla oblongata) which causes an increase in parasympathetic activity. This will include reducing the heart rate and vasodilating the blood vessels. Blood pressure is then increased and restored to normal limits.

Chemoreceptors play an important role in maintaining blood pressure. Chemoreceptors are found in similar places as the baroreceptors but are distinct from them. They are stimulated by a fall in the oxygen concentration or a rise in the Carbon dioxide levels. The receptors will not be fully activated unless the systolic pressure has fallen to below 80mm Hg. Chemoreceptors follow the same pathway as the baroreceptors but excite the sympathetic centre promoting vasoconstriction.

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Capillary shift mechanisms aid in the maintenance of the blood pressure. Due to the permeability of the capillaries, when there is an increase in blood pressure, the high pressure forces fluid to move out of the capillaries into the interstitial space thus reducing the volume thus reducing the pressure. The opposite takes place when there is a decrease in blood pressure. Due to the low pressure, fluid diffuses into the capillaries increasing the volume thus the pressure. Vascular stress relaxation is another mechanism in which blood pressure is regulated. Blood vessels are able to stretch when there is an increase ...

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