Physiology of Blood Pressure

Blood pressure is a vital sign that is made up of two values; the systolic and the diastolic blood pressure. It mainly refers to the arterial blood pressure and is a representation of the pressure of blood exerted on the arterial walls while circulating through the body.

The average ranges of blood pressure are considered for systolic between 90-130 mmHg and for diastolic blood pressure between 60-85 mmHg. Measurement is obtained traditionally by means of a sphygmomanometer using the auscultatory and palpatory methods or obtained by using a digital blood pressure measurement device (which is of course, the more recent method). It is expressed using the unit “millimeter mercury”.

Systolic pressure is the representation of the force of the blood on the walls of the aorta and other blood vessels throughout the body during ventricular systole. Ventricular systole is the contraction of the ventricles (the lower, larger and stronger chambers of the heart) leading to ejection of blood into the aorta and then to the entire body.

Diastolic pressure is the representation of the force of blood on the walls of the aorta and blood vessel when the ventricular chambers relax. This phase of relaxation is to allow the heart to be refilled with blood returning from the body to the vena cava of the right side of the heart and then pumped through the lung to be re-oxygenated. This blood will then pass to the left side of the heart, pumped to the body and so on and so forth as the normal cardiac cycle continues.

The pulse pressure is the difference between the systolic and the diastolic blood pressure. It typically ranges between 40-50 mmHg.

The mean aortic pressure (MAP) is the average blood pressure measured over a complete cardiac cycle.

Maintenance of the blood pressure within a normal range is achieved through changes of cardiac output and peripheral resistance. Cardiac output is the product of the stroke volume of the left ventricle and the heart rate normally ranging between 60-90 beats per minute. The peripheral resistance is the resistance of the blood vessels to the blood flowing through them which changes by their widening, known as dilatation or their narrowing, known as constriction.

Short-term maintenance is also maintained by baroreceptors in the larger vessels of the body present in the thorax and the neck. Through the baroreceptor reflex, signals are sent to the heart to modify its activity according to fluctuations in the blood pressure.

A process known as the up-regulation of baroreceptors occurs in individuals suffering from hypertension. The normal values at which the blood pressure is maintained by baroreceptors are gradually increased in this process leading to maintenance of the blood pressure at higher values.

The renin-angiotensin system is a system that increases the blood pressure. It is activated by decreased blood flow to the kidneys which causes the release of angiotensin II and aldosterone. Angiotensin II is a powerful vasoconstrictor that raises blood pressure by narrowing blood vessels. Aldosterone is a steroid hormone which increases sodium and water retention in the body by modifying their excretion in the kidney. It therefore causes an increase in the blood pressure by increasing the blood volume.

This system aims at increasing the blood pressure in order to compensate loss of blood volume as in cases of hemorrhage.