# Blood Pressure: Definition, measures, mean arterial pressure and Physiological changes

## Blood Pressure

Blood pressure is the pressure of blood in the arteries. Also known as blood pressure because this pressure is the force exerted by blood against the walls of arteries, it stretches the artery wall (see section tension); strict sense, the "tension" resulting from 'pressure' and the 'elasticity of the wall.

The international unit for measuring pressure is the pascal (Pa). However, the use that blood pressure is often measured in inches of mercury (cmHg), sometimes in millimeters of mercury (mm Hg).

It is expressed by 2 measures:
The maximum pressure during the contraction of the heart (systole)
The minimum pressure at the time of "relaxation" of the heart (diastole).

If one sets the voltage as a single figure, without unity, there is then the mean arterial pressure (MAP) expressed in mmHg. It is calculated as follows:

MAP = (systolic pressure + 2 × diastolic pressure) / 3

Note: in everyday language (eg the doctor), the voltage is indicated by two numbers corresponding to the systolic pressure followed by diastolic pressure expressed in mm Hg; example "twelve / eight" is not 12.8 but pressure systolic 12 mm Hg and a diastolic pressure of 8 mm Hg, and is usually displayed on a meter with the numbers 120 and 80 mmHg.

### Physiological changes

Blood pressure is the result of complex interactions between different systems. For simplicity, we can compare the fluid mechanics with the electric circuit and Ohm's law: U = R × I:

• U corresponds to the difference in pressure between two places;
• I, electrical current can be compared to blood flow, it depends on the pump function of heart and the total quantity of blood in the body (called the blood volume):

cardiac pump function depends directly on the frequency of contraction, but also the strength of the latter;
blood volume is the result of a balance between water intake and physiological losses (urine, feces, sweat, breath);

• R, resistance, actually corresponds to the resistance of small vessels to blood flow:

if they shrink (vasoconstriction), the resistance rise;
instead, they expand (vasodilation), the resistance decrease.

Other elements physiological pulse pressure or differential pressure, Poiseuille's law.

The regulatory mechanisms like:

• Nervous vasodilation by the vagus nerve to slow the heart rate
• humoral (that is to say, by producing chemicals that are spread throughout the body and act as distance):
• Adrenaline and noradrenaline (produced by the adrenal medulla glands) leading to inter alia an acceleration of the heart.
• The renin-angiotensin-aldosterone system, causing vasoconstriction of the renal arteries, renin produced in the juxtaglomerular cells, allows the conversion of angiotensinogen (produced by the liver) to angiotensin.
• Anti-diuretic hormone (ADH or vasopressin peptide produced in the neurohypophysis).
• Factors tissue vasodilators.

This explains the great variability of blood pressure numbers from one minute to another in the same individual: basically, the effort and stress are increased blood pressure, lowers the rest.

Taking voltage is subject to many artifacts, it should ideally be taken lying down, the patient is at rest, we must not ignore the "white coat effect" (the patient's blood increases due to the nervousness induced by the measure). It must also verify the match between the size of the cuff and the arm: if the first is too small, we may have a false elevation of blood pressure data (effect "big arm").