blood pressure

Blood pressure, what it is and how it is measured

Blood pressure is the force with which blood is pushed through the vessels .

It depends on the amount of blood that the heart pushes when it pumps and the resistances that oppose its free flow

What is blood pressure

PHYSICS teaches that pressure is directly proportional to the force acting in a direction perpendicular to a surface and inversely proportional to the area of ​​the surface to which the force is applied (P = F / S). Consequently, the more the surface is small (needle of a pin, blade of a knife, etc.) and the more the pressure increases (with the same applied force).

We notice this physical law when, for example, we walk on fresh snow and we sink down. In this situation our body exerts a force F on the ground through a support surface S given by the size of the soles. When moving on skis the sinking is much less evident as the contact surface S increases.

The pressure can be expressed through different units of measurement (Pascal, Torr, Atmosfera, Bar, ata).

When it comes to arterial pressure, the reference scale is the millimeter of mercury (abbreviation mmHg)

The PHYSIOLOGY teaches that the heart is a very effective pump capable of lifting a ton at a height of ten meters in 24 hours. By contracting and relaxing, this precious organ sends blood to all body tissues. The work done by the heart is so remarkable that over the course of its life it pumps about 190 million liters of blood which would be enough to lift an entire aircraft carrier by three meters.

Each time this muscle contracts (systole) the blood is circulated at a considerable speed (about 50 cm / second). The walls of the aorta, the main arterial vessel that comes out of the heart, are stretched strongly by the passage of blood. Fortunately, these walls are not rigid but have the possibility of dilating and contracting in relation to the amount of blood that passes through them. This mechanism allows the blood pressure to be regulated effectively.

The maximum pressure therefore depends on the efficiency of the cardiac pump (quantity of blood expelled at each contraction) and on the elasticity of the walls of the arteries. Under normal conditions the maximum or systolic pressure is 120 mmHg. When the lumen of the arteries shrinks or decreases the elasticity of the walls, the blood encounters greater difficulties to flow and the maximum pressure increases beyond the normal values.

When the emptying of the heart ends the filling phase begins (diastole). In this period the blood flow in the arteries decreases as well as the pressure that reaches its minimum value (diastolic or minimal pressure) a moment before the beginning of the new systole.

The minimum arterial pressure therefore depends on the resistance that the blood meets in the peripheral tissues. The more the flow is hindered and the more slowly the pressure drops. In this situation the minimum value that is reached before the subsequent systole is higher than the normal value of 80mm Hg.

Arterial pressure = cardiac output x peripheral resistance.

Arterial pressure is therefore determined by three main factors:

  • the amount of blood that is released into the circulation during systole and its viscosity (hematocrit)
  • the force of contraction of the heart
  • the resistance offered by the vessels (arteries and veins) to the passage of the blood flow;

These three elements undergo external control mediated above all by hormonal and nervous stimuli. Our body is indeed able

to autonomously regulate cardiac pressure according to the metabolic needs of the various organs. Due to circadian rhythms, blood pressure varies during the day, reaching maximum values ​​during the early morning and late afternoon

So, for example, as we go up the stairs, the pressure increases both because the muscles and the respiratory system need more oxygen (increase in stroke volume and heart rate) and because muscle contraction tends to occlude the vessels, increasing peripheral resistance. On the contrary, while we sleep the pressure is lowered because the metabolic demands of the various organs are lower. Even a hot bath, thanks to the heat dilatation effect, is able to decrease the arterial pressure.

Blood pressure must remain within a range of predetermined values ​​to ensure oxygen and nutrients to all tissues. This range varies from 75 to 80 mmHg for the minimum pressure and from 115 to 120 mmHg for the maximum pressure.

Below these values, blood is not circulated effectively and peripheral tissues tend to receive less oxygen and nutrients. The sense of dizziness, blurred vision and fainting felt by those suffering from low blood pressure is precisely due to the reduced supply of oxygen to the brain cells. Even "healthy" people notice these effects when, for example, they suddenly rise from their lying position (orthostatic hypotension). In these cases there is a sudden drop in pressure due to the force of gravity which draws blood into the lower vessels while at the same time causing a temporary hyperflow of blood at the local level. In normal conditions, the vessels respond to this phenomenon by contracting and thus hindering the flow downwards; at the same time the pressure increase is favored by the acceleration of the heart rate.

When a person suffers from hypertension the vessel walls are forced to withstand strong stresses which, when they become particularly high, can cause them to break. This predisposes the individual to arteriosclerosis and to dangerous organ damage which generally involves the kidneys, heart, vessels, brain and in some cases even the eye. The heart, just to mention an example, is forced to contract against high resistance and can "give in" (heart attack) due to excessive effort.