Regulation of blood pressure
Arterial blood pressure exhibits a pulsatile character, with the maximum value of these variations called systolic (highest blood pressure value resulting from the systole (contraction/extension and ejection phase of the heart) of the heart) and the minimum called diastolic (lowest blood pressure value occurring during the diastole (relaxation and filling phase) of the heart) blood pressure. The parameter that is actually relevant to arterial blood pressure is mean arterial blood pressure (MAP). It is the product of cardiac output (CV) and total peripheral vascular resistance (TPW).
BP = HZV x TPW
Peripheral vascular resistance depends on the diameter of the arterioles and the viscosity of the blood.
Cardiac Output
Cardiac output (HRV) is expressed as cardiac output per minute (HMV) and is the volume of blood expelled from the heart within one minute. Thus, HMV is dependent on heart rate and stroke volume (SV).
HMV = heart beat volume x heart beats per minute. In healthy humans, the cardiac output per minute is 4.5-5 1/min.
Cardiac index
Another important parameter is the cardiac index (CI): the cardiac index is calculated from the cardiac output (HMV) and the body surface area. It is expressed as cardiac output in liters per m2 of body surface area. It can be measured using a PiCCO, for example.
In healthy individuals, the cardiac index is between 4.2 and 3.3 l/m2.
Since measuring mean arterial blood pressure is very time-consuming, only systolic and diastolic blood pressure are measured in medical practice.
Cardiac index plays an important role in monitoring hemodynamics (fluid mechanics of blood and the forces affecting it) and circulation data of patients in intensive care units.
Blood pressure regulation
Cardiac output and total peripheral vascular resistance, and thus arterial blood pressure, are subject to rapid and slow regulatory mechanisms.
Rapid changes in blood pressure are controlled by increased sympathetic activity, which causes a demand-driven increase in cardiac output with an increase in arterial blood pressure during physical exertion. The baroreceptor reflex (reactions triggered by the baroreceptors/pressure receptors to a change in blood pressure) triggered by an increase in blood pressure leads to a drop in blood pressure again within seconds.
Slow changes in blood pressure are mainly controlled by electrolyte and water balance, i.e. renal function and various hormonal systems (renin-angiotensin-aldosterone system (RAAS), kinin-kallikrein system, atrial natriuretic peptide, respectively).
The local regulation of blood pressure takes place with the help of messenger substances such as endothelin and nitric oxide (NO = endothelium derived relaxing factor) through constriction (“narrowing”) or dilation (“expansion”) of the vessels by the endothelial cells (“inner vessel cells”) in the vessel wall.
Of great clinical-practical importance is the day-night rhythm of blood pressure, which is controlled by sympathicovagal activity.
