18.1 Basic laws of hemodynamics
18.2 Functional characteristics of the vascular
bed
18.3 Regulation of vascular tone
18.4 Blood pressure as an indicator of systemic hemodynamics
18.5 Regulation of systemic hemodynamics
18.6 Studying hemodynamics
18.7 Lymphatic system
Profile materials Control questions Situational tasks
18.1. Basic laws of hemodynamics
Circulation occurs as a result of pumping by the heart, which creates blood pressure at the beginning of the systemic circuit (left ventricle) and the pulmonary circuit (right ventricle). In accordance with the laws of hydrodynamics, the blood flows from a zone of higher pressure to a zone of lower pressure and is redistributed from a zone of higher resistance to a zone of lower vascular resistance. The formulas below describe general patterns in hemodynamics, but they only roughly reflect the real quantitative picture.
Volumetric blood flow (Q=ΔР/R) is directly proportional to the gradient of blood pressure (for the systemic circuit it is almost equal to aortic pressure). It is inversely proportional to the total systemic vascular resistance or SVR (application of Ohm?s law to hemodynamics, where the blood flow is analogous to current strength and the pressure is analogous to voltage; this analogy underlies the modeling of blood circulation us-
ing electrical circuits). SVR is the total resistance of all series and vessels connected in parallel in the systemic and pulmonary circuit. To calculate the value for the systemic or pulmonary circuit it is necessary to determine ΔР (mean arterial pressure in the aorta or pulmonary artery and subtract the pressure in the right or left atrium) and Q (cardiac output, l/min, it is the same in both ventricles). In young people SVR in the systemic circuit is about 1400 dyne×s×cm-5 (or ≈140 Pa×s×ml-1), in the pulmonary circuit it is 10 times less.