Plan
► Basic laws of hemodynamics.
► Vessel tone and mechanisms of its control.
► Modern concepts of mechanisms of control of arterial blood pressure.
Basic Laws of Hemodynamics
Basic laws of passage of fluids through tubes are described by a branch of physics called hydrodynamics. According to the laws of hydrodynamics, the flow of fluid through a tube depends on pressure difference between the inflow and the outflow ends of the tube, on the diameter of the tube and on resistance to the flowing fluid. The more the pressure difference, the higher the velocity of fluid flowing through the tube. The higher the resistance, the lower the velocity. To characterize the process of passage of fluid through a tube, a concept of volume flow rate, or volume flow, is used.
Volume flow rate is a volume of fluid passing through the cross section of a tube per unit time. Volume flow can be calculated from the equation of Poiseuille:
Q = (P1 - P2)/R,
where Q - is volume flow rate, P1 - inflow pressure; P2 - outflow pressure; R - resistance to the flow of fluid in the tube.
Flow of blood through vessels obeys the law of hydrodynamics with some corrections. Flow of blood through vessels is called hemodynamics. According
to the laws of hemodynamics, resistance to blood flowing through vessels depends on the length of vessels, on their diameter and viscosity of blood:
R = (8ηl)/πr4,
where R - resistance, η - viscosity of blood, L - length of vessels, r - vessel radius.
Viscosity of blood is determined by the amount of cell elements in it and by protein composition of plasma. Volume flow depends on the diameter of vessels. The highest volume flow is in the aorta, and the lowest in a capillary. However, volume flow of blood in all capillaries of the systemic circulation equals that in the aorta, which means that at any given moment equal amounts of blood are flowing through different regions of the vascular bed.