92. Principles of Hemodynamics – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Hemodynamics is the study of the physical principles governing blood flow through the cardiovascular system. Blood flow depends mainly on pressure gradient, vessel diameter, vascular resistance, and blood viscosity. Among these, vessel radius is the most important determinant because resistance varies inversely with the fourth power of the radius.
• Blood flow is the volume of blood passing through a vessel per unit time, whereas blood velocity is the distance traveled per unit time. Velocity is directly proportional to flow and inversely proportional to the total cross-sectional area. Therefore, blood velocity is highest in the aorta, lowest in the capillaries, and increases again in veins. Slow capillary flow facilitates exchange of gases and nutrients.
• According to Bernoulli’s principle, an increase in blood velocity is associated with a decrease in lateral (static) pressure, while dynamic pressure increases. Thus, blood flowing through a narrowed vessel has increased velocity but reduced lateral pressure, decreasing tissue perfusion distal to the stenosis. This explains ischemia in arterial stenosis.
• Blood flow follows Ohm’s law: it is directly proportional to the pressure gradient and inversely proportional to vascular resistance. Arterioles are the major resistance vessels and therefore play the most important role in regulating tissue perfusion and arterial pressure.
• Blood normally exhibits laminar (streamline) flow, in which blood moves in parallel layers with a parabolic velocity profile. When velocity exceeds the critical velocity, flow becomes turbulent. Turbulence is favored by increased velocity, large vessel diameter, reduced viscosity (e.g., anemia), and irregular vessel surfaces. Reynolds number predicts turbulence: <2000 indicates laminar flow, whereas >3000 indicates turbulent flow. Turbulent flow produces murmurs, bruits, and Korotkoff sounds.
• Poiseuille’s law states that blood flow is directly proportional to the pressure gradient and the fourth power of vessel radius, and inversely proportional to blood viscosity and vessel length. Small changes in vessel radius therefore produce marked changes in blood flow and peripheral resistance.
• Peripheral resistance depends mainly on vessel radius and blood viscosity. Blood viscosity increases with polycythemia, increased plasma proteins, reduced red cell deformability, and low temperature, whereas it decreases in anemia and at higher temperatures. Plasma skimming occurs because red blood cells occupy the center of the vessel, allowing smaller branches to receive relatively plasma-rich blood and reducing capillary viscosity.
• Critical closing pressure is the intravascular pressure below which blood flow stops despite a positive pressure because vessels collapse under surrounding tissue pressure.
• Laplace’s law states that wall tension is proportional to pressure × radius (T = P × r). Larger vessels and dilated ventricles require greater wall tension, whereas capillaries rarely rupture because of their very small radius. Increased wall thickness reduces wall stress; therefore, ventricular hypertrophy compensates for pressure overload, while a dilated heart has increased wall tension, higher oxygen demand, and progresses more rapidly to heart failure.