Haemodynamics

Haemodynamics describes the physical factors governing blood flow. Blood flow (Q) is the volume passing a point per unit time (ml/min); total flow equals cardiac output (~5000 ml/min at rest). Flow is driven by a pressure gradient and opposed by resistance: Q = ΔP/R.

Blood normally flows in laminar fashion: concentric layers slide over each other in a parabolic velocity profile (fastest in the centre, slowest at the wall). A marginal plasma layer keeps red blood cells away from the endothelium, reducing friction. Shear stress — the friction between laminar layers — is normally 0.5-1.5 Pa. Pathologically elevated shear stress (from hypertension, atherosclerosis, or Marfan syndrome) predisposes to aneurysm formation, most commonly in the abdominal aorta.

Turbulent flow occurs when the Reynolds number (Re) exceeds ~2000. Re is proportional to velocity (v), vessel diameter (D), and blood density (ρ), and inversely proportional to viscosity (η): Re = ρvD/η. Turbulence generates audible sounds: murmurs in the heart, bruits in vessels. Turbulence is physiologically present in the left ventricle (mixing) and the aortic root at peak ejection, and is detected during BP measurement (Korotkoff sounds).

Resistance to flow follows Poiseuille's Law: R ∝ 1/r4. Radius has the dominant effect — halving the radius increases resistance 16-fold. Arterioles have the smallest radius and therefore the greatest resistance, causing the steepest pressure drop across the circulation.

Veins are highly distensible capacitance vessels. Venous pressure is determined by sympathetic tone (venoconstriction), blood volume, the respiratory pump, and skeletal muscle pumps. Arterial compliance (distensibility) affects pulse pressure: stiff arteries (reduced compliance) produce a higher systolic pressure for the same stroke volume, widening pulse pressure. Pulse pressure correlates clinically with stroke volume. Aortic elastic recoil during diastole (Windkessel) maintains diastolic pressure and smooths flow.

Bolus flow occurs in capillaries — erythrocytes are slightly larger than capillary diameter and must deform to pass in single file. Sickle cell RBCs are rigid and interrupt this bolus flow.