Hemodynamics

Hemodynamics is the branch of physiology that studies the circulation of blood and the forces that govern it within the cardiovascular system. It focuses on quantities such as blood flow (volume per unit time), pressure, resistance, and the compliance of vessels. A central concept is that flow results from a pressure gradient and is opposed by vascular resistance, analogous to Ohm's law: flow equals the pressure difference divided by resistance. In narrow, long, laminar vessels, Poiseuille's law describes how flow increases with radius to the fourth power and decreases with viscosity and vessel length.

Key determinants of systemic hemodynamics include cardiac output and systemic vascular resistance, which together determine mean arterial pressure (MAP roughly equal to CO × SVR). Venous return and the heart's pumping ability set stroke volume. The autonomic nervous system, endocrine signals, and local autoregulatory mechanisms adjust heart rate, contractility, and vascular tone to match tissue demand. Vessels' compliance and the distribution of blood between arterial and venous compartments influence pulse pressure and filling pressures.

Clinical hemodynamics employs invasive and noninvasive methods to assess pressures, flows, and volumes, such as arterial lines, central venous pressure measurements, echocardiography, and Doppler flow studies. Abnormal hemodynamics underlie many conditions, including shock (various types), heart failure, hypertension, and sepsis. Fluid status, vascular tone, and cardiac function must be considered together, as perturbations in one compartment often affect the whole circulatory system and tissue perfusion.