hemodynamics

Hemodynamics is the study of how blood moves—the pressures, flows and resistances that govern the circulation. It is, in a sense, the ‘plumbing physics’ of the cardiovascular system: how forcefully the pump pushes, how the pipes resist, and how much fluid actually moves where.

At its heart is a simple relationship borrowed from fluid mechanics: flow equals the pressure difference divided by resistance (an analogue of Ohm’s law). Applied to the body, cardiac output (flow) equals the pressure gradient across the circulation divided by vascular resistance. From this small set of ideas—pressure, flow, resistance, plus volume and compliance—clinicians reason about why blood pressure is high or low, why an organ is under-perfused, or how a failing heart and stiff vessels interact.

Hemodynamics can be assessed non-invasively (blood pressure, echocardiography) or invasively, as during cardiac catheterization where pressures are measured directly inside the chambers and great vessels. The framework is powerful but a simplification: real blood is not a simple fluid, vessels are living and reactive, and flow is pulsatile, so the equations are guides to thinking rather than exact predictions.