Haemodynamics

Haemodynamics is the study of blood flow and the forces that govern circulation. It integrates cardiac output, vascular resistance, and pressure profiles throughout the circulatory system. It encompasses systemic and regional regulation, microcirculation, and interactions with the lymphatic system.

Flow results from pressure differences; F = ΔP / R. Resistance depends largely on vessel radius (Poiseuille's law: R ∝ 1/r^4), as well as blood viscosity and vessel length. The arterial system stores energy as pulse wave due to heart contractions; veins act as capacitance vessels with high compliance.

Cardiac output (CO) is the product of heart rate and stroke volume. Systemic arterial pressure is often described by mean arterial pressure (MAP), approximated as MAP ≈ CO × systemic vascular resistance (SVR). Changes in CO, SVR, or both alter perfusion. Pulse pressure and central venous pressure reflect cardiac and venous conditions; venous return is influenced by intrathoracic pressure, muscle pump, and venous tone.

In microcirculation, capillary exchange is governed by Starling forces: capillary hydrostatic pressure, interstitial hydrostatic pressure, and oncotic pressures. Perfusion is regulated by autoregulation (myogenic, metabolic, endothelial signals) and neural/endocrine inputs to maintain tissue oxygen delivery.

Pathophysiology and measurement: Abnormal haemodynamics underlie shock, heart failure, hypertension, and venous insufficiency. Clinically, haemodynamics are assessed with imaging and invasive monitoring (e.g., arterial lines, central venous or pulmonary pressure measurements) and noninvasive methods such as Doppler ultrasound and echocardiography. Frank-Starling mechanism links ventricular filling to stroke volume.