Anaes · Applied cardiovascular & respiratory physiology

Microcirculation & control of blood flow

Also known as Microcirculation · Capillary exchange · Starling forces · Autoregulation · Local blood flow control · Hypoxic pulmonary vasoconstriction

The microcirculation — the arterioles, capillaries and venules — is where blood flow is actually distributed and where exchange with the tissues occurs. The framework rests on five exam-critical ideas: local blood flow is controlled chiefly at the arteriolar and precapillary sphincter level, by metabolic feedback (CO2, hydrogen ion, potassium, adenosine, lactate) that matches flow to demand, by myogenic tone, and by endothelial factors (nitric oxide, prostaglandins, endothelin); capillary exchange is governed by the Starling forces — the balance of hydrostatic and oncotic pressures across the capillary wall — with filtration at the arterial end and reabsorption at the venous end; oedema results from a disturbance of one of four factors (raised hydrostatic pressure, low plasma oncotic pressure, raised capillary permeability, lymphatic obstruction); the lung is the exception that proves metabolic control, constricting rather than dilating in response to hypoxia (hypoxic pulmonary vasoconstriction); and microcirculatory dysfunction — a leaky, leaky-flow state with shunting — explains why a normal macroscopic blood pressure does not guarantee tissue oxygenation in sepsis. Built on the microvascular-failure-in-septic-shock review (Popovich 2026), the carbon-dioxide-vasodilator review (Duse 2026), the transvascular-exchange/revised-Starling review (Alamilla-Sanchez 2023), the capillary-hydration study (Pstras 2022), the perioperative-renal-microcirculation review (Li 2026), and the pulmonary-vascular-tone study (Maier 2026).