systemic vascular resistance

Systemic vascular resistance (SVR) is how hard it is to push blood through all the vessels of the body—the circulation’s overall ‘friction.’ Think of breathing through a wide straw versus a narrow one: the narrow straw means more resistance, and the heart has to work harder against it.

Most of this resistance lives in the small arteries and arterioles, whose muscular walls constrict or dilate to fine-tune it. Quantitatively, SVR links pressure and flow: it roughly equals the pressure drop across the systemic circuit (mean arterial pressure minus central venous pressure) divided by cardiac output. Because resistance is exquisitely sensitive to vessel radius, small changes in arteriolar tone have outsized effects.

SVR is one of the two main levers, alongside cardiac output, that set blood pressure, so it is central to understanding both hypertension (often a high-resistance state) and shock. In septic (distributive) shock, for instance, vessels dilate inappropriately and SVR plummets, so blood pressure can collapse even when the heart is pumping vigorously.