Blood flow through vessels is determined by Poiseuille's law: flow = (pressure gradient) / resistance. Vascular resistance is proportional to blood viscosity and vessel length, but inversely proportional to the fourth power of vessel radius. This fourth-power relationship means that small changes in arteriolar diameter produce enormous changes in resistance and thus redistribute blood flow between tissues. Arteriolar smooth muscle contraction is continuously adjusted by sympathetic neural signals, metabolic factors (decreased O2, increased CO2 or H+), and endothelial-derived factors (nitric oxide), enabling dynamic redistribution of blood to active tissues.
Measure blood flow velocity and vessel diameter using Doppler ultrasound or video microscopy. Observe vasodilation in response to metabolic demands (exercise, hypoxia) or vasoconstriction with sympathetic stimulation. Calculate resistance from Poiseuille equation.
Vascular resistance is not uniformly distributed; arterioles (small diameter, thick smooth muscle) account for ~50% of total resistance and are the primary site of metabolic control, while capillaries contribute minimally to resistance despite their small size.
From your understanding of passive transport and the cardiovascular system, you know that substances move along gradients and that the heart pumps blood through a closed circuit of vessels. Vascular resistance and blood flow control explains the physics of how blood actually moves through that circuit and, crucially, how the body directs blood to where it is needed most at any given moment.
The fundamental relationship governing blood flow is an analogy to Ohm's law in electricity: Flow = Pressure gradient / Resistance. Just as current flows through a wire proportional to voltage and inversely proportional to resistance, blood flows through a vessel proportional to the pressure difference between its two ends and inversely proportional to the vessel's resistance. Poiseuille's law makes this more precise: resistance depends on blood viscosity (η), vessel length (L), and — most importantly — the fourth power of the vessel radius (r⁴). The radius⁴ relationship is the single most important concept in hemodynamics. If a vessel's radius doubles, its resistance drops to 1/16th and flow increases 16-fold. Conversely, even a modest 20% narrowing of radius nearly doubles resistance. This extreme sensitivity to radius means that small adjustments in vessel diameter produce enormous changes in blood flow.
The arterioles — small muscular vessels just upstream of capillary beds — are the body's primary flow-control valves. They have thick walls of smooth muscle relative to their small lumens, giving them a large range of adjustable diameters. Three control systems regulate arteriolar tone simultaneously. Local metabolic control is the most intuitive: when a tissue is metabolically active, it produces vasodilatory metabolites — CO₂, H⁺, K⁺, adenosine, and lactate — that relax nearby arteriolar smooth muscle, reducing local resistance and increasing blood flow to match the tissue's oxygen demand. This is why exercising muscle turns red and warm — local metabolites have dilated its arterioles, flooding it with blood. Neural control comes from sympathetic vasoconstrictor fibers that tonically constrict most arterioles via norepinephrine acting on alpha-adrenergic receptors; increased sympathetic activity (as during hemorrhage or the fight-or-flight response) constricts arterioles in the skin, gut, and kidneys, redirecting blood toward the heart and skeletal muscles. Endothelial control involves signals from the cells lining the vessel itself — most notably nitric oxide (NO), released in response to shear stress from flowing blood, which causes local vasodilation.
The interplay of these control systems enables the remarkable redistribution of cardiac output based on demand. At rest, the gut receives about 25% of cardiac output, the kidneys about 20%, and skeletal muscle about 20%. During vigorous exercise, skeletal muscle's share can rise to 80% or more — not because total cardiac output merely increases, but because arteriolar constriction in the gut and kidneys actively diverts flow toward the dilated vascular beds of working muscles. The total peripheral resistance across all these parallel vascular beds determines the mean arterial blood pressure (MAP = cardiac output × total peripheral resistance), which is why widespread arteriolar dilation (as in septic shock) causes a dangerous drop in blood pressure even if cardiac output is maintained. Every clinical intervention for blood pressure — from vasopressors in the ICU to antihypertensive medications — ultimately works by manipulating this relationship between cardiac output, arteriolar resistance, and the fourth-power physics of vessel radius.