Blood flow follows pressure gradients from high (arterial) to low (venous) pressure. Poiseuille's law states that flow is proportional to pressure difference and inversely proportional to resistance, which increases dramatically with decreasing vessel radius. Arterioles serve as primary resistance vessels, allowing the body to redirect blood flow between organs by controlling vascular tone through smooth muscle contraction.
Calculate vascular resistance using the relationship Flow = ΔP/R. Consider how halving arteriolar radius increases resistance 16-fold, demonstrating why arteriolar diameter is the critical control point.
From your study of blood vessel structure, you know that arteries have thick, muscular walls and veins have thinner, more compliant walls. From hemodynamics, you understand the basic relationship Flow = ΔP/R: flow through any tube equals the pressure difference divided by resistance. Vascular physiology applies these ideas to the living circulation, where the "tubes" can actively change their own resistance and where the body must continuously redistribute blood among organs with wildly different demands.
The key formula is Poiseuille's law: flow is proportional to the pressure gradient and to the fourth power of the vessel radius (F ∝ r⁴ × ΔP). The r⁴ relationship is the most important insight in the entire topic. It means that if an arteriole's radius halves — which happens readily when smooth muscle contracts — resistance increases 16-fold and flow through that vessel drops to 1/16 of its former value. This enormous sensitivity to small changes in radius is why arterioles are the primary resistance vessels and the body's main tool for controlling blood distribution. A small sympathetic signal or local metabolic cue that constricts arterioles feeding one organ can nearly shut off that organ's blood supply while leaving neighboring organs unaffected.
The systemic circulation is organized in parallel, not in series. Each organ receives branches off the aorta at nearly the same arterial pressure (~90 mmHg mean arterial pressure). This means the kidneys, gut, brain, and muscles all see similar input pressures, and each can independently control its own blood flow by adjusting local arteriolar tone. Compare this to a series circuit: if the organs were arranged in series, flow reduction to one organ would require reducing flow to all downstream organs. The parallel arrangement gives the body organ-level control. Total peripheral resistance is the sum of the reciprocals of each parallel branch's resistance — adding a new branch always *decreases* total resistance.
Veins are not passive reservoirs. At rest, about 65% of the body's blood volume sits in the venous system, which is highly compliant (stretches easily at low pressure). When sympathetic nerves fire during exercise or hemorrhage, veins constrict, reducing this unstressed volume and shifting blood toward the heart. Increased venous return stretches the ventricle, increasing the force of the next contraction (the Frank-Starling mechanism). This is why the venous side of the circulation is often called the capacitance side — it acts as an adjustable reservoir. Together, arteriolar resistance and venous capacitance give the cardiovascular system moment-to-moment control over both the distribution and the total delivery of blood flow.