Physiolog | Clinical Physiology, Muscle Centered

Circulation and resistance

This model turns the circulation chapter's Poiseuille equation into a visual experiment. Start near baseline, move the radius slightly, and compare that change with viscosity or vessel length.

Read the Circulation chapter

Live model

Poiseuille's Law: Why Radius Matters

Change the vessel and the driving conditions. The radius has an outsized effect because flow changes with the fourth power of radius.

Vessel radius1.00 × baselineThe most powerful adjustable variablePressure gradient100 mmHgThe force driving blood through the vesselBlood viscosity1.00 × baselineResistance from the fluid itselfVessel length1.00 × baselineLonger vessels create more resistance

Values are normalized to a baseline vessel. This keeps the relationship visible without implying that a single vessel represents whole-body blood flow.

Relative flow

5.0 L/min

Relative resistance

1.00×

Radius effect

1.00×

Flow vs baseline

1.0×

Vessel cross-section

Relative radius, shown at scale

r = 1.00×

Resistance rises as radius narrowsFlow follows r⁴

Pressure to preserve 5 L/min

100 mmHg

Read the change

Radius is near baseline. Move it slightly and watch resistance change faster than the vessel drawing.

Poiseuille's Law: flow is proportional to pressure gradient and radius⁴, and inversely proportional to viscosity and vessel length.

Teaching model, not a clinical calculator. Real vascular networks include branching vessels, pulsatile flow, elastic walls, and active regulation that this idealized model does not represent.