Discover how resistance (R) in blood vessels changes with vessel radius (r), length (L), and blood viscosity (η) according to Poiseuille's Law.
* Red particles on the canvas represent blood cells (erythrocytes).
In the formula, the radius (r) is in the denominator to the 4th power. This means that a very small change in the radius will have a massive effect on the resistance (R).
For example; if the vessel radius is halved, the resistance increases 16 times!
E.g.: Dehydration (fluid loss) increases viscosity.
Scenario: Exercise. Tissue metabolism increases (O2 drops, CO2 and Lactate rise). These metabolites locally relax the vascular smooth muscles (Vasodilation). The radius widens, resistance drops, and more blood goes to the tissue.
Scenario: Stress/Blood Loss. The sympathetic nervous system activates, releasing Noradrenaline. It binds to receptors in vessels outside the target organ, constricting the vessel (Vasoconstriction). The radius narrows, resistance increases rapidly, and blood pressure is maintained.