vascular resistance
Summary
Vascular resistance is the opposition to blood flow in vessels, caused by viscous friction, and ΔP = QR. For idealized laminar flow, R = 8ηL/(πr⁴), so resistance varies as 1/r⁴: halving a vessel's radius raises its resistance 16-fold.
Detail
Meaning: Vascular resistance is the opposition a vessel or network offers to blood flow, caused by viscous friction. For steady laminar flow it rises with viscosity and length and falls steeply with radius. Resistances in series add; in parallel, reciprocals add, so the total is smaller than any single branch. Arterioles supply most of the systemic resistance because they are narrow and smooth muscle adjusts their radius.
Equation: R = 8ηL/(πr⁴) and ΔP = QR, where R is resistance (Pa·s/m³), η is viscosity (Pa·s), L is length (m), r is radius (m), ΔP is the pressure drop (Pa), and Q is volume flow rate (m³/s).
On the MCAT: Expect a ratio question. Halving the radius multiplies R by 16, so at constant ΔP the flow falls to 1/16. Another common setup is a narrowed artery: to keep Q constant, ΔP must rise. The circuit analogy also appears, with ΔP like voltage and Q like current.
Common confusion: Capillaries are the narrowest vessels, but so many lie in parallel that their combined resistance is lower than that of the arterioles. Their slow flow comes from the largest total cross-sectional area (v = Q/A). Poiseuille's law is also an idealization, since real blood is not perfectly Newtonian.
Sources
- Physics (Giancoli)
- Physics (Cutnell and Johnson)
- Fundamentals of Physics (Halliday, Resnick, and Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.