Flow, pressure and resistance: Poiseuille and turbulent flow !
- Concept
- Exam priority: medium
Blood flows from high to low pressure; narrow vessels resist flow enormously, and fast, disturbed flow becomes noisy and turbulent.
Updated 2026-11-01
Also called: Poiseuille's law, Reynolds number
See it in 3D: Ascending aorta · Abdominal aorta · Carotid arteries
What it is
Blood flow through a vessel equals the pressure difference divided by resistance. Resistance depends very strongly on radius (to the fourth power), and on vessel length and blood viscosity. Flow is normally smooth (laminar); at high velocity, at narrowings or with low viscosity it turns turbulent, which wastes energy and makes bruits and murmurs.
Key facts
- Viscosity rises with hematocrit: polycythemia (including high-altitude polycythemia in Bogotá residents, mild) increases resistance.
- Blood behaves as less viscous in small vessels (Fåhræus–Lindqvist effect) because red cells line up in the centre.
- In laminar flow, velocity is highest in the centre and zero at the wall; shear stress on the endothelium releases NO.
Equations
- Q
- flow
- ΔP
- pressure difference
- R
- resistance
- η
- viscosity
- L
- vessel length
- r
- radius
Exact only for steady laminar flow of a simple fluid in a rigid tube; a good guide for blood.
- ρ
- density
- v
- mean velocity
- D
- diameter
Turbulence tends to begin above ≈ 2000 in straight tubes, lower at branches.
- v
- velocity
- A
- cross-sectional area
Velocity rises where the area falls (stenosis).
Clinical notes
- A 50% reduction in diameter (75% in area) begins to limit flow at high demand; a stenosis beyond ≈ 70% limits it at rest.
- Carotid bruit and the murmur of aortic stenosis are turbulence at a narrowing; turbulent sites also favour atherosclerosis.
Test yourself
Think of the answer, then tap the question to see it.
An arteriole constricts so its radius falls by half. By how much does its resistance change?
It rises about 16-fold, because resistance is inversely proportional to the radius to the fourth power.
Why does adding a parallel vascular bed lower total resistance?
In parallel the conductances (1/R) add, so total resistance is less than that of any single bed; each organ also gets the same driving pressure.
Why can a patient with severe anemia have a systolic flow murmur without valve disease?
Low viscosity and high cardiac output raise velocity and Reynolds number, so flow becomes turbulent across normal valves.
Where in the circulation is the largest pressure drop, and why?
Across the arterioles: they have the highest resistance of any segment because of their small radius and muscular tone.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Systolic, diastolic, mean and pulse pressure
- Vascular compliance and capacitance
- Local and humoral control of blood flow
- Starling forces at the capillary
- Murmurs and the hemodynamics of valve disease
Related conditions
- Arterial hypertension
- Peripheral artery disease
- Aortic Valve Stenosis
- Carotid artery stenosis
- Atherosclerosis and dyslipidaemia
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 20.2 Blood Flow, Blood Pressure, and Resistance (OpenStax (Rice University), 2022); 20.1 Structure and Function of Blood Vessels (OpenStax (Rice University), 2022); Carotid Artery Stenosis (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 14). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.