Blood flow, pressure, and resistance
A blood flow, pressure, and resistance calculator applies the core hemodynamic relationship that flow depends on the pressure difference driving blood forward and the resistance opposing that movement. The main quantity may be blood flow, pressure difference, or resistance, depending on which variable is selected as the unknown.
This relationship is the circulatory analog of Ohm’s law. It is useful for understanding why constricted vessels reduce flow, why dilated vessels increase flow, and why a larger pressure gradient is needed when resistance is high.
Core definitions and formulas
Blood flow is commonly written as Q, pressure difference as ΔP, and resistance as R. The central equation is:
\[
\begin{aligned}
Q &= \frac{\Delta P}{R}
\end{aligned}
\]
This equation can be rearranged depending on what must be solved:
\[
\begin{aligned}
\Delta P &= Q \cdot R
\end{aligned}
\]
\[
\begin{aligned}
R &= \frac{\Delta P}{Q}
\end{aligned}
\]
In this calculator, pressure difference is typically entered in mmHg, resistance in mmHg·min/L, and flow in L/min. A larger pressure difference increases flow when resistance is fixed, while a larger resistance decreases flow when pressure difference is fixed.
How to interpret results
If flow rises while resistance stays constant, the pressure difference must also rise. If resistance rises while pressure difference stays constant, flow falls. This makes vessel constriction a simple way to model harder flow, while vessel dilation models easier flow.
The comparison mode is especially useful because it shows how one vessel state differs from another. A normal vessel, constricted vessel, and dilated vessel can be compared side by side to make the pressure–flow–resistance tradeoff easier to see.
Common pitfalls
- Using the wrong variable as the unknown in solve mode.
- Entering zero or negative resistance or flow values.
- Forgetting that higher resistance lowers flow when pressure is unchanged.
- Mixing up pressure difference with absolute pressure.
Micro example: if \(\Delta P = 90\) mmHg and \(R = 18\) mmHg·min/L, then \(Q = 90/18 = 5.00\) L/min. If resistance increases to \(30\) mmHg·min/L at the same pressure difference, flow falls to \(3.00\) L/min.
This tool is best used for introductory hemodynamics, vascular physiology, and simple vessel comparisons. It is not a full circulation model; the next step for deeper analysis is often total peripheral resistance, venous return, or mean arterial pressure.