Flow, Pressure and Resistance
Core cards on the universal Flow = ΔP/R logic applied across circulation, ventilation, filtration and ion movement.
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Questions Covered in This Set
10 cards to master
The universal physiology equation
Flow = ΔP / R — flow equals the driving pressure difference divided by resistance (Ohm's law in a lab coat).
Why is ΔP, not absolute pressure, what matters?
Flow needs a gradient. MAP 100 with right atrial pressure 95 (e.g. tamponade) gives ΔP of only 5 — the patient is dying despite a 'normal' pressure.
Poiseuille's law for resistance
R = 8ηL / πr⁴ — resistance rises with viscosity and length, and falls with the fourth power of radius.
Effect of halving vessel radius
Resistance rises 16-fold; flow at constant pressure falls to 1/16.
Which variable does the body actually control?
Resistance — arterioles change radius in seconds, whereas the heart cannot easily change generated pressure minute to minute.
Driving force and resistance site in the circulation
ΔP = MAP − right atrial pressure ≈ 98 mmHg; main resistance = arterioles (smooth muscle tone). CO ≈ 5 L/min.
Driving force in ventilation
Atmospheric − alveolar pressure ≈ 1 cmH₂O, created by diaphragm descent; main resistance at medium bronchi. Minute ventilation ≈ 6 L/min.
Driving force for glomerular filtration
Net filtration pressure ≈ 10 mmHg from Starling forces; resistance at afferent/efferent arterioles and the filtration barrier. GFR ≈ 125 mL/min.
Ohm's law at the membrane
I_ion = g_ion(V_m − E_ion): conductance g = 1/R = number of open channels; (V_m − E_ion) is the electrochemical driving force.
Septic shock explained by the equation
TPR halves (~19.6 → ~10 mmHg·min/L) from vasodilation; cardiac output doubles, giving a bounding, warm, high-output circulation with still-low blood pressure.
