Resistance, Pressure, Flow & Cardiac/Vascular Function Curves

Rapid Review·Cardiovascular

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PHYSIOLOGY

T3Take a glance

FA P290

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Why radius dominates resistance, and how two intersecting curves find the heart’s operating point.

Key takeaways

Normal cardiac pressures

The values

marked on the Normal cardiac Pressure figure.

Right atrium < 5 mm Hg
Right ventricle (pulmonary artery pressure) 25/5 mm Hg
Left atrium (pulmonary capillary wedge pressure) < 12 mm Hg (higher than left ventricular pressure in mitral stenosis)
Left ventricle 130/10 mm Hg

Flow and resistance

Flow

volumetric flow rate Q = flow velocity (v) × cross-sectional area (A).

Resistance

R = ΔP / Q = 8ηL / πr⁴, so R is inversely proportional to r⁴. Radius overwhelms every other term.

Pressure drop

ΔP = Q × R, which is the exact analogue of Ohm's law (V = I × R).

Series and parallel

in series R total = R1 + R2 + R3. In parallel 1/R total = 1/R1 + 1/R2 + 1/R3. Never asked, just to understand.

Viscosity

depends mostly on hematocrit. It rises in hyperproteinemic states (multiple myeloma) and polycythemia, and falls in anemia.

Where each vascular segment dominates

Arterioles account for most of total peripheral resistance.
Capillaries have the highest total cross-sectional area and therefore the lowest flow velocity, which is what allows time for exchange.
Veins provide most of the blood storage capacity.

Where each vascular segment dominates
Segment	Dominant property
Arterioles	Most of total peripheral resistance
Capillaries	Highest total cross-sectional area, lowest flow velocity
Veins	Most of blood storage capacity

Cardiac and vascular function curves

What this shows

Cardiac and vascular function curves, plotted against right atrial pressure; their intersection is the operating point of the heart. Panel A: raising or lowering inotropy pivots the cardiac function curve. Panel B: changing blood volume or venous tone shifts the vascular function curve, moving its x-intercept (mean systemic filling pressure). Panel C: changing total peripheral resistance rotates the vascular curve, so cardiac output changes while the effect on right atrial pressure is unpredictable.

The operating point

the intersection of the cardiac and vascular function curves is the operating point of the heart. Venous return and cardiac output must be equal, because the circulation is a closed system. The three panels show what each intervention moves.

Changes usually occur in tandem, in one of two patterns

Reinforcing: exercise raises inotropy and lowers TPR, both acting to maximise cardiac output.
Compensatory: heart failure lowers inotropy, so fluid is retained to raise preload and defend cardiac output.

The three curve shifts
Graph	Effect	Raised by	Lowered by
Inotropy	Changes contractility, so SV and cardiac output change	Catecholamines, dobutamine, digoxin, exercise	HFrEF, narcotic overdose, sympathetic inhibition
Venous return	Changes circulating volume, so RAP and then SV change	Fluid infusion, sympathetic activity, AV shunt	Acute hemorrhage, spinal anesthesia
Total peripheral resistance	Changes cardiac output; effect on RAP is unpredictable	Vasopressors	Exercise, AV shunt

A vessel's radius is halved. By how much does its resistance change?

Sixteen-fold, because resistance is inversely proportional to r⁴. Length only enters linearly, which is why a larger-bore catheter beats a longer one.

How it's tested

Resistance is inversely proportional to the fourth power of the radius, so halving a vessel's radius raises resistance sixteen-fold. That is why arterioles, not the aorta, set total peripheral resistance. The same relationship is tested clinically: asked how to speed up large-volume fluid resuscitation, with the options being a second catheter, a longer catheter, or a larger-bore catheter, the answer is the larger bore, because radius enters to the fourth power while length only enters linearly. Mehlman's hypertension point is a good check on understanding: high arterial pressure does not flood the interstitium, because increased pre-capillary resistance shields the capillary bed.

Go deeper

First Aid 2026 — CV/Physiology (p.290) · B&B — Vascular function curves · Mehlman — HY Cardio (Poiseuille, pre-capillary resistance and edema)

High-yield images2

Cardiac and vascular function curves, plotted against right atrial pressure; their intersection is the operating point of the heart. Panel A: raising or lowering inotropy pivots the cardiac function curve. Panel B: changing blood volume or venous tone shifts the vascular function curve, moving its x

Resistance, Pressure, Flow & Cardiac/Vascular Function Curves

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What does the intersection of the cardiac function curve and vascular function curve represent?

(...) of the heart

PreviousCardiac Output Equations & Starling Curves NextCardiac Cycle, JVP & Pressure-Volume Loops

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