The Capacitance Problem
A term that means everything and explains nothing.
I’m increasingly convinced we should stop using the term “vascular capacitance” in haemodynamics.
Not because the circulation cannot accommodate volume. It clearly can.
The problem is that “capacitance” is used a lot while meaning different things to different people. It sounds precise, but in cardiovascular physiology it often lacks a stable, agreed meaning.
In electrical circuits, capacitance has a clear definition: how much charge is stored for a given voltage.
In hydraulic terms, the closest equivalent would be how much volume changes for a given change in pressure.
But in haemodynamics we already have the correct word for that.
Compliance.
Compliance is:
dV / dP
If a vascular compartment accepts a large change in volume with only a small change in pressure, it is compliant. If a small change in volume produces a large change in pressure, it is stiff.
If we mean the opposite relationship — how much pressure changes for a given change in volume — then the correct word is:
Elastance.
Elastance is:
dP / dV
And if we mean how much volume a compartment can contain, that is not capacitance either.
That is:
Capacity.
A rigid one-litre bucket has a capacity of one litre. But it does not have high compliance. If it is closed and truly rigid, it cannot accept extra volume without a large rise in pressure.
Capacity and compliance are not the same thing.
This is where “vascular capacitance” becomes a problem.
Sometimes it is used to mean compliance.
Sometimes it is used to mean capacity.
Sometimes it is used to mean venous tone.
Sometimes it is used to mean unstressed volume.
Sometimes it is used to mean the broader ability of the venous system to contain blood without increasing the effective filling state.
Those are not the same thing.
So when someone says “venous capacitance increased”, what do they actually mean?
Did the venous system become more compliant?
Did elastance fall?
Did venous tone decrease?
Did the available containing volume increase?
Did unstressed volume increase?
Did blood redistribute into regions that could accommodate volume more easily?
Did the same circulating volume now generate a lower effective filling state?
Those are different mechanisms with different consequences.
This matters because haemodynamics is already full of shorthand that sounds causal but is often only descriptive. “Capacitance” is one of those terms. It is technical enough to sound explanatory, but vague enough to hide the actual mechanism.
A better approach is to say what we actually mean.
If we mean volume-containing size, say capacity.
If we mean volume change for pressure change, say compliance.
If we mean pressure change for volume change, say elastance.
If we mean venous tone, say venous tone.
If we mean unstressed volume, say unstressed volume.
If we mean redistribution of blood into a more accommodating vascular region, say that. This is when a broader phrase can be useful as long as we know what we mean. I use vascular accommodation. This is the distributional consequence of the vascular elastic state: the ability of the circulation, especially the venous circulation, to contain blood volume without presenting it to the heart as effective filling volume. It should not be treated as a single physical property. It describes the net behaviour of the system. The next question should always be:
what mechanism produced it?
That is the problem with “capacitance”. It can sound like a mechanism when it often isn’t one. It’s generally blurring capacity, compliance, elastance, tone, unstressed volume and accommodation into one word, so it is probably not helping us think clearly.
Use the mechanism when you know it.
Use broader language only when you mean broader behaviour. And use a term that people will precisely understand
Use the mechanism, not the metaphor.