# Compliance and Elastance.

Compliance and Elastance.
By Jon Trister MD

Volume/Pressure= Compliance
Pressure/Volume= Elastance

Elastance and Compliance express the same properties of the connective
tissue from different points of view.

Compliance:Dilation of the arteries, veins in response to mechanical load.

Elastance: Resistance, stiffness of the arteries, veins in response to mechanical load

Arteries have lower compliance and high elastance
Veins heave high compliance and low elastance

Compliance: reflects ability to change the shape of the structure when mechanical load applied.
Elastance: reflects resistance to change the shape when mechanical load applied.

When compliance is less than elastance – rigidity predominates.
When elastance is less than compliance – hypermobility predominates

Bernoulli’s equation

Total Fluid Energy +Pressure energy + Movement Energy

Total Fluid Energy= P + 1/2pv^2 + pgh

P=Pressure energy

1/2pv^2= Movement or Kinetic energy (p=density; v=velocity)

pgh=Potential energy (p=density;g=gravity;h=height)

Combine value of pressure energy and kinetic energy remains the same at any point of blood flow.

In any two points on a streamtube, like an artery, the total mechanical energy will be the same. If we assume that potential energy does not change, we can write the energy of the fluid in any part of the streamtube as: Pressure+ 1/2(density)(velocity)^2= a constant. Since the total flow is always the same at any point in a streamtube, the velocity must be higher in narrow area than in a wide area. According to the formula, as velocity increases, pressure will decrease so that the constant remains the same.

Another word : In a wide section where velocity is low, each particle of fluid will on average have a smaller portion of its kinetic energy in the same direction as the flow, and more of its kinetic energy in other directions. As the velocity increases in a narrow section, each particle will have more of its kinetic energy directed with the flow, and less in other directions. The particles in the narrow sections will therefore be colliding with each other and the walls of the streamtube less, and pressure will therefore be lower.

Stored elastic energy in large and middle sized arteries (windkessel)

During systole pressure (elastance) and volume (compliance) of the large arteries are increased.

Energy Generated by Heart:
-Movement energy (blood flow)

-Pressure energy (pressure of fluid)

-Elastic energy ( arterial wall)

During systole pressure energy converts to elastic energy and movement energy.

During diastole elastic energy of the arterial wall converts to movement energy.

The heart contracts, a bolus of blood leaves it and moves to the aorta which expands to accept this larger volume of blood. The heart relaxes to fill again with the blood and now the stored elastic fiber energy in the aorta pushes that first volume of blood on down the artery.

Compliance of the arterial wall decrease blood pressure by converting pressure energy to elastic energy of the arterial wall.

Compliance increase blood flow by converting elastic energy stored in arterial wall to movement energy.