Hemodynamics

Question 1

What is perfusion pressure?

Answer 1

The pressure driving blood down a vessel length (upstream pressure - downstream pressure). Also called the driving pressure for flow.

Question 2

What is transmural pressure?

Answer 2

The pressure across a vessel tending to distend the vessel (inside pressure - outside pressure). It is the force across the wall of a vessel, stretching it open. It may also serve as a driving pressure for movement into or out of a vessel across the vessel wall through a pore: P(in) - P(out) across vessel wall drives flow through the pore.

Question 3

What is compliance?

Answer 3

a. Compliance is the flexibility of the wall of the heart or a vessel (V of heart/transmural pressure). Elastance in the inverse - the stiffness of a vessel or heart.
b. Compliance describes how “accommodating” a vessel can be - how much pressure must rise for an increase in volume. Compliance is the change in volume divided by the change in transmural pressure. A very compliant vessel will only have a small increase in pressure for an increase in volume; a stiff vessel will show a large increase in pressure for that same increase in volume.

Question 4

What is Poiseuille’s Law?

Answer 4

This describes the fact that flow is extremely sensitive to diameter. As diameter decreases, flow decreases. The relationship of flow to radius is as the 4th power.

“Resistance varies directly with viscosity and the length of the flow path, and inversely with radius^4 (this radius is the structure size mentioned above)”

Excerpt From: “Hemodynamics.” v1.1. iBooks.
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Question 5

What factors determine viscosity?

Answer 5

Hematocrit
Flow rate
RBC
Membrane deformities
Temperature

Question 6

What is the Ohm’s Law analogy? How does it apply to blood flow?

Answer 6

Q=(change in P)/R. Blood flow is dependent on pressure and resistance in a vessel.

Question 7

Ohm’s Law

Answer 7

Flow rate = (driving pressure) / resistance