Lecture 2: Pressure-flow relationships

Basic Principles of Circulatory Function

1. Under normal conditions, blood flow to most tissues is controlled according to tissue needs

2. Cardiac output (= heart rate x stroke volume) is the sum of all the local tissue blood flows

3. Arterial pressure is tightly regulated across wide changes in cardiac output and local tissue blood flows

Flow

The movement of substances or heat from one point to another, driven by energy gradients between those two points

Molecules

Concentration energy gradient

Heat 

Temperature energy gradient 

Gases

Partial Pressure gradient

Fluids

Hydrostatic pressure gradient

Ions

Concentration and voltage gradient

What is blood pressure? 

• pressure if a force

• Blood pressure is a force exerted by the blood against the vessel all

• Increased pressure pushing against vessel containing blood, that energy drives flow of blood

Blood flow

The blood pressure difference between two points in the circulation (P1-P2), and the resistance (based on Ohm’s law)

Blood flow equation

F = ∆P/R

Conductance equation

1/Resistance

Cardiac output (CO)

The total blood flow through the aorta.

∆P

The difference in mean blood pressure between the aorta and the right atrium

Total peripheral resistance (TPR)

The resistance of the entire systemic circulation

Laminar flow

Blood usually flows smoothly in ‘streamlines’ through vessels, with relatively little mixing in a radial direction

Turbulent flow

• Blood flow can become disorderly, whirling in a radial (crosswise) direction form what are called ‘eddy currents’.

• This usually only occurs in largest vessels, at highest flow rates, or around obstacles

• Turbulence increases the resistance to blood flow compared to laminar flow

Poiseuille’s Law

Describes the effect of vessel radius on Flow under laminar conditions

Anemia

Can occur during menstrual cycle, in response to blood loss, etc —> decrease in red blood cells

Polycythemia (excessive erythrocytosis)

Occurs at high elevation, in response to sleep apnea, in pulmonary diseases, etc —> excessive production of red blood cells

Isovolumic hemodilution

Removing blood and replacing it with saline