Laminar and turbulent flow
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what is the relationship between pressure, flow and resistance in the cardiovascular and respiratory system?
Flow = pressure / resistance
How can we calculate the flow of the cardiovascular system?
CO = blood pressure (ABP - CVP) / resistance (TPR)
How can we calculate the flow of the respiratory system?
Air flow = pressure gradient (PB - PA) / Airway resistance (Raw)
What factors determine flow?
Radius (airway/blood vessel) Viscosity of fluid (air/blood) Length (of airway or blood vessels)
What is flow proportional to?
Driving pressure RS - dependent on work of chest wall muscles CVS - dependent on cardiac work
What is flow inversely proportional to?
Resistance
What is the haematocrit?
Proportion of red blood cells compared to the total volume of blood plasma
What does a change in the radius do to resistance?
An increase in the radius from 1 to 2 will reduce the resistance 16 fold (2^4)
How do we physiologically control the radius of airways or blood vessels?
RS - bronchoconstriction/dilation CVS - vasoconstriction/dilation
What does poiseuilles law only apply to?
Only applicable to laminar flow
What is laminar flow?
Organised parabolic flow profile Formed through layers being set up within the vessel/airway.
What are the layers that set up laminar flow?
Immobile layer blood layers slipping over each other (each successive layer slips past faster than the previous one) flow is fastest at the vessel centre
What is turbulent flow?
Disorganised flow No structured pattern Cross-currents that uses up loads of kinetic energy in the set up of the random flow patterns
Why is turbulent flow less energy efficient?
Requires more energy to set up cross-flows and currents to mix the fluid streams and therefore energy is used up. Some energy is lost as sound.
What is the relationship between laminar and turbulent flow and resistance?
Laminar flow - Inversely proportional to resistance Turbulent flow - inversely proportional to the square root of resistance
what factors determine whether flow is laminar or turbulent?
Turbulent flow has a high value Reynolds number.
What factors decrease turbulence?
Viscosity of the fluid. more viscous makes the flow less turbulent.
What does a Reynolds number <2000 mean
Most likely to be laminar flow. >2000 more likely to be turbulent flow
Where is turbulent flow found in the respiratory system?
Turbulent airflow - Trachea since there is a high velocity and large radius.
What is transitional air flow?
Mixture of flow types (most of the branchial tree is transitional)
Where is laminar flow found in the respiratory system?
Very small airways since there is a low velocity of fluid here.
Where is laminar flow found in the cardiovascular system
in most blood vessels (but vessel branches disrupt causing eddy currents) RBCs form concentric layers
What is the key idea about laminar flow?
The idea that laminar flow is highly energy efficient and SILENT
Where is turbulent flow found in the cardiovascular system
Where there is an irregularity in a blood vessel (eg at the valves) When the blood velocity is high (aorta) When there is stenosis (narrowing of blood vessels) Branch points in vessels