Physical Principles of Respiratory Care
I.
II.
III.
IV.
States of Matter
Change of State
Gas Behavior Under Changing Conditions
Fluid Dynamics
Fluid Dynamics
1.
2.
3.
4.
5.
6.
Pressure in Flowing Fluids
Patterns of Flow
 Laminar Flow
 Turbulent Flow
 Transitional Flow
Flow,Velocity, and Cross-Sectional Area
Bernoulli Effect
Fluid Entrainment
Fluidics and the Coanda Effect
Fluid Dynamics
 The study of fluids in motion is called
hydrodynamics.

The pressure exerted by a liquid in motion depends
on the nature of the flow itself.

A progressive decrease in fluid pressure occurs as the
fluid flows through a tube due to resistance.
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Patterns of Flow
Patterns of flow
 Laminar flowfluid moving in discrete cylindrical
layers or streamlines
 Poiseuille’s lawpredicts pressure required to
produce given flow using
ΔP = 8nl V./ πr4

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https://www.youtube.com/watch?v=G3bO8RcRgxQ
Conditions that cause laminar flow to
become turbulent
1.
2.
3.
4.

High linear gas velocity (causes unorganized Brownian
movement)
High gas density (slows down velocity)
Low gas viscosity (such as Helium)
Large tube diameter (more space for gas to disperse)
*A liquids viscosity is measured by its resistance to flow.
Density is the amount of mass occupying a space.
Patterns of flow

Turbulent flowloss of regular streamlines; fluid
molecules form irregular eddy currents in chaotic
pattern.
 Predicted by using Reynold`s number (NR)
NR = v d2r / h
Patterns of Flow
Transitional Flow

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Poiseuille’s Law
(only applies to laminar flow)

Flow of fluid through a tube:
 Driving pressure
 Resistance
 Viscosity
 Length of the tube
 Radius of the tube
*The more the resistance, the higher the driving
pressure must be to overcome the resistance
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Poiseuille’s Law
1.
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The more viscous the fluid the more pressure is
required to cause it to move through a given tube
Poiseuille’s Law
Resistance to flow is directly proportional to the
length of the tube
2.
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
If the length of a tube is increased four times, the
driving pressure to maintain a given flow must
be increased four times

*Why then in the lung, is the resistance to flow
not increased in the lower airway?
Poiseuille’s Law
Resistance to flow is inversely proportional to the
fourth power of the radius of the tube
3.

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If the inside diameter of the tube is decreased by
one half, the driving pressure must be increased
16 times to maintain original flow
Poiseuille’s Law
 Respiratory
Care
Application:
ETT,
trachestomy…
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Poiseuille’s Law

Asthma
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Pressure in Flowing Fluids
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Law of Continuity
The speed of flow in a closed system will be inversely
proportional to the area of the tubes through which it
flows
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Law of Continuity
2.54


If the area of flow is decreased, then the velocity must
increase
If the area of flow in increased, then the velocity must
decrease
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Law of Continuity
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The Bernoulli Effect
As the speed of the fluid increases, the
pressure in a fluid decreases
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Venturi Principle of Fluid Entrainment
If the increase in velocity at a constriction is so great
that is causes the pressure of the fluid to fall below
atmospheric (becoming negative) it can pull another
fluid into the primary flow
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Fluid Entrainment
 Respiratory
Care
Application:
Air
injector/venturi
device
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Fluid Entrainment
 Respiratory
Care
Application:
Entrainment mask
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Fluid Entrainment
 Respiratory
Care
Application:
Small Volume
Nebulizer
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Fluid Entrainment
 Respiratory
Care
Application:
Large volume jet
nebulizer
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Fluid Dynamics
Fluidics and the Coanda effect
 Fluidics is a branch of engineering that applies
hydrodynamics principles in flow circuits.

The Coanda effect (wall attachment) is observed
when fluid flows through a small orifice with properly
contoured downstream surfaces.
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Advantages
 Operate
without moving parts, minimizing
maintenance expenses
 Generally cost less than electronic
counterparts
 Don’t break down as often as their
electronic counterparts
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Disadvantages
 Not
easily interfaced with
microprocessors
 Not as accurate as their electrical
counterparts
 Difficult to measure tidal volume because
tidal volume exits with source gas
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