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Physical Principles of
Respiratory Care
Egan Chapter 6
Physical Principles of Respiratory Care
I.
II.
III.
IV.
States of Matter
Change of State
Gas Behavior Under Changing Conditions
Fluid Dynamics
III. Gas Behavior Under Changing Conditions
A.
Gas Laws
1. Effect of Water Vapor
 Corrected Pressure Computations
 Correction Factors
2. Properties of Gases at Extremes of Temperature
and Pressure
3. Critical Temperature and Pressure
A. Gas Laws



Boyle’s Law
Charles’ Law
Gay-Lussac’s Law
Boyle’s Law
 Robert
Boyle
 1627-1691
 British chemist
 Pressure-volume
relationship
5
Boyle’s Law


6
If temperature
(T) remains
constant,
pressure (P) will
vary inversely to
volume (V)
P1V1 = P2V2
Boyle’s Law

If temperature (T) remains constant, pressure (P)
will vary inversely to volume (V)

If gas P , the V occupied by the gas 

If gas V , the P exerted by the gas 
7
Boyle’s Law
P
8
T
V
Ventilation is an example
of Boyle’s Law






As the diaphragm drops at the
beginning of inspiration
The volume of the lungs
increases
As lung volume increases what
happens to the pressure inside
the lungs?
It _________
And air flows into the lungs
http://www.youtube.com/watch?
v=q6-oyxnkZC0
9
Boyle’s Law

A snorkel diver is preparing to descend a freshwater
pond to a depth of 66 feet. At sea level, his lungs contain
about 3000 mL of air. As he descends below the surface
of the water, what will happen to the gas volume in his
lungs as he descends to 66 feet below the surface of the
pond?
10
Charles’ Law
 Jacques
Charles
 1746-1823
 French chemist
 Volumetemperature
relationship
11
Charles’ Law


12
If pressure (P)
remains constant,
the volume (V) will
vary directly with
temperature (T)
V1 = V2
T1 T2
Charles’ Law

If pressure (P) remains constant, the volume (V) will
vary directly with temperature (T)

If gas T , the V occupied by the gas also 


If gas T , the V occupied by the gas also 

13
As T , kinetic energy , the gas molecules move more
vigorously and the gas expands
As T , kinetic energy , the gas molecules slow down and
the gas contracts
Tire volume is an example of
Charles’ Law
14
Gay-Lussac’s Law
An oxygen cylinder has a pressure of 2200 psi at a
temperature of 25 C, what will its pressure be if it is
heated to 50 C? (Must first convert temperature to
Kelvin)

15
Gay-Lussac’s Law
 Joseph
Gay-Lussac
 1778-1850
 Expanded on Charles’
work
 Pressure-temperature
relationship
16
Gay-Lussac’s Law
17

If volume (V)
remains constant,
pressure (P) will
vary directly with
temperature (T)

P1 = P2
T1 T2
Gay-Lussac’s Law

If volume (V) remains constant, pressure (P) will
vary directly with temperature (T)

If gas T , the P exerted by the gas also 

If gas T , the P exerted by the gas also 
18
Gas Cylinder Pressures and Temp is an
example of Gay-Lussac’s Law

An alarm signals that there is a fire in the
basement of the hospital. Although the fire is
confined to an area that is approximately 300
feet from the room where the compressedgas cylinders are stored, you are asked to
move the cylinders to a safer location. Why
is it necessary to move the cylinders?
Keep cylinder temperature below 125°F
(51.7°C)
http://www.youtube.com/watch?v=xceBXe5YH
j0

19
Charles’ Law
Given: 100 mL of an unknown gas measured at 28° C
and 760 mmHg. Find the gas volume measured at 37°
C and 760 mmHg.(Must first convert temperature to
Kelvin)

20
III. Gas Behavior Under Changing Conditions
A.
Gas Laws
1. Effect of Water Vapor







In clinical practice, most gas law calculations must take into
account the presence of water vapor.
Water vapor, similar to any gas, occupies space.
The dry volume of a gas at a constant pressure and temperature
is always smaller than its saturated volume.
The opposite is also true.
Correcting from the dry state to the saturated state always yields
a larger gas volume.
The addition of water vapor to a gas mixture always lowers the
partial pressures of the other gases present.
This fact becomes relevant when discussing the partial pressure of
gases in the lung where the gases are saturated with water vapor
at body temperature.
III. Gas Behavior Under Changing Conditions
A.
Gas Laws
1. Effect of Water Vapor


The addition of water vapor to a gas mixture always lowers
the partial pressures of the other gases present.
This fact becomes relevant when discussing the partial
pressure of gases in the lung where the gases are saturated
with water vapor at body temperature.
III. Gas Behavior Under Changing Conditions
A.
Gas Laws
1. Effect of Water Vapor
 Corrected Pressure Computations
 PC = (PT – PH 0)
2
III. Gas Behavior Under Changing Conditions
A.
Gas Laws
1. Effect of Water Vapor
 Correction Factors
Correction from ATPS to BTPS
Correction from ATPS to STPD
Correction from STPD to BTPS

Table 6-3
Clinical Example: Spirometry



Spirometry: laboratory evaluation of lung function using a
spirometer
Spirometer: a device for measuring lung volumes and/or
flows
When a patient exhales into a spirometer the gas is
measured at room temperature



About 25°C
ATPS: Ambient Temperature Pressure Saturated
But inside the lungs the gas is at body temperature


26
About 37°C
BTPS: Body Temperature Pressure Saturated
Clinical Example: Spirometry

As the patient exhales and the temperature of the gas
decreases to room temperature

What will happen to the volume of gas?

Whose law tells us this?
27
III. Gas Behavior Under Changing Conditions
A.
Gas Laws
1. Properties of Gases at Extremes of Temperature
and Pressure
 See Mini Clini: Variations from Ideal Gas Behavior:
Expansion Cooling and Adiabatic Compression
page 121.
III. Gas Behavior Under Changing Conditions
A.
Gas Laws
2.
Critical Temperature and Pressure
 Critical temperature: the highest temperature at which a
substance can exist as a liquid.
 The temperature above which the kinetic activity of its
molecules is so great that the attractive forces cannot
keep them in a liquid state.
 Critical pressure: the pressure needed to maintain
equilibrium between the liquid and gas phases of a substance
at this critical temperature.
 Together, the critical temperature and pressure represent
the critical point of a substance.
2. Critical Temperature and Pressure




Liquid oxygen is produced by
separating it from a liquefied air
mixture at a temperature below its
boiling point (−183° C or −297° F).
After it is separated from air, the
oxygen must be maintained as a liquid
by being stored in insulated containers
below its boiling point (-183 ° C)
If higher temperatures are needed,
higher pressures must be used.
If at any time the liquid oxygen
exceeds its critical temperature of
−118.8° C, it converts immediately to
a gas.

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