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Physical Principles
RET 2274
Respiratory Therapy Theory I
Module 1.0
Molecules and States of Matter

Atom, Molecules, Compounds are the
building blocks of all matter
Molecules and States of Matter

Three Primary States of Matter
Solid
Liquid
Gas
Molecules and States of Matter

Solid
A solid is a condensed
structure in which
strong intermolecular
bonds determine a
definite shape and
volume.
Molecules and States of Matter

Liquid

Composed of molecules
that can move about freely,
has a definite volume
without definite shape.
Liquids are denser than
gases. Like solids, liquids
are difficult to compress.
Molecules and States of Matter

Gas

The intermolecular bonds of
a gas are weak. A gas is
compressible and
completely fills an enclosed
space. Both gases and
liquids are considered
fluids, that is, substances
that can flow.
Molecules and States of Matter

All three states of matter have a
characteristic elasticity, or reversible
deformability
Solid
Liquid
Gas
Units of Measurement

Systeme' International d’Unites (SI
system)

An international system of measurement that
is universally accepted but not always used

SI unit for measuring pressure


Pascal (Pa; 1 newton/m2)
Common units for pressure used in the clinical
setting


cm H2O for gases
mm Hg for liquids
Units of Measurement


Pressure can also be measured
indirectly as the height of column of
liquid:

Centimeters of water pressure (cm
H2O)

Millimeters of mercury (mm Hg)
Both mercury and water columns are
still used in clinical practice, especially
when vascular pressures are being
measured
Units of Measurement
Units of Measurement

Familiarity with the
symbols used in
respiratory physiology
is essential to
understand common
terminology
Mass, Force, Stress, Pressure, and Work

Mass

The amount of substance determined by the
number and type of molecules

The molecular mass of a substance is the
number of moles (mol) of a substance, and 1
mole is Avogadro’s number (6.023 X 1023)
Mass, Force, Stress, Pressure, and Work

Force

A mechanical energy applied to a body.


Force = (mass x acceleration)
Weight describes the force due to the
acceleration of gravity (9.8 m/s2) acting upon
a mass

Weight = (m x g)
Mass, Force, Stress, Pressure, and Work

Density (p)

A mass per unity volume, or m/V.

Weight

p (density) x V (volume) x g (acceleration of gravity)
Mass, Force, Stress, Pressure, and Work

If an object has a mass
of 1 kg on the earth, it
would have a mass of
______ on the moon.
However, it would
weigh one-sixth as
much.
Mass, Force, Stress, Pressure, and Work

If an object has a mass
of 1 kg on the earth, it
would have a mass of
1 kg on the moon.
However, it would
weigh one-sixth as
much.
Mass, Force, Stress, Pressure, and Work

Stress

The force applied to an area


Force applied at an angle
generates shear stress
Pressure

Is force per unit area e.g., 1lb/in2

When measuring pressure within
an oxygen cylinder, pounds per
square inch (psi) is used
Mass, Force, Stress, Pressure, and Work

Strain


The physical deformation or change is shape
of a structure or substance – usually caused
by stress
Elasticity

The amount of reversible deformability that
can be generated by a stress yet allow the
structure or substance to return to its original
shape
Mass, Force, Stress, Pressure, and Work

Elasticity

Gas is highly elastic, its volume can be
compressed relatively easily

Fluids, such as liquids, are less elastic and
behave as if incompressible

Solids lack elasticity compared with gases or
liquids
Mass, Force, Stress, Pressure, and Work

Viscosity

The resistance to movement between
adjacent fluid molecules
Mass, Force, Stress, Pressure, and Work

Viscosity

Example: When there is an increase in red
blood cells (polycythemia), the heart must
work harder to circulate the blood because it
is more viscous
Mass, Force, Stress, Pressure, and Work

Work

A force causing displacement of matter does
work

For gases, a force can be measured as pressure,
and the displacement is the volume change to the
lungs

Using SI units, work is expressed in (N.m) or joules
(J) or joules per liter
Mass, Force, Stress, Pressure, and Work

Work of Breathing

The work necessary to move air (or other
gases) through breathing cycles
(inspiration/expiration)
Mass, Force, Stress, Pressure, and Work

Work of Breathing

Mechanical ventilators do the work of
breathing for patients who cannot do it on
their own
Mass, Force, Stress, Pressure, and Work

Pascal’s Law

Liquid pressure depends only on the height
(h) of the vessel and not on the vessel’s
shape or the total volume of liquid.
Mass, Force, Stress, Pressure, and Work

Hydrostatic Pressure

The weight of a fluid generates
static fluid pressure due to the
force of gravity, which varies
according to the density and
depth within the fluid container.

Atmospheric (barometric)
pressure is an example of static
fluid pressure – as elevation
increases, atmospheric
pressure decreases (shorter
column of atmospheric gas)
Mass, Force, Stress, Pressure, and Work


Compliance

The volume change to sphere-like structures such as
lungs or alveoli caused by a pressure change is the
compliance, or stiffness, of the sphere.

Compliance =
Volume
Pressure
Respiratory System Compliance

Is a composite of two compliances, lung compliance
and chest wall compliance
Mass, Force, Stress, Pressure, and Work

Elastance

The quality of recoiling or returning to an original form
after the removal of pressure. The reciprocal of
compliance (Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier.)
Elastance =

Pressure
Volume
A measure of the tendency of a hollow organ to recoil
toward its original dimensions upon removal of a
distending or compressing force. It is the reciprocal of
compliance. (The American Heritage® Medical Dictionary Copyright © 2007, 2004
by Houghton Mifflin Company)
Mass, Force, Stress, Pressure, and Work

Surface Tension

Surface tension is the
property of a liquid that
tends to reduce the
surface of a liquid toward
a minimum, pulling the
surface molecules inward.
This is what causes water
to bead up rather than
spread out.
Mass, Force, Stress, Pressure, and Work

Surface Tension

Laplace’s Law: In a liquid sphere, the pressure
required to distend the sphere is directly proportional to
the surface tension of the liquid and inversely
proportional to the sphere’s radius
Mass, Force, Stress, Pressure, and Work

Surface Tension
Relationship described by
Laplace’s law. Bubble A (left),
which has the smaller radius, has
the greater inward or deflating
pressure and is more prone to
collapse than is bubble B (right).
Because the two bubbles are
connected, bubble A would tend to
deflate and empty into bubble B.
Conversely, because of bubble A’s
greater surface tension, it would be
harder to inflate than bubble B.
Mass, Force, Stress, Pressure, and Work

Surfactant

A fluid that reduces surface tension

Surfactant is produced in the lungs (it can also
be manufactured synthetically). It reduces the
surface tension of fluid in the lungs. This keeps
them from collapsing when an individual
exhales and makes them easier to inflate when
inhaling.
Mass, Force, Stress, Pressure, and Work

Surfactant

Surfactant also reduces the pressure
differences between alveoli of different
diameters

Without surfactant, smaller alveoli would empty
into larger ones because of the greater surface
tension in the smaller alveoli
Mass, Force, Stress, Pressure, and Work

Surface Tension

The lungs resemble clumps of
bubbles. It follows therefore that
surface tension plays a key role in the
mechanics of ventilation

Abnormalities in alveolar surface
tension occur in certain clinic
conditions, e.g., Babies that are born
very prematurely often lack adequate
surfactant and must receive surfactant
replacement therapy immediately after
birth in order to breathe.
Mass, Force, Stress, Pressure, and Work

Surface Tension

Abnormalities in alveolar
surface tension occur in
certain clinic conditions.

Babies that are born very
prematurely often lack
adequate surfactant and
must receive surfactant
replacement therapy
immediately after birth in
order to breathe.
Temperature

Temperature describes the amount of
heat, or thermal energy, present in a
system

Three temperature scales are common:
o
o
o
Fahrenheit (° F)
Celsius (° C)
Kelvin (absolute) (K)
Temperature

Fahrenheit


Used in Healthcare
Divides the temperature range between
freezing and boiling into 180 gradations, or
degrees

Freezing point of water = 32º F

Boiling point of water = 212º F
Temperature

Celsius (C)


Used in Healthcare
Divides the temperature range between
freezing and boiling into 100 gradations, or
degrees

Freezing point of water = 0º C

Boiling point of water = 100º C
Temperature
Temperature

Kelvin (K)

Zero degrees K = absolute zero

Absolute zero is the concept that a temperature
exists at which there is no kinetic energy (energy
of motion) – exists in theory only

Freezing point of water = 273 K

Boling point of water = 373 K

Note: To covert degrees Celsius to degrees
Kelvin, simply add 273

Example: 25º C = 25 + 273 = 298º K
Temperature
Temperature

Linear relationship between gas molecular activity, or
pressure, and temperature. The graph shows comparable
readings on three scales for five temperature points
Freezing point
of water
Boiling point
of water
Thermodynamics and Heat Exchange

Thermodynamics describes changes in the thermal state
of a system by adding or removing energy, such as when
changes in pressure, volume, or temperature alter the
state of the substance.

When a change of state requires the addition of energy,
the process is called endothermic.

An exothermic process gives off energy.
Thermodynamics and Heat Exchange
Gas Laws

The Ideal Gas Law defines a
relationship between pressure, volume,
temperature, and the number of
molecules of a gas.

Pressure and volume are inversely
related, whereas temperature is directly
proportional to volume or pressure
Gas Laws
(A)
A mass of gas in the resting
state exerts a given pressure at
a given temperature in a cylinder
(B)
As the piston compresses the
gas, the molecules are crowded
closer together, and the
increased energy of molecular
collisions increases both the
temperature and the pressure
(C)
Conversely, as the gas expands,
molecular interaction diminishes
and the temperature and
pressure fall
Gas Laws
Gas Laws
•
Gay-Lussac’s Law The pressure of a gas of
fixed mass and fixed volume is directly
proportional to the gas' absolute temperature.
•
The pressure in an oxygen tank will change
directly with changes in temperature.
•
P1/T1 = P2/T2
Told ya!
Gas Laws
Two escape trailer before explosion
The Associated Press
Tuesday, September 4, 2012
HENDERSONVILLE — Henderson County fire officials say a Bat Cave woman and her
friend narrowly escaped serious injury when they left a burning trailer before it exploded.
The Times-News of Hendersonville reports that a family member said Elizabeth Lawter
awoke to find her bed on fire after a discarded cigarette caught a nearby trashcan on fire
and spread to the bedding.
Fire Marshal Wally Hollis said several oxygen tanks in the trailer exploded shortly after
the two occupants left Sunday morning. Hollis said one large tank was exposed to the
flames, leading to the explosion. Her mother said Lawter suffers from diabetes and
chronic emphysema.
The blast threw the trailer's roof to the edge of U.S. Highway 64 and also blew out the
windows of an adjacent trailer.
Gas Laws
•
Boyle’s Law states that pressure is inversely
proportional to volume.
•
If a volume of gas is halved, pressure will
double, given a constant mass and temperature
Gas Laws



Let’s try and understand Boyle’s law using a simple example.
At the surface we are subjected to 1 ATM (atmosphere) of
pressure. At 33ft underwater, we are subjected to 2 ATM; i.e. 1
ATM of Air pressure and 1 ATM of water pressure.
So if we take a 1 liter Coke bottle filled with air faced down with
no cap on, to 33ft (10m) underwater, we would see that the
volume of air decreases to around ½ a liter of air, and water
would begin filling into the bottle without any of the air
escaping. Because at 33ft the pressure has increased of 2
ATM or has doubled, thereby halving the volume of the air. If
we take the bottle down to 66ft (20m), we would be at 3
atmospheres of pressure and the air in the bottle would be 1/3
of a liter and so on.
Now assume we add air into the coke bottle from our scuba
tanks at the depth of 33ft (10m) topping off the half full bottle,
cap the bottle tightly, then begin to ascend.(remember the air in
our scuba tank is also being subjected to Boyle’s law ) As we
rise, the pressure decreases, causing the already compressed
air to expand. At the surface the volume of the air in the 1 liter
bottle would have doubled to 2 liters probably causing the
bottle to burst on the way up.
Gas Laws
•
Boyle’s Law states that pressure is
inversely proportional to volume.
Gas Laws
•
Charle’s Law states that at constant pressure,
the volume of a given mass of an ideal gas
increases or decreases by the same factor as its
temperature on the absolute temperature scale
(i.e. the gas expands as the temperature
increases).
Gas Laws
•
Charle’s Law predicts the effect of
temperature on a fixed amount of dry
gas.
Gas Mixtures and Partial Pressures
•
Dalton’s Law of Partial Pressure
describes the behavior of physical
mixtures of gases and vapors.
•
The partial pressure of each particular
gas is equal to the fractional
concentration times the total atmospheric
pressure.
Gas Mixtures and Partial Pressures
•
Dalton’s Law of Partial Pressure
•
Many gases exist together as mixtures, for example
air, which contains mostly oxygen and nitrogen
•
The pressure exerted by a single gas is called its
partial pressure
•
The total pressure of a mixture of gases must equal
the sum of the partial pressures of all component
gases
PressureTotal = Pressure1 + Pressure2 ... Pressuren
Gas Mixtures and Partial Pressures
•
Dalton’s Law of Partial Pressure
Properties of Gases

Composition of Earth’s Atmosphere
Gas Mixtures and Partial Pressures
•
Physical combinations of gases mix uniformly
and are evenly distributed in any particular
confined space.
•
The same fractions of oxygen and nitrogen
are present in Death Valley (86 m below sea
level) as on Mount Everest (elevation 8850
m), although their partial pressures vary
greatly according to their respective altitude.
Humidity, Water Vapor, Evaporation
•
Most gases encountered in physiologic
conditions are combination of various dry gases,
but they also contain water vapor (gas), which
combines with the other gases according to
Dalton’s law of partial pressures.
•
Water is particularly important as a vapor under
conditions encountered in respiratory care.
Humidity, Water Vapor, Evaporation
•
Evaporation and Condensation
•
A water surface emits molecules of vapor
continuously by evaporation
•
As vapor molecules hit the surface of a
liquid, some are absorbed into the liquid by
condensation
Humidity, Water Vapor, Evaporation
•
Between 100°C and 0°C
there is saturation
pressure (or partial
pressure of water) at any
given temperature at
which water will
condense
•
Temperature defines a
limit to the maximum
amount of water vapor
that can be contained in
air at that temperature
Humidity Therapy and Humidifiers

Absolute Humidity

Is the actual content or weight of water present
in a given volume of gas

Expressed as:


Milligrams per liter (mg/L)
Also know as water content
Humidity Therapy and Humidifiers

Relative Humidity (RH)

Is the ratio of actual content or weight or the
water present in a gas relative to the sample’s
capacity to hold water at that temperature

Expressed as a percentage
%RH = absolute humidity X 100
humidity capacity

When the amount of water that a gas contains at a given
temperature is equal to the gas’s capacity, the RH is 100%
- described as saturated
Humidity Therapy and Humidifiers

Relative Humidity (RH)

If absolute humidity is held
constant, increasing the
temperature of the gas will
decrease the RH
(Temp RH)

If absolute humidity is held
constant, decreasing the
temperature of the gas will
increase the RH or it will
remain at 100%
(Temp RH)
Humidity Therapy and Humidifiers

Condensation


Cooling a gas that has an RH of 100%
decreases its capacity to hold water, which
results in water being squeezed out of the gas
Temp RH
Condensation
Humidity, Water Vapor, Evaporation
•
At 1 atm, fully humidified or saturated air
at body temperature has a PH2O of 47 mm
Hg. Other gases account for the
remainder of the 760 mm Hg, or 713 mm
Hg.
Humidity, Water Vapor, Evaporation
•
Percentage of Body Humidity (%BH)
•
%BH is the ratio of actual water vapor
content to the water vapor capacity in a
saturated gas at 37°C.
%BH =
Content
x 100%
Capacity (43.8 mg/L)
•
The water content (absolute humidity) of fully
saturated gas at body temperature is 43.8
mg/L
Humidity, Water Vapor, Evaporation
•
A humidity deficit occurs whenever inspired
gas is not fully saturated at body temperature,
requiring the body to add water to inspired
gases to achieve full saturation.
Humidity deficit = water vapor content – 43.8 mg/L
•
The difference is the burden on the airway to
humidify the inspired gas
Properties of Gases

Henry’s Law states that at a constant
temperature, a gas dissolves in solution in
proportion to its partial pressure
William Henry (chemist)
Properties of Gases

Henry’s Law also states that the capacity of a
liquid to carry a gas decreases as a temperature
increases

High temperatures decrease solubility

Low temperatures increase solubility
Leave a carbonated drink open and out of
the refrigerator and it will quickly go flat
Properties of Gases

Diffusion is the process whereby molecules
move from areas of high concentration to areas
of lower concentration

Graham’s law states that the rate of diffusion of
a gas (D) is inversely proportional to the square
root of its density:
Lighter gases diffuse rapidly, whereas heavy gas
molecules diffuse more slowly

In a liquid medium, both Graham’s law and Henry’s
law affect the rate of diffusion of gases.
Gases in Solution, Diffusion, Osmosis
•
Osmosis is the
movement of a solvent
by diffusion, primarily,
through a
semipermeable
membrane that does
not permit movement of
larger solute molecules.
•
A solvent diffuses across
the membrane from an
area of lesser to greater
concentration
Gases in Solution, Diffusion, Osmosis
•
Fick’s law relates the factors that affect the
transmembrane transfer of solute during
osmosis
•
The total diffusion rate of a gas across a barrier (such
as the alveolar membrane in the lung) is directly
proportional to the:
•
•
•
Cross-sectional area available for diffusion (Lung size)
Difference in concentration gradients of the diffusing
gases
Thickness of the barrier (e.g., alveolocapillary
membrane)
Conversion of Gas Volumes
•
Several sets of conditions are commonly
encountered in respiratory therapy because of
the conditions under which certain stored (dry)
or measured gases (body temperature,
humidified).
Flow of Gases and Other Fluids
•
Flow is the movement of a specified volume of
fluid (gas or liquid) in a particular period of time
(Volume/Time)
•
•
e.g., liters of air / minute
The flow of gas through tubes is a key physical
phenomenon in respiratory physiology:
•
Flow of air into and from the lungs
•
Flow of gas through a ventilator circuit
Flow of Gases and Other Fluids
•
Principle of Continuity is the concept that
states that if any liquid flows through a rigid
pipe, the mass of fluid entering a tube must
equal the mass leaving the tube
•
Flow Velocity is the distance a fluid moves over
a time period (Distance/Time)
•
e.g.,
Velocity = centimeters/second
Flow of Gases and Other Fluids
•
The product of velocity and the area of the tube through
which a fluid moves defines the volume of fluid moving
over time.
•
If the diameter (hence area) of a section of the tube
increases, the velocity decreases through that segment
because the same mass entering must equal the mass
exiting the tube (principle of continuity).
•
Diameter and velocity are therefore inversely related.
Flow of Gases and Other Fluids
Figure 50-7: Principle of continuity. Note that fluid velocity is related
inversely to the cross-sectional area.
Flow of Gases and Other Fluids
•
The Bernoulli principle describes the pressure in a fluid
as the velocity changes. According to the Bernoulli
theorem, a flowing fluid’s lateral pressure must vary
inversely with its velocity
Flow of Gases and Other Fluids
•
The Venturi principle is an application of the Bernoulli
principle and the law of continuity that explains the
entrainment of fluids through an open port in a tube.
•
When a flowing fluid encounters a very narrow passage,
its velocity can increase greatly and cause the fluid’s
lateral pressure to fall below that exerted by the
atmosphere and pull another fluid into the primary flow
stream.
Flow of Gases and Other Fluids

The Venturi principle

The amount of air
entrained depends on
both the diameter of the
jet orifice and the size
of the air entrainment
ports
Flow of Gases and Other Fluids
•
Viscosity is described as the internal friction of
a fluid and is independent of the density of the
fluid (gas or liquid).
•
For gas movement, viscosity increases with
temperature because the frequency of collisions
between molecules is greater at higher temperatures
•
Viscosity in liquids is increased at lower temperatures
Flow of Gases and Other Fluids
•
Laminar Flow is the orderly flow of a fluid
through a straight tube as a series of concentric
cylinders slide over one another – friction is
decreased during laminar flow
Flow of Gases and Other Fluids
•
Turbulent Flow is a jumbled mixture of
velocities across the section of tube – friction is
increased during turbulent flow
Flow of Gases and Other Fluids
Figure 50-8: (A) Laminar flow. (B) Turbulent flow.
Flow of Gases and Other Fluids
•
Transitional Flow is mixture of laminar and
turbulent flow patterns

Flow in the respiratory tract is mainly transitional
Flow of Gases and Other Fluids
•
Reynolds Number describes factors associated
with the generation of laminar or turbulent flow,
e.g., velocity, radius, density, viscosity
•
Reynolds Number
o >3000 = Turbulent
o 2000 – 3000 = Transitional
o <2000 = Laminar