Chapter 6
Physics is the branch of science that
deals with the interactions of matter
and energy.
http://www.youtube.com/watc
h?v=51RCyBr_nGk
States
that the atoms & molecules that
make up matter are in constant motion.
http://www.youtube.com/watch?v=guoU_
cuR8EE
Solids – have a high degree of internal order; their atoms
have a strong mutual attractive force
Liquids – atoms exhibit less degree of mutual attraction
compared with solids, they take the shape of their
container, are difficult to compress, exhibit the
phenomenon of flow
Gases – weak molecular attractive forces; gas molecules
exhibit rapid, random motion with frequent collisions,
gases are easily compressible, expand to fill their container,
exhibit the phenomenon of flow
All matter possesses energy. There are 2 types of internal energy:
The energy of position, and the energy of motion.
Internal energy of matter
• Potential energy (Position) The strong attractive forces between
molecules that cause rigidity in solids
• Kinetic energy (Motion) Gases have weak attractive forces that
allow the molecules to move about more freely, interacting with
other objects that they come in contact with
Internal energy and temperature
• The two are closely related: internal energy can be increased by
heating or by performing work on it.
•Absolute zero = no kinetic energy


Potential energy is stored energy.
Kinetic energy is the energy that an object
possesses when it is in motion.
http://www.youtube.com/watch?
v=vl4g7T5gw1M

Possess the least amount of KE

Mostly Potential Energy in
intermolecular forces holding
particles together

Can maintain their volume &
shape

Intermolecular, cohesive forces
are not as strong

They exhibit fluidity (particles
sliding

They exhibit a buoyant force

Essentially incompressible

Assume the shape of their
container
Extremely weak – if any – cohesive
forces
 Possess the greatest amount of KE & the
least amount of Potential Energy
 Motion of atoms & molecules is random
 Do not maintain their shapes & volumes
but expand to fill the available space
 Exhibit the phenomenon of flow
 Exhibits the least thermal conductivity
 Uses: Gas therapy (Oxygen, Heliox,
Nitrous oxide…HHN/SVN…)

http://www.youtube
.com/watch?v=Xre9
2Ap0vrA
 Liquid-solid phase changes (melting and freezing)
Melting = changeover from the solid to the liquid state
Melting point = the temperature at which melting occur.
Freezing = the opposite of melting
Freezing point = the temperature at which the substance
freezes; same as its melting point
13
Fahrenheit Scale
the freezing point of water at 32 degrees
and the boiling point at 212 degrees.
These two points formed the anchors for
his scale.
Celcius Scale
the freezing temperature for water to be
0 degree and the boiling temperature 100
degrees. The Celsius scale is known as a
Universal System Unit. It is used
throughout science and in most
countries.
Kelvin Scale
There is a limit to how cold something
can be. The Kelvin scale is designed to go
to zero at this minimum temperature. At
a temperature of Absolute Zero there is
no motion and no heat. Absolute zero is
where all atomic and molecular motion
stops and is the lowest temperature
possible. Absolute Zero occurs at 0
degrees Kelvin or -273.15 degrees
Celsius or at -460 degrees Farenheit.

Properties of liquids
◦ Pressure – depends on the height and weight density.
◦ Buoyancy – occurs because the pressure below a
submerged object always exceeds the pressure above it
◦ Viscosity – the force opposing a fluid’s flow. The greater
the viscosity of a fluid, the greater the resistance to flow.
 Blood has a viscosity five times greater than that of
water
15


Pressure is measured in cmH2O, mmHg or PSI
Atmospheric pressure is the force per unit
area exerted into a surface by the weight of
air above that surface in the atmosphere of
Earth (or that of another planet). In most
circumstances atmospheric pressure is
closely approximated by the hydrostatic
pressure caused by the mass of air above the
measurement point.


Many techniques have been developed for the
measurement of pressure and vacuum.
Instruments used to measure pressure are
called pressure gauges or vacuum gauges.
A manometer could also refer to a pressure
measuring instrument, usually limited to
measuring pressures near to atmospheric.
The term manometer is often used to refer
specifically to liquid column hydrostatic
instruments.

Static pressure is uniform in all directions, so
pressure measurements are independent of
direction in an immovable (static) fluid. Flow,
however, applies additional pressure on
surfaces perpendicular to the flow direction,
while having little impact on surfaces parallel
to the flow direction. This directional
component of pressure in a moving (dynamic)
fluid is called dynamic pressure.

Heat transfer
◦ Conduction – transfers heat in solids
◦ Convection – transfers heat in liquids and gases
 (Example: heating homes or infant incubators)
◦ Radiation – occurs without direct contact between
two substances - example: microwave oven
◦ Evaporation/Condensation: requires heat energy to
occur
◦ Sublimation - change from a solid to a gas without
an intermediate change to a liquid - example dry ice
turning into CO2
http://www.youtube.com/watch?v=lxPDAh4je54
19
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
Conduction:
http://www.youtube.com/watch?v=9UxU0ELgYO
A
Convection
http://www.youtube.com/watch?v=oFRMvVNia00
&feature=relmfu
Radiation
http://www.youtube.com/watch?v=RmFJOVOia_4
&feature=relmfu
Condensation/evaporation
http://www.youtube.com/watch?v=QjSfIDARTik&
feature=related
Pascal’s Principle. Liquid pressure depends only on the height
and weight density of the liquid and not the shape of the
vessel or total volume of a liquid.
21


Pascal's law states that pressure exerted anywhere in a
confined incompressible fluid is transmitted equally in all
directions throughout the fluid such that the pressure ratio
(initial difference) remains the same.
Pascal’s Law states that when you apply pressure to
confined fluids (contained in a flexible yet leak-proof
enclosure so that it can’t flow out), the fluids will then
transmit that same pressure in all directions within the
container, at the same rate.
The simplest instance of this is stepping on a balloon; the
balloon bulges out on all sides under the foot and not just
on one side. This is precisely what Pascal’s Law is all about
– the air which is the fluid in this case, was confined by the
balloon, and you applied pressure with your foot causing it
to get displaced uniformly.

Cohesion and adhesion
◦ The attractive force between like
molecules is cohesion.
◦ The attractive force between unlike
molecules is adhesion.

The shape of the meniscus depends
on the relative strengths of adhesion
and cohesion.
H20: Adhesion > Cohesion
Mercury: Cohesion > Adhesion


H20
Mercury
23




Cohesion: Water is attracted to water
Adhesion: Water is attracted to other substances
Adhesion and cohesion are water properties that affect every
water molecule on earth and also the interaction of water
molecules with molecules of other substances. Essentially,
cohesion and adhesion are the "stickiness" that water
molecules have for each other and for other substances.
The water drop is composed of water molecules that like to
stick together, an example of the property of cohesion. The
water drop is stuck to the end of the pine needles, which is
an example of the property of adhesion. Notice I also threw in
the all-important property of gravity, which is causing the
water drops to roll along the pine needle, attempting to fall
downwards. It is lucky for the drops that adhesion is holding
them, at least for now, to the pine needle.
http://www.youtube.com/watch?v=VHnFMPxteGo

Liquid to vapor phase changes
◦ Boiling – heating a liquid to a temperature at which
its vapor pressure equals atmospheric pressure.
◦ Saturation – equilibrium condition in which a gas
holds all the water vapor molecules that it can.
◦ Dew point – temperature at which the water vapor
in a gas begins to condense back into a liquid.
◦ Evaporation – when water enters its gaseous state
at a temperature below its boiling point.
25



Evaporation: Heat is taken from the surrounding air by the liquid via convection
thereby cooling the air
This heat transfer increases the KE in the liquid thus more molecules will have
sufficient energy to escape from a liquid to a gaseous state, and vaporize.
If air temperature increases, KE increases and more evaporation occurs.
Condensation (conversion
from a gas to a liquid) is the
opposite of evaporation
The hygroscopic humidifier (artificial nose)
traps the condensation from the patient’s
exhaled gas and re-humidifies the dry
incoming air on inhalation
http://www.youtube.com/watch?v=Oi0ECrYNtS8
http://www.youtube.com/watch?v=Jo5B4MKQAfQ

Vaporization: the change of matter from a liquid to a
gaseous form

Water vapor pressure – the direct measure of the
kinetic activity of water vapor molecules

Reducing the pressure above a liquid lowers its
boiling point. Ex. water boiling in mountains




When a gas is in contact with a liquid, and is in equilibrium (saturated)
with the liquid, the partial pressure of the gas is a function of
temperature. The one gas to which this applies in a normal respiration is
water. The lungs and airways are always moist, and inspired gas is
rapidly saturated with water vapor in the upper segments of the
respiratory system. The temperature in the airways and lungs is almost
identical with deep body temperature (approximately 37°C); at this
temperature water vapor has a partial pressure of 47 mmHg. (Note that
the gaseous form of a liquid frequently is termed a "vapor").
Using the value of 47 mmHg, we can calculate partial pressure of oxygen
and nitrogen in inspired air, after the gas mixture becomes saturated
with water vapor in the upper airway (so-called tracheal air):
Ptotal = 760 mmHg
PH20 = 47 mmHg
--713 mmHg for remaining inspired gases (21% O2 and 79% N2)
PO2 = 0.21 · 713 = 150 mmHg
PN2 = 0.79 · 713 = 563 mmHg


That is, since water vapor partial pressure
must be 47 mmHg in a saturated gas mixture
at 37°C, the total pressure remaining for the
inspired gases is only 760-47 or 713 mmHg.
The composition of this remaining gas is 21%
O2 and 79% N2, giving the partial pressures
indicated above which is then substrated by
the partial pressure of PaCO2 (PACO2, is a
product of the amount of CO2 diffused into
the lung)
PAO2 = FIO2 (Pb-PH2O) – (PaCO2/0.8)
 Absolute humidity: the actual content or water vapor present
in a given volume of air
 Relative humidity: the actual water vapor present in a gas
compared with the capacity of that gas to hold the vapor at a
given temperature
 If the water vapor content of a volume of gas equals its capacity,
the relative humidity of the gas equals 100%

Both are essential in effective ventilation.

Prevents drying of airway mucosa and
irritation.

Various respiratory care devices are used to
ensure adequate humidification of inspired
gases.

http://www.youtube.com/watch?v=CL5cgX
wKUXc

The NOSE is the bodies natural humidifier and
filter, when bypassed we must use a artificial
humidifier

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








Vapor pressure – Pressure water as a vapor or gas exerts and is part of the
total atmospheric pressure. Water vapor pressure in the lungs exert 47
mmHg
Absolute Humidity – the actual amount (in mg./l) of water vapor in the
atmosphere
Relative Humidity – the percent of water vapor in the air as compared to the
amount necessary to cause saturation at the same temperature.
% Body Humidity – the relative humidity at 37 degrees Celsius
Humidity Deficit – the amount of water vapor needed to achieve full
saturation at body temperature (44 mg/l - A.H)
Isothermic Saturation Boundary – At or just below carina (end of trachea) The
point at which inspired gases are fully 100% saturated and warmed to body
temperature (44 mg/L at 37oC)






Uses of Humidity therapy
Humidification of inspired gases
Thinning of bronchial secretions
Sputum induction
Solutions Used
 Sterile water used in humidifiers and continuous
nebulizers (Hypotonic)
 (Normal) Isotonic saline (.9% Na) with (Aerosol /
Medicine) Treatments
 Hypertonic saline (10%) (for sputum induction)
0
A gas is flowing thru a ventilator circuit at 50 C with a
relative humidity of 100%. As it flows thru the tubing it
0
is cooled to 37 C by the surrounding ambient
temperature of the room.
What effects will occur within the tubing? What will
occur to the ambient temperature of the air surrounding
the tubing?
1.
Condensation will occur on the inside surface of the
tubing as the water vapor reaches its dew point
2.
There will be visible droplet formation when dew point
is reached
3.
There will be warming of the adjacent air due to
convection


Critical temperature:
o The temperature reached in which gaseous
molecules cannot be converted back to a liquid,
no matter what pressure is exerted on them.
o The highest temperature at which a substance
can exist in a liquid state.

Critical pressure:
o The critical pressure of a substance is the
pressure required to liquefy a gas at its critical
temperature.


Gases can be converted to liquids by
compressing the gas at a suitable
temperature.
Gases become more difficult to liquefy as the
temperature increases because the kinetic
energies of the particles that make up the gas
also increas
Gas
Liquid


The critical temperature of a substance is the
temperature at and above which vapor of the
substance cannot be liquefied, no matter how
much pressure is applied.
Every substance has a critical temperature.
substance
critical temperature (oC)
NH3
132
O2
-119
CO2
31.2
H2O
374

Tubes containing water at several
temperatures. Note that at or above 374oC
(the critical temperature for water), only water
vapor exists in the tube.

The critical pressure of a substance is the
pressure required to liquefy a gas at its
critical temperature. Some examples are
shown below.
substance
critical pressure (atm)
NH3
111.5
O2
49.7
CO2
73.0
H2O
217.7
Water boils at 100 C and has a critical temperature
of 374 C.
 Oxygen has a boiling point of -183 C and a critical
temperature of about -119 C
Below -183 C, oxygen can exist as a liquid.
Above – 183 C, liquid oxygen becomes a gas.
Above 217 atm, and a temperature of 374 C gaseous
water cannot be converted back to a liquid no matter
how much pressure is added


Here, the liquid O2 is allowed to exceed its critical
temp & convert to gas.

Adding heat to a thermometer
changes its physical properties.

A mercury (nonelectrical thermometer) expands or
contracts as temp. changes.

A thermistor (electrical thermometer) operates by
the electrical resistance of metal changing
with changes in temp. As the temp.
increases, resistance to current flow
decreases and is shown as an increased
temp. reading
Viscosity
 The internal force that opposes the flow of fluids (equivalent to
the frictional forces between solid substances)
 The greater the viscosity, the greater the opposition to flow
 The stronger the cohesive forces, the greater the viscosity
46
Surface Tension
 A force exerted by like molecules at a liquids surface
 For a given liquid, surface tension varies inversely with
temperature
 Surface tension, like a fist compressing a ball, increases the
pressure inside a liquid drop or bubble
 The smaller the bubble, the greater the inflation pressure
 Inflation pressure can be lowered if surface tension is lowered
 The smaller the bubble, the greater the surface tension
 When connected, small bubbles tend to empty into larger
bubbles
 ETOH has a low surface tension and
is used to treat pulmonary edema
47


http://www.youtube.com/watch?v=u5AxlJSiE
Es
The pressure difference between the inside and outside of a
bubble depends upon the surface tension and the radius of
the bubble. The relationship can be obtained by visualizing
the bubble as two hemispheres and noting that the internal
pressure which tends to push the hemispheres apart is
counteracted by the surface tension acting around the
circumference of the circle.

The amount of net pressure required for
inflation is dictated by the surface tension
and radii of the tiny balloon-like alveoli.
Capillary Action
 A phenomenon in which a liquid in a small tube moves
upward, against gravity
 Involves both adhesive and surface tension forces
 Small capillary tubes create a more concave meniscus and thus
create a greater area of contact with the liquid along its glass
surface.
 The strong adhesive force of the liquid to the glass coupled
with the surface tension properties of the liquid combine to
cause the liquid be pulled upward.
 Liquid will rise higher in tubes with smaller cross-sectional
areas
 http://www.youtube.com/watch?v=wOOY1szbcX4
51
Temperature
 Temperature Scale Calculations:
oConversion of Celsius to Kelvin: °K = °C +273
oConversion of Fahrenheit to Kelvin: °K = °F + 460
oConversion of Celsius to Fahrenheit: °F = (9/5 x °C) + 32
•(°C x 1.8 ) + 32
oConversion of Fahrenheit to Celsius: °C = 5/9(°F – 32)
•(°F – 32) divided by 1.8
52
Temperature
 Temperature Scale Calculations:
oConversion of Celsius to Kelvin: °K = °C +273
oConversion of Fahrenheit to Kelvin: °K = °F + 460
oConversion of Celsius to Fahrenheit: °F = (9/5 x °C) + 32
•(°C x 1.8 ) + 32
oConversion of Fahrenheit to Celsius: °C = 5/9(°F – 32)
•(°F – 32) divided by 1.8
53
First-year students at Med School were receiving their first Anatomy
class with a real dead human body. They all gathered around the surgery
table with the body covered with a white sheet.
The professor started the class by telling them: "In medicine, it is
necessary to have 2 important qualities as a doctor. The first is that
you not be disgusted by anything involving the human body." For an
example, the professor pulled back the sheet, stuck his finger in the EYE
of the corpse, withdrew it and stuck his finger in his mouth."
“Go ahead and do the same thing," he told his students. The students
freaked out, hesitated for several minutes, but eventually took turns
sticking a finger in the EYE of the dead body and sucking on it.
When everyone had finished, the Professor looked at them and told them,
"The second most important quality is observation. I stuck in my Middle
finger and sucked on my index finger. Now learn to pay attention to your
patients, their life may depend upon it."
Gas Laws

During mechanical ventilation, volumes,
pressures, flows & the temperature of delivered
gas are routinely manipulated to better match
the patient’s condition.
=
Force
Area
Caused by collision of gas molecules with
solid or liquid surfaces.
 Measurements reported in psi, mmHg, torr,
cmH2O, and kPa.

diffusion – the movement of molecules
from areas of high concentration to areas of
lower concentration
 Gaseous
 Gas
pressure
◦ All gases exert a pressure.
◦ Gas pressure in a liquid is known as gas
“tension.”
◦ Atmospheric pressure is measured with a
barometer.
58
Components of a mercury barometer
59

Gas pressure (cont.)
◦ Partial pressure = the pressure exerted by a single
gas in a gas mixture
◦ Dalton’s law – the partial pressure of a gas in a
mixture, is proportional to its percentage in the
mixture. So the greater percentage that a gas
occupies in a mixture, the greater its partial pressure

Solubility of gases in liquids (Henry’s law)
◦ The volume of a gas dissolved in a liquid is a function
of its solubility coefficient and its partial pressure.
◦ Solubility coefficient: The volume of gas dissolved per
unit volume of liquid at standard atmospheric pressure
and at a specified temperature
60

Boyle’s Law: The volume that a gas occupies when it
is maintained at a constant temp. is inversely
proportional to the absolute pressure exerted on it.
http://www.youtube.com/watch?v=J
_I8Y-i4Axc
http://www.youtube.com/watch?v=
eYeRA--Xq0E&feature=related
Body box plethesmography uses Boyle’s law
to determine the volume of air remaining in
the lungs after a full expiration. This is used
to determine Residual Voume, Total Lung
Capacity and Functional Residual Capacity
A factor in breathing:
 For inhalation to occur,
1. Diaphragm muscle flattens/moves down
2. Increases the volume of the chest cavity
3. Pressure within the chest decreases
4. Air pressure within the lung is now less than
atmospheric air pressure
5. Air flows into your lungs.

Charles Law: If pressure remains constant,
the volume of a gas varies directly with the
temperature, expressed in Kelvin.

As the temperature increases, the
volume of the gas will increase.

As the temp. decreases, the
volume will also decrease.

In both parts of this diagram the gas is at the same
pressure, as the temperature increases, the volume
of the gas also increases
If the gas expands exponentially, the kinetic energy
will also increase to the same degree
 http://www.youtube.com/watch?v=bWhs5L_gBTI



APPLIED IMPORTANCE:
Temperature also plays a role in the solubility of a
gas in a liquid. As the temperature is increased
the solubility of a dissolved gas is actually
decreased.
Clinical example:
o When an ABG is iced, the temperature of the
plasma decreases. This decreases the amount of
oxygen that can be displaced off the RBC and
dissolved into the solution

Gay-Lussac’s Law: “With volume
remaining constant, pressure and
temperature are directly related”

Example:
◦ Drive to Las Vegas what happens to tire
pressure? What is constant? What varies?
P 1 x V1
T1


P 2 x V2
T2
States: “The state of an amount of gas is
determined by its pressure, volume, and
temperature”
=
The absolute pressure of a gas is inversely related
to the volume it occupies & directly related to its
absolute temp.



It describes the macroscopic behavior of gases
when any or all of the variables change
simultaneously.
It is useful in determining pressure, volume or
temperature corrections in arterial blood-gas
measurements and during PFTs.
http://www.youtube.com/watch?v=QcAQ8GEHE
mM&feature=related

States: “The sum of the partial pressures of a

Example:
gas mixture equals the total pressure of the
system and that the partial pressure of any
gas within a gas mixture is proportional to its
% of the mixture”.
OXYGEN = 21%
NITROGEN = 78%
TRACE GASES = 1%
100% Atmospheric
At 100% atmospheric,
these gases exert a pressure
of 760mmHg at sea level
Denver, CO
 640 mm Hg x 21%
 = 134 mm Hg

Seattle, WA
 760 mm Hg x 21 %
 = 152 mm Hg


PO2 = (PB – PH2O) (FIO2)
PO2 = (760 mm Hg – 47 mm Hg) (0.21)
PO2 = 150 mm Hg

States “the rate of diffusion of a gas through a liquid is
directly proportional to its solubility coefficient and inversely
proportional to the square root of its density”.

Describes the diffusion rate of one gas into another gas.

Gram molecular weight equals the number of particles in
a given amount of matter.
o
Molecular weight of CO2 = 44.01
Molecular weight of Oxygen = 31.99
CO2 diffuses 20x faster than O2

Describes the diffusion rate, or dissolving of
a gas molecules into liquid.

It states “For a given temperature, the rate of

http://www.youtube.com/watch?v=8yU5y-cFXoo
a gas’s diffusion into a liquid is proportional
to the partial pressure of that gas and its
solubility coefficient”.
Relates to the solubility of gases, such as oxygen into
and carbon dioxide out of the blood.
 http://www.youtube.com/watch?v=L41KhBPBymA
 It is known that 0.023 mL of oxygen dissolve in every
milliliter of blood at a temp. of 37 C and 1 atm of
pressure.


Oxygen and Carbon Dioxide transport can change
significantly with changes in body temperature.

The normal Pa02 at 37°C is approx. 80-100 torr. As a
patient’s temperature rises from 37°C to 39°C, Pa02
increases to 110 torr due to the increased solubility.
Likewise PCO2 increases 10% from 40 to 44 torr.
3.
States: The flow of a gas across a semi-permeable
membrane into a membrane fluid phase is directly
proportional to:
The surface area available for diffusion,
the partial pressure gradient between the two
compartments &
The solubility of the gas.
4.
http://www.youtube.com/watch?v=Hmfnolr47Zw

1.
2.
Alveolus
CO2
O2
O2
O2
CO2
Pulmonary
capillary
O2

When a fluid flows through a tube of uniform diameter,
pressure decreases progressively over the tube length.

As fluid passes thru a constriction, the pressure drop is
much greater
Jet Entrainment
Source Gas
Area of negative pressure

States: “The pressure drop that occurs as the fluid flows
thru a constriction in the tube can be restored to the
preconstriction pressure if there is a gradual dilation of the
tube”.

http://www.youtube.com/watch?v=Wokswr_KHXQ

http://www.youtube.com/watch?v=Yj7jszHWlK8&feature=re
lated
Va = Flow before restriction.
Vc = Flow from entrainment plus driving flow.
Pa = Original lateral pressure
Pb = Falling lateral pressure at the restriction.
Pc = Restored lateral pressure passed the restriction.

The reduced pressure within the restriction may be used to
introduce gases (usually air) into a low-pressure region of
gas flow.
Fluid viscosity, tube length and radius
determine resistance to flow.
 As the radius of a tube decreases by ½,
resistance increases 16 times.
 Increased resistance to flow can be caused by
a decreased airway size secondary to an
increase in airway secretions, bronchospasm,
intubation, etc.

http://www.youtube.com/watch?v=
-jyBY726np4


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
Poiseuille’s Law: the law that the velocity of a liquid
flowing through a capillary is directly proportional to the
presence of the liquid and the fourth power of the radius
of the capillary and is inversely proportional to the
viscosity of the liquid and the length of the capillary.
The variables in Poiseuille’s Law are:
The driving pressure gradient (the heart pumping)
Viscosity of the fluid (a person who is anemic can affect
the viscosity of his/her blood)
Tube length (the veins and arteries)
Fluid flow (depending on the pressure and viscosity)
Tube radius (can be affected by clogged arteries)
And the constants (8 and 3.14)

Mucous plug removed from a patient’s airway.

The changeover from laminar to turbulent
flow depends on several factors including:
◦
◦
◦
◦
•
Fluid density
Viscosity
Linear velocity
Tubing length
In a smooth bore tube, laminar flow becomes
turbulent when the Reynolds Number > 2000
• Factors that favor turbulent flow include:
•
•
•
•
High gas velocity
High gas density
Low gas viscosity
Large tube diameter

Fluidics and the Coanda effect
o
Fluidics is a branch of engineering that applies
hydrodynamics principles in flow circuits.
o
The Coanda effect (wall attachment) is observed when
fluid flows through a small orifice with properly contoured
downstream surfaces.
90
91
 Basis
for fluidic devices
used in several
mechanical ventilators.
 Main
advantage: fewer
valves and moving parts
that can break.