Pressure
Week 3
Definition of Pressure
Defined as force (F) acting
perpendicularly to a surface area (A)
 P = F/A
 In the atmosphere the gas we breathe
(Nitrogen, Oxygen, Co2 and trace gases)
are in constant motion, creating kinetic
energy and FORCE applied against the
surface of the earth. This is the
atmospheric pressure

Atmospheric pressure




Atmospheric pressure is usually measured
by the height of a liquid in a closed,
evacuated tube as shown on pages 178-179
in the text book.
You have already learned the normal values
at sea level for one atmosphere in several
different pressure scales.
At altitudes above sea level the atmospheric
pressure is lower because there is less air
pushing down on the surface.
At sea level, 1 ATM = 760 torr, In Denver, 1
mile above sea level, 1 ATM = 680 tor.
Atmospheric pressure units

At sea level:
◦
◦
◦
◦
◦
◦
◦
760 mmHg
760 torr
1034 cmH2O
33 ft H2O
14.7 PSI
29.9 inHg
101.33 kPa
◦ *in respiratory we will use mmhg/torr
Positive pressure
Pressure applied that is above the current
atmospheric pressure
 In respiratory we do this all the time with
devices such as mechanical ventilators
 We do this to overcomes a resistance or
disruption in normal pressure gradients
between the atmosphere and the patients
lung

Negative pressure
"Negative" pressure is a gauge pressure that is
below atmospheric pressure. Since atmospheric
pressure measured in gauge pressure is 0, any
gauge pressure that is below atmospheric
pressure will be negative. It is very important to
remember that if you are measuring pressure on
an absolute scale, you can not have a pressure
that is less than 0 or a negative number.
 Negative is also called vacuum pressure
 We use Negative pressure when suctioning
patients.
 For example: a suction pressure of 60 mmHg is a
vacuum, or 60 less that the current atmospheric
pressure

The air is made up of molecules.
Gravity pulls the air molecules
toward the earth, giving them
weight. The weight of the air
molecules all around us is
called the air pressure.
Your weight is the result of
gravity pulling your mass down
on the bathroom scales. Note
that weight has units of a force,
such as pounds.
High altitudes = lower pressure
Low altitudes = higher pressure
Atmospheric pressure

Air pressure can be thought of as the
column of air rising above us. As we go
up in altitude, we get closer to the top of
the column. Thus there are fewer
molecules of air above us to be pulled
down by gravity, so the air “weighs” less.
Therefore, pressure always decreases as
one goes up.
Atmospheric Pressure
Gas pressure
depends on both
density and
temperature.
Adding air
molecules
increases the
pressure in a
balloon.
Heating the air
also increases
the pressure.
Air pressure is
equal in all
directions.
Because air is a fluid, force
applied in one direction is
distributed equally in all
directions. Thus the downward
pull of gravity on air molecules
produces air pressure in all
directions.
Pressure = force per unit area
As
elevation
goes up
Barometric
pressure
goes
down.
This is an inverse relationship.
A Barometer
is
used
to
measure
air
pressure.
Torricelli’s barometer
used a glass column
suspended in a bowl of
mercury. The pressure
of the air molecules
pushed the mercury up
into the glass tube.
The weight of the mercury in
the tube was equal to the
weight of the air pressing
down on the mercury in the
dish.
As
atmospheric
pressure
increases…
The mercury in
the tube rises.
The Mercury Barometer
Good:
Bad:
•Simple to construct
•Glass tube is fragile
•Highly accurate
•Mercury is very toxic!
Although mercury has been used for hundreds of years, its toxic effects
have only been fully realized in the last few decades. Students should
NEVER handle mercury or broken mercury thermometers or
barometers. Mercury should also never be thrown in the trash or washed
down the drain, since it moves easily up the food chain from fish to
humans.
The Aneroid Barometer
•No fragile tubes!
•No toxic chemicals!
•No batteries!
•Never needs winding!
MILLIBARS
An aneroid barometer
uses a cell which has
had most of the air
removed.
As the air pressure
around the cell
increases, it presses
on the cell, which
causes the needle to
move.
Television weather forecasters usually give barometric
pressure in inches of mercury. However, meteorologists
measure atmospheric pressure in millibars.
Most aneroid
barometers have a
needle which can be
set to remember the
previous reading.
Atmospheric Pressure
Although pressure varies with altitude,
the concentration of gases do not change.
 Meaning there is 21% O2 and 78%
Nitrogen everywhere in the atmosphere.
 The difference is the pressure. Less
pressure= less pressure change from in
the lung to the outside.
 High altitudes = increased work of
breathing

Changing Pressure
A rising barometer = increasing air pressure.
This usually means:
Rising barometer readings indicate that a high
pressure system is approaching. Higher
atmospheric pressure is usually associated
with fair weather and clearing skies.
Changing Pressure
A falling barometer = decreasing air pressure.
This usually means:
Falling barometer readings usually
indicate the approach of an area of low
pressure. Low pressure readings are
usually associated with storm systems.
Tornadoes and hurricanes can produce
very low barometric readings.
Air Movement and Flow
6-1 flow from areas of high
Fluids (air and water)
pressure to areas of low pressure.
 Change in pressure across a horizontal distance
is a pressure gradient.
 Greater the difference in pressure and the
shorter the distance between them, the
steeper the pressure gradient and the high
the velocity of flow
 In the lung, flow travels to the alveoli based
on the diameter of the airway and also the
pressure gradient. We will learn about this in
another lecture

Pressure gradient in lung
speed, velocity, and flow.

Speed is a measurement of an object’s movement
in units of distance or length /time, (ie cm/sec,
miles per hour).

Velocity is the speed of an object with its
direction at a given instant, (ie 50 miles/hour
South).

Fluid flow is a special kind of velocity expressed in
units of volume/time, ie ml/sec, l/min. Remember
that a fluid is a substance capable of flowing ---a
gas or liquid.
Kinetic Theory of Matter

The Kinetic Theory of Matter is the
theory that all molecules are in constant
motion resulting in kinetic energy. This
theory applies to all three states of
matter -- solids, liquids and gases.
States of Matter
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
States of Matter
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
Physical properties of
a solid:
 Possess
the least amount of KE
 Mostly
Potential Energy in
intermolecular forces holding
particles together
 Can
maintain their volume &
shape
Physical properties of
a liquid:
 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
Physical properties of
a gas:
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…)

Change of State
 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
38
Change of State (cont.)

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
39
Change of State (cont.)
 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
40
Heat Transfer







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&f
eature=related
Change of State (cont.)
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.
42
Pascal's law
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.
Change of State (cont.)

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
44
Cohesion and Adhesion




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
Change of State (cont.)
 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.
46
The Gas Law
Ideal Gas follows kinetic molecular theory, made up of large
number of molecules that are in rapid random motion following
perfect elastic collitions losing no momentum
 How the Kinetic Molecular Theory Explains the Gas Laws

 The pressure of a gas results from collisions between the gas particles
and the walls of the container.
 Each time a gas particle hits the wall, it exerts a force on the wall.
 An increase in the number of gas particles in the container increases the
frequency of collisions with the walls and therefore the pressure of the
gas.
 Avogadro's Hypothesis
 As the number of gas particles increases, the frequency of collisions with
the walls of the container must increase.
 This, in turn, leads to an increase in the pressure of the gas.
 Flexible containers, such as a balloon, will expand until the pressure of
the gas inside the balloon once again balances the pressure of the gas
outside.
 Thus, the volume of the gas is proportional to the number of gas
particles.
The Gas Laws

Charles Law
◦ The volume of a gas increased with the temperature
◦ The volume of a given amount of dry ideal gas is directly
proportional to the Kelvin Temperature provided the amount of gas
and the pressure remain fixed.
◦ When we plot the Volume of a gas against the Kelvin temperature it
forms a straight line.
◦ V1 / T1 = V2 / T2

Boyle’s Law
◦ the product of the pressure and volume are observed to be nearly
constant.
◦ The product of pressure and volume is exactly a constant for an
ideal gas.
◦ p * V = constant
WATER VAPOR
(9.8oC)