25th April
Physiology #2
Pulmonary volumes & capacities
Dr. Nayef AL-Gharaibeh
KHALED MOUSA BACHA
We will start this lecture by explaining an important concept from the previous one:
Intrapleural pressure is equal to -5 cmH2O at resting state, while during inspiration phase it
will be around -8 cmH2O; so why there is an increase in this pressure negativity?
There are two forces that affect the pleural space
which are:
1- The chest expansion and diaphragm contraction
which will pull the parietal layer "out and down"
creating more negativity in the pleura (the red thick
arrow).
2-Air filling force in the lungs that would push the visceral layer in the same direction which
will decrease pleural suction (the blue thin arrow).
If both forces have the same rate, there won't be a change in the pleural resting pressure, but
the rate of chest expansion is more than air filling rate in the lungs which will validate
the creation of more negativity in this cavity (e.g. the net pressure is equal to the negative
pressure -suction force- that is created by chest expansion minus the positive pressure that is
created by air filling in the lungs),
Like if we close the glottis and try to expand the chest by doing forceful inspiration, the
parietal layer will move while there is no air filling in the lungs which means that the
visceral layer won't move, and that will create much more negativity in the pleura ( -15 or -16
cmH2O).
Now let's join this biophysical concept with another
one:
As this curve illustrates, intra-alveolar pressure starts
from zero, then decreases to reach -1 cmH2O, after
that it starts to increase in the middle of inspiratory
phase to reach zero again, while intrapleural pressure
keeps decreasing through the whole phase. Why?
At the beginning of inspiratory phase -which is a
suction force-, a negative pressure will be created
due to inspiratory muscles contraction, and this
negative pressure will appear at the curve (from zero
to -1) but in the middle of inspiration there is another
force which comes from the air filling of the alveoli
which is positive pressure, now the airflow filling the alveoli force is more than the suction
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force of the chest muscles, so the pressure will go up instead of keep decreasing, this is
similar somehow to intrapleural pressure changes, but the pleural cavity is enclosed
compartment which means that there is no air filling inside the pleura whereas the pushing
force comes from the lungs!!!
Now let's explain an another concept from this curve, the "Transpulmonary Pressure"
Obviously it’s the difference between the
intra-alveolar pressure and the intraplural
pressure.
 At the resting state – the glottis is
opened and before the inspiration = 0 – (-5) = 5 cmH2O
 At the middle of inspiration phase
= (-1) – (-6.5) = 5.5 cmH2O
 At the end of inspiration
= 0 – (-8) = 8 cmH2O
When the transpulmonary pressure is equal
to 5 cmH2O, the lungs are holding "x" volume of air, and as the transpulmonary pressure
increases (from 5 to 5.5 then to 8 cmH2O), this indicates an increment in the air volume in
the lungs "more than x"
Because "transpulmonary pressure" definition: is the pressure which holds a certain volume
of air in the lungs.
The opposite scenario will happen during expiratory phase (from 8 to 5.5 then to 5 cmH2O),
so the volume inside the lungs will decrease. And whenever the air volume in the lungs got
fixed, the transpulmonary pressure won't change –by its definition-.
Pulmonary volumes and
capacities
This is another field of respiratory
physiology and it is important for the
evaluation of the respiratory
functions. As the tests like ECG which is done to evaluate
cardiovascular functions-, blood
pressure, temperature and chemicals
concentration in the plasma. There is a "pulmonary function test; PFT" which we can explore
the function of respiratory system through it.
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The machine used in such test is called a spirometer, the classical one looks like the one in
this picture, but we are going to use a high-tech, electrical and computerized one in the lab!
How does this machine work?
Spirometer consists of a chamber that is filled partially -at the edges- by water, and there is
another chamber (floating drum) that is floating on the water on the top of the first chamber
creating a cavity of air, at the base of it, there are two openings for tubes that the air will
move through them (in & out), if the air increased in this cavity (by expiration) the floating
drum will go up, while if it decreased (by inspiration) the drum will go down, and there is a
connection to this drum with a pen that will move against a paper that is wrapped around
rotating cylinder, this pen is going to draw lines on the paper according to the movement of
the drum.
Patient should close his nose and breathe from his mouth via the spirometer tube, during
expiration, the volume of the air in the cavity will increase leading to raise the drum and
drawing a line up, and the opposite thing will happen during inspiration, this lines will be
proportional to the air volume inside the cavity, and because the cylinder is rotating, the lines
will be drawn as waves making a "Spirograph" (while they would appear as overlapped lines if the
cylinder was a static one)
First of all, the lungs are floating in the chest with a certain volume of air that’s always
present in the lungs -the first breathe outside the uterus after the delivery is the hardest one in
our entire life (excluding pathological situations) because it inflates the lungs, after that a
fixed amount of air will remain in the lungs making the breathing process much easier!Under a resting condition, we breathe normally and quietly because we do not need too much
oxygen, while upon an exposure to any activity or stressful conditions we will start to
breathe forcefully. Actually, we are able to breathe forcefully in a voluntary way, without a
stress condition or doing exercises! And by that we can measure the different volumes inside
the lung.
TV = Tidal volume (500ml)
IRV = Inspiratory reserve volume
(3,000 ml)
IC = Inspiratory capacity (3,500
ml)
ERV = Expiratory reserve volume
(1,000 ml)
RV = Residual volume (1,200 ml)
FRC = Functional residual
capacity (2,200 ml)
VC = Vital capacity (4,500 ml)
TLC = Total lung capacity (5,700
ml)
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Now let's talk about these different pulmonary volumes:
 Tidal Volume
The volume of air that we inspire or expire during a resting state with a normal quiet
breathing is called the Tidal Volume, we can approach it by asking the patient to breathe
normally in the spirometer and that will be reflected as a small wave on Spirograph, the
height of this wave represents the TV.
The average value of TV is 0.5 L, and for females it's less than that!
 Inspiratory reserve volume
Now, can we take a forceful inspiration after a normal one?
Yes, because you are trying to do it now and it really works! actually that’s done by sending
signals by an action potential from the brain to all the muscles of inspiration (external
intercostal muscles, diaphragm and accessory muscles) to contract, at first we inspire the half
liter which is the TV, after that by increasing the contraction, chest expansion and the suction
around the lungs, we will inspire much more than the TV, this inspired volume which
entered after the TV is called the Inspiratory reserve volume.
Its value is 3 L, which is a gift from god to take the amount of the oxygen that we need by
increasing the volume of air in our lungs up to 3 liters on the top of TV upon demand!
So physiologically, the human body is able to accommodate oxygen demand 6 times more
than that required amount at resting state, which is a safety factor.
 Expiratory reserve volume
We can approach it by doing a forceful expiration after we expel the TV by quiet expiration,
and that is done by the contraction of internal intercostal muscles and abdominal recti. This
extra amount of volume that's been expelled after the TV is called the Expiratory reserve
volume and it reaches up to 1000 ml (1 L) – it equals to 1100 milliliters according to
Guyton which is our reference Residual volume
Which is the volume that remains in the lungs after the maximum ability for expiratory
muscles contraction and reaching the point that we cannot expel more air; it equals to 1.2 L
Humans lose this volume after a death or in the pathological situations like penetration of the
lung layer or the chest layer which is called in medicine a "pneumothorax" (air inside the
chest); like when a sharp object penetrates the chest cavity (pleural cavity) turning
intrapleural pressure into zero, which means that there is no air inside the lung.
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Now let's explain these volumes in physiological terms:
Figure A represents the lungs after a forceful expiration (Minimal lung volume
-residual volume- at maximum deflation).
A
The continuous line in Figure B represents the volume of air inside the lungs
after a normal expiration (which is RV+ERV; 1000+1200 ml) it's called the
resting condition volume, and during normal breathing the volume will start to
go up and down between the continuous line and the dashed
B
one (± TV)
Figure C illustrates the volume inside the lungs after a
forceful inspiration, which is the maximum inflation of the
lungs (RV; 1200 ml + ERV; 1000 ml + TV; 500 ml + IRV;
3000 ml = 5700 ml)
Why is that important? Because sometimes we need more oxygen and we
are able to take it from IRV, other times we need to expel more CO2 from
the lungs and we are able to get rid of more CO2 through expelling the ERV
from lungs; and by that we get rid of the "contaminated" air and get a fresh
air as an ERV, TV and IRV leading to renewal of the air in our lungs.
C
Also having this amount of air in the lungs which is the "Functional residual capacity"
(RV+ERV) gives us beauty to our chests; the chest won't be a good looking if it's fully
collapsed after the expiration and fully inflated after the inspiration! And that meets the
function also, since the blood is continuously flowing through the lungs, there should be an
amount of air inside them to maintain the gas exchange function, so the blood can take up
oxygen gas even during the expiratory phase -from ERV and RV-.
To sum up:
Residual volume: the minimum amount of air remaining in our lungs after the maximum
forceful expiration.
Expiratory reserve volume: the maximum amount of air a person can expire by forceful
expiration after a normal expiration.
Tidal volume: the amount of air a person inspires or expires under a resting condition.
Inspiratory reserve volume: the maximum amount of air a person can inspire from the
atmosphere by forceful inspiration after a normal inspiration.
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Pulmonary Capacities
A capacity is a value that results from the summation of two or more volumes; like:
 Inspiratory capacity
Mathematically is TV + IRV (3.5 L)
Physiologically, it is the maximum amount of air a person can inspire by forceful inspiration
after a normal expiration.
 Functional residual capacity
Equals to ERV + RV (2.2 L)
And it is the maximum amount of air remaining inside the lungs after a normal expiration.
 Vital capacity
Equals to IRV + TV +ERV (4.5 L)
It is the maximum amount of air a person can expire forcefully after a forceful inspiration, or
the maximum amount of air a person can inspire forcefully after a forceful expiration.
And it's called a "vital" because it is vital for our life, since it is responsible for the renewal
of the air in the lungs by increasing the oxygen and decreasing carbon dioxide contents.
 Total lung capacity
The total value of all respiratory volumes; IRV + TV + ERV + RV (5.7 L)
All these capacities are measurable by spirometry except the FRC and TLC since both of
them contain the residual volume, which is unmeasurable by spirometer because it stays in
the lungs and could not be expelled out or inspired!
But there is a simple method called "Helium dilution method" to calculate FRC, RV and
TLC.
Helium dilution method
Helium is a nontoxic gas to be inspired, so we can use it.
A spirometer of known volume is filled with atmospheric air mixed with helium at a known
concentration (this is called the initial concentration of helium; Ci He). Before breathing from
the spirometer, the person expires normally. At the end of this expiration, the remaining
volume in the lungs is equal to the functional residual capacity. At this point, the subject
immediately begins to breathe from the spirometer (with a closed nose), and the gases of the
spirometer mix with the gases of the lungs.
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As a result, the helium becomes diluted by the functional residual capacity gases and evenly
distributed between FRC and the spirometer, and the volume of the functional residual
capacity can be calculated from the degree of dilution of the helium, using the following
formula: where FRC is functional residual capacity, CiHe is initial concentration of helium in
the spirometer, CfHe is final concentration of helium in the spirometer, and ViSpir is initial
volume of the spirometer.
𝐹𝑅𝐶 = (
CiHe
− 1) ViSpir
CfHe
Once FRC is determined, we can calculate the residual volume by this equation:
RV = FRC – ERV
Also the total lung capacity could be calculated through this one:
TLC = FRC + IC
*Actually, we can predict the FRC without these calculations, like if we try this method on a
child female, and an adult male, the final concentration of the helium in spirometer would be
much less for the adult male because the FRC is much larger, and the opposite thing for the
child female.
Finally, the dead space
It’s a volume on top of all the previously discussed ones, that is cannot be measured by
simple spirometry (it needs a special method to be measured by spirometer).
It is defined as the amount of air located in the air conducting channels; trachea, bronchi and
bronchioles, where no gas exchange can happen (so called "dead space"), because gas
exchange can occur only in the respiratory spaces; respiratory bronchioles, ducts and alveoli.
there are two types of the dead space:
 the anatomical dead space: is the
volume of air located in the
bronchial tree (150 ml)
it can be measured by a simple method
as following:
as we know, atmospheric air contains an
oxygen, nitrogen, and a small amount of
carbon dioxode, and the lungs contains
the same gases!
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at first the object has to expire forceflly then take a deep breath from a pure oxygen, allowing
this oxygen to fill the lungs and bronchial tree (pulmonary spaces as well as the dead space),
after that he have to expel the air in the spirometer that is connected with a device to measure
the nitrogrn concentration, the first portion will be a pure oxygen (which was in the bronchial
tree – dead space- since there is no gas exchange there), after that the next portion of air will
contains carbon dioxide and nitrogen due to gas exchage in the pulmonary spaces and
mixing of the pure oxygen with pre-existing gases in the lungs. and the nitrogen
concentration in this portion will be detected by the device that records and plotts nitrogin
concentration to make a curve, and by that we can calculate the dead space volume (the first
portion which consists of pure oxygen).
 The physiological dead space: some area in the respiratory spaces which has a
ventilation but doesn’t have a blood flow, it's considerd as a dead space because it
doesn't exhibit any gas exchange , this space doesn't exist in healthy people, it might
exist in tall people or those who have low blood pressure at the upper parts of their
lungs (just 10-15 ml), but it becomes a significant volume in abnormal pathological
situations.
THE END!
Edited by: Cyrine katanani.
Done by: Khaled Mousa Bacha.
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