9. Lungs and Breathing
This material is in Chapter 7 of the text.
The primary function of the lungs is to supply the O2 required for the “combustion”
of the body’s fuel and then to remove the resulting CO2. We will talk of the exchange
process below. The lungs also act as a heat exchanger for the body by warming and
moisturizing the cooler and dryer air we normally inhale. Of course the lungs are also the
source of the airflow we use to talk, cough, sneeze, whistle, etc. The lungs are normally
under the control of involuntary breathing controls. Under that control, adult males
breathe about 12 times per minute, while women about 20 and infants about 60
As during sound production, breathing is often under our voluntary control. We
inhale more and faster when speaking. While speaking, we spend about 80% of the
breathing cycle exhaling and speaking. There is a substantial amount of work done by
the lungs in moving air in and out while relatively little is actually converted to sound
power. The normal voice produces about 1mW (1/1000 watt). There are many other
times when breathing is clearly under voluntary control.
For the average resting male, the inspiration volume is about ½ liter, not to be
confused with the actual volume of the lungs. With about 12 breaths per minute, the
average male breathes about 6 liters of air per minute. The atmospheric air inhaled is
about 20% O2 and 80% N2 while the exhaled air is 80% N2, 16% O2, and 4% CO2. The
exhaled air is also saturated with H2O. We exhale about 0.5 kg (about 1 lb). of CO2
and about 0.5 kg of water each day. More on that later.
With about ½ liter/breath we take in about 1022 molecules of air per inhalation.
With the total number of molecules in atmosphere at about 10 44, we take in about 10-22 of
the earth’s atmosphere each time we breathe. So for each ½ liter (10 22) we take in, there
is on average at least one molecule of any ½ liter of the atmosphere at any prior time. If
you think about the 150 million breaths taken by Christ, we could expect that each of our
breaths has about 150 molecules that Christ breathed in His lifetime. Of course this
calculation can be made for just about anyone who lived long ago enough for the
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atmosphere to be thoroughly mixed. If you wanted to consider all the molecules that
made up any individual that died a long while ago, you could make the case that all of
their molecules eventually made their way into the atmosphere and we breathe molecules
of any of these individuals also. In fact our own bodies all have some molecules of
essentially anyone who lived a long time ago. Talk about recycling!!!
Air enters the body normally through the noise and the nasal passages. The hairs
in the noise first filter the air where relatively large dust and dirt are removed. The
air is moisturized and warmed before entering the trachea (the windpipe). It is
therefore a very good idea to breathe through your nose if you really have a choice. We
tend to inhale through our mouth when we need a lot of air in a hurry as during strenuous
exercise. While the air can enter faster, more irritants can get into the airways as well as
the airways drying somewhat by the rapidly moving, relatively dry air that has bypassed
the nose and sinus cavities.
The real business of the lungs occurs in the millions of alveoli at the end of each
air passage. These sacs are the interface between the air and the blood. The total
surface area of these sacs is about 80 m2, about half a tennis court!
Refer to Section 7.5 of the text.
How the Blood and Air in the Lungs Interact
The lungs offer little resistance to the flow of blood so the heart doesn’t need
to develop much pressure to push the blood into and through the lungs. Thus the right
side of the heart, that is responsible for pumping blood through the nearby lungs, is
smaller and less powerful than the left side that has the huge job of pumping blood
through the rest of the body. So while the pressure from the left side is of the order of
120 mm of Hg. the pressure from the right side going to the lungs is of the order of 20 mm
Hg or about 1/5 as large.
A very large amount of blood is in the lungs at any given moment, about 1/5
of the total blood supply. However, while a liter or so might be in the lungs, less than
10% of that, only about 70 ml, is in the capillaries where the gas exchange is taking
place. With so little of the blood in these capillaries, the gas exchange must take place
very rapidly, in fact, on average, a particular chunk of blood is only in the alveoli
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about 1 sec. In order to affect the rapid transfer, the walls of alveoli must be and are
extremely thin. With only 70 ml of blood spread over the total area of the alveoli area of
80 m2, the thickness of the film of blood is about 1 m, the thickness of a single red blood
cell. What this means is that every red blood cell in the capillaries of the alveoli is in
contact with the walls of a capillary in the alveoli. The very short distance over which
gas molecules must pass from the airside to the blood cell contributes to the very short
time over which the necessary gas exchange takes place.
Two processes are necessary to for the gas exchange. First, there must be a
good blood supply, a process called perfusion. The second is getting a good airflow
in and out, a process called ventilation. Fortunately about 90% of the lungs, the alveoli,
have both good perfusion and good ventilation. The remaining 10% are areas that only
have one of the two necessary processes. As far as the proper functioning of the lungs is
concerned, an obstruction in the blood supply or an obstruction in the air supply
can cause serious problems. A blood clot, a pulmonary embolism, can block or reduce
the blood supply and fluid in the lungs, pneumonia, smoking residue, etc. can reduce the
air supply. In short, the blood and the air really have to get very close and do it quickly.
The actual transfer of O2 and CO2 is a diffusion process. On the gas side of the
interface, the capillary wall in the alveoli, the gases must diffuse a small fraction of a
mm as they cross the air sack of the alveoli. Since diffusion depends upon the speed
and the mean-free-path of the particles through which they are diffusing, the O 2 and CO2
must diffuse in or out respectively through the predominately N2 gas. Indeed, with the
very small distance to travel, the passage can be made easily during the average time
stay of blood in the alveoli. As far as diffusing through the ultra thin capillary wall,
that takes far less time.
Under normal breathing, only a fraction of the total air volume in the lungs is
exhaled or inhaled in any single breath. Under normal breathing, we expel about 60% of
the air that was in the lungs and then inhale a similar volume of fresh air. Thus the
percentages of O2 and CO2 in the alveoli are not the same as normal air. In effect, about
30% old air is being mixed with the newly inhaled air. So expelled air has a slightly
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higher O2 and lower CO2 percentages than are in the alveoli. The expelled air of
course is lower in O2 and higher in CO2 then the atmosphere.
The diffusion process across the membrane wall is driven by concentration
differences of the molecules in question. The actual solubility of O2 in the blood is
quite small. The O2 requirements could never be satisfied by this small amount of
gas dissolved in the blood. However, the blood has some really efficient “blood
carrying freight cars”. These carriers are molecules of hemoglobin that chemically
combine with oxygen. In fact each red blood cell, which is passing single file through the
capillaries, can carry about 1 million molecules of O2. This means that about 1 liter of
blood can carry about 200 cm3 of O2 or about 1/5th of a liter of O2. Without the
hemoglobin, the same one liter of blood could only carry about 2.5 cm3 of dissolved O2 at
standard pressure and temperature.
The name of the game is bring O2 to the cells where it can participate in the
combustion process of food derivatives. The oxygen, from the oxygen-rich blood reaches
the cells by dissociating from the hemoglobin and diffusing into this intercellular fluid that
baths both the capillaries and cells. CO2, that has diffused out of the cells where it was
produced into the intercellular fluid. As CO2 diffuses into the blood, the CO2 stimulates
the release of O2 from the hemoglobin. Thus the oxygen, stimulated by the CO 2, is
dumped off where it is needed. The dissociation and diffusion processes depend upon
the gas concentrations in and around the cells. When the oxygen levels are very low
and CO2 levels high, a larger amount of oxygen is delivered. For non-working
muscles, the fluid around the cells is still quite rich in O2 since the demand for oxygen by
the cells is low and thus a relatively small amount of oxygen is delivered. The remaining
portion continues to circulate and a lesser amount is added into the blood stream by the
lungs. When the muscles are active, the local oxygen supply in the muscles is depleted
and the circulating blood delivers more. The O2 depleted blood is returned to the lungs
for a fresh load. The lower O2 concentrations in and around the cells that cause the
delivery of more O2 for the working cells is also accompanied by a higher CO2 level which
diffuses into the blood where a lower concentration exists.
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This dissociation process of O2 from the hemoglobin depends on the local
concentrations of CO2, the acidity and the temperature. These factors increase with
muscle activity thereby increasing the delivery of O2 to the muscles. Like O2, CO2 is also
transported by coupling itself to chemicals in the blood, and like the oxygen, a certain
concentration of CO2 remains circulating with the blood. This CO2 serves to regulate
breathing and therefore oxygen levels.
Carbon monoxide, CO, when inhaled, attaches itself to the O 2 attachment points of
the hemoglobin. In doing so, it dramatically reduces the blood’s ability to carry oxygen
and can therefore be fatal.
Measurement of Lung Volumes
With normal breathing, the lungs inspire and expire about ½ liter of air, the so-
called tidal volume. Thus there is always about a 2-liter reserve at the beginning or end
of a normal breathing cycle. As we become more active, the inspiration and expiration
volume increases to about a liter, leaving about 1 liter in reserve, an amount that will
remain even with a maximum effort to expel as much as we possibly can, called the vital
capacity. Under normal circumstances, the amount of air breathed in 1 minute is called
the respiratory minute volume. The maximum volume breathed in 15 seconds is the
maximum voluntary ventilation. A normal person can expire about 70% of the vital
capacity in ½ sec. increasing to almost all of it, 97%, in about 3 seconds.
The velocity of the expelled air can vary over a very wide range. The velocity
of a hard sneeze or cough can cause a velocity of the expelled air near the speed of
sound! Sometimes that’s what it takes to expel something that should not be in your air
When measuring these various lung capacities, it is important to provide a system
that captures the air expelled without creating a pressure that would stop the airflow. A
volume of air like any other mass will not accelerate (change its velocity) unless a force
acts on it. Recall Newton’s 2nd Law, F = m a = m (change in velocity per unit time). Thus
a volume of air will not move unless a force is acting on the volume. Consider the air in
your windpipe as illustrated below.
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Volume of Air
From the definition of pressure, force / area, the force acting on the left side
pushing to the right is p1 A while the force on the right side pushing to the left is p2 A. The
net force acting on the volume of air is the difference between these two forces. Thus the
air will move from the higher pressure side towards the lower pressure side. Not
surprising but we should always keep in mind that it takes a force to change the velocity
of a mass, whether it’s stopping a skull in a collision or accelerating air into or out of the
Occasionally someone may suggest that we can measure the volume of the lungs
by inflating a balloon and then measuring the volume of the balloon. This is not true.
Recall our demonstration of how much pressure you could muster by attempting to
exhale into a tube that was attached to a vertical water column. Once the water rose to a
height of about four feet, the pressure created at the base of the water column by the
weight of the water column equaled the maximum pressure of the person exhaling. At
that point, no further exhaling was possible regardless of how much air was moved. In
fact, with the size of the tube used, a very small volume of air was actually expelled
before the maximum pressure point was reached. So if you tried to inflate a balloon as
measure of your lungs volume, you would only be measuring the volume of air required in
that particular balloon to cause a pressure sufficient to stop the flow of air from your
lungs. What is required to measure the lung capacities during breathing cycles is a
device that will capture and measure the volume of air moved while maintaining a
zero gauge pressure in the measuring vessel. Recall a zero gauge pressure means
that the pressure in the vessel is at atmospheric pressure. Thus either a positive or
negative gauge pressure in the lungs is the only issue in driving the air out or in
respectively. We can make such a “weightless” piston by providing a counter weight
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equal in weight to the piston itself. The counter weight is connected to piston by a rope
and pulley system so the piston has an effective zero weight as the volume of air below is
changed. The piston must be weightless so that it does not create any pressure on the
volume of gas. If we attach a movable pen to the piston so we can record its motion we
will have a measure of the volume below the piston. This device is called a spirometer.
See figure 7.7 in the text.
We inhale and exhale by changing the air pressure within our lungs. The
control of the lungs is usually on “auto-pilot” and takes place without our having to think
about it. We can over-ride this automatic control and of course we do that all the time
for a variety of reasons. In order to talk or make other sounds we control the breathing to
make the airflow in an appropriate manner. We “hold” our breath to swim underwater. Of
course we can only override the autopilot as long as there is no higher priority for
You can only hold your breath as long as the body’s need for oxygen will allow. It
is impossible for you to hold your breath to the point of doing any significant damage to
yourself. You might perhaps momentarily faint but as soon as you did the autopilot would
start breathing and restore you. You probably all have see children try to get attention by
attempting this with the result that a parent (who has taken this course) will let nature take
Another example of the autopilot taking control is the situation of a particle of food
“tickling” your throat because of ingestion into the windpipe. The cough reflex will take
control to expel the potentially dangerous intruder. In a case like this, your lungs can
generate airflow velocities that are extremely high, a requirement to dislodge and eject
the offender. This high velocity is not only generated by a sudden increase in the
pressure in the lungs but by a constriction of the air passage. The constriction is caused
a decrease in pressure inside the tube because of the higher velocity. That decrease in
cross-section of the tube further increases the velocity of the air being forced through.
The decrease in pressure resulting from a higher velocity of a fluid is called the
Bernoulli principle. This is the same principle that allows planes to fly. The velocity of the
air flowing over the top of a wing is higher than the air flowing underneath due to the
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shape of the wing. The higher velocity over the top causes a lower pressure than the
slower air moving under so there is a net upward force that supports the weight of the
plane. In the air passage, a significantly higher velocity inside can cause the partial
collapse of the somewhat flexible air passage and thus reduce the cross-section that in
turn increases the velocity through the passage. Let’s see how this works.
It’s pretty safe to say that the amount of air flowing into one end of a pipe is
equal to the amount of air flowing out the other end or in fact past any cross-
section of the tube. Thus if a fluid is flowing through a pipe under the influence of a
pressure difference at each end, the flow through any cross-section of the pipe will be
inversely proportional to the cross-sectional area. So the flow is faster through the
narrower portions of the pipe. So a good cough, driven by a sudden, large pressure
increase in the lungs, can create a very high air velocity, particularly in a region where a
particle might already be constricting the cross-section of the tube. An example of this
relationship between the velocity and the cross-section is when you place your thumb
over the end of a garden hose to increase the velocity of the exiting water. A more
germane example that will be discussed later is the change in the blood’s velocity in
regions of blood vessels that are constricted by plaque in the vessels.
The breathing process In the normal breathing process, a very small pressure
difference between the lungs and the outside are required to move air in and out, a
pressure difference of perhaps 200 Pa (an inch or so of water as might be measured by
that water column we had in class). Recall that atmospheric pressure is about 100 KILO
Pascals, i.e. about 500 times larger. Atmospheric pressure can push water up about 33
feet if the pressure at the top of the tube is reduced to zero.
It is interesting to see how one might measure the pressure in the lungs. Recall
how blood pressure is measured using Pascal’s principle inside the closed volume
created by the blood pressure cuff around an arm making the pressure in the cuff equal to
the pressure in the vessel. For the closed chest cavity, the pressure in the lungs is equal
to the pressure elsewhere in the cavity. That pressure will tend to collapse the rather
flexible although muscular tube (the esophagus) passing through it. This tube is normally
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closed at both ends and therefore becomes a convenient place to measure the pressure
in the chest cavity.
The largest portion of the lung volume is bounded by the alveoli that are as
we have seen, extremely thin and elastic air sacks. They are like tiny balloons at the end
of each air passageway. Like any inflated balloon, they would deflate if they had the
opportunity to do so. The outer boundaries of the lungs are in contact with the inner
chest wall that by virtue of being held more or less in place, keep the lungs from deflating.
If however the space between the lungs and the chest wall were allowed to be open to
the atmosphere say by a puncture of the chest cavity, the lungs would indeed deflate by
virtue of the elasticity of all those air sacks. At the same time, the chest cavity would
expand as the surrounding muscles relaxed and air filled the space between the lungs
and chest cavity.
Under normal circumstances, the muscles around the chest cavity are maintaining
an inward pressure like a spring. The lungs themselves are being “held” inflated by the
inability of the sealed chest cavity to collapse. So the breathing is accomplished by
changing the volume of the chest cavity by movement of the lower boundary, the
diaphragm, and the expansion the chest walls themselves and the lungs. Refer to figure
7.20 page 171 for a somewhat more graphical description of the process. In order to
exhale, we relax the muscles in the diaphragm so it can bulge upward while the
elasticity of the lungs partially deflates the lungs as if it were a balloon. To inhale,
the muscles of the diaphragm contract, causing it to flatten thereby increasing the
volume of the chest cavity, creating the required negative pressure for inhalation.
Further we can contract “intercostal muscles in the chest wall that cause the chest walls
to expand. Inhalation can therefore take place with either the muscles of the diaphragm
or chest walls or both.
The maximum inspiration and expiration pressures are roughly + or – 100 cm. of
water. That + or – one meter is just about what we measured in our simple classroom
demonstration! I guess sometimes those demos really work!
Ref: Pages 160 -161
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The alveoli, with their incredibly thin, “wet” walls act more like soap bubbles than
balloons. The elasticity we have been speaking about results from the surface tension of
the fluid like membrane. The surface tension of a soap bubble is in fact the real force that
keeps the bubble in tact. It is the force that allows bugs to literally walk on water! If you
were to stick a straw in some soapy water and blow a bubble at the end of the straw
without allowing it to fly off as a closed volume, the bubble at the end of the straw would
collapse if given the opportunity to do so. Thus if you allowed air to escape from the
straw, the bubble would just collapse under the surface tension force that is acting to
minimize the amount of surface area. The surface tension in its attempt to minimize the
surface area is what causes soap bubbles to assume a spherical shape whenever
possible. The alveoli are therefore like little soap bubbles at the end of a tube. They
are trying to collapse but the negative pressure created by the chest cavity keep them
from doing so. Actually, the alveoli are coated with a substance called a surfactant
whose surface tension is not constant. As a result, the alveoli will collapse but as
they do, the surface tension decreases to the point that the residual air pressure
becomes equal to the pressure created by the lower surface tension. The alveoli
therefore collapse to about ¼ of their original size. Normal soap bubbles have a
surface tension that is more or less constant with volume and therefore given the chance,
will collapse to essentially zero volume. For a bubble, the higher the surface tension the
faster it will collapse. Bubbles expelled from lung tissues have been observed to last
literally for hours yielding the conclusion that these tiny bubbles must have a very low
Another aspect of the negative pressure in the chest cavity is the help it provides
to keep open the major blood vessel.veins, returning blood to the heart, the vena cava.
The returning blood pressure is only about 50 Pa, so the negative pressure of the chest
cavity makes it easier for this low pressure to keep that vein inflated and thereby allow
blood to return to the heart more easily.
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The work of breathing
In order to move air in and out of our lungs mechanical work must be done.
Recall, work is the product of force x displacement. Thus if a force is acting over some
distance, then work is done. In this case, the various muscles involved in the breathing
process do the work. As far as the work of moving the air, it is also a force x distance. If
we consider a tube with a quantity of air within, then the force is the pressure difference
end-to-end x the cross-sectional area. The distance D, over which this force acts, is the
distance the air volume is moved. So we can write
Work FD ( p1 p 2 ) AD
But A x D is the volume moved so the work done = p V where p is the pressure
difference across the volume V being moved.
In order to do this mechanical work we have to burn fuel in the muscle cells. The
overall efficiency of our breathing system is in the 5 – 10% range so we have to burn
about 10x the number of joules (calories) to get the needed mechanical work done. As
we have already seen, this 5 – 10% efficiency for our bodies is typical of most of the
mechanical work done by our muscles.
If the air path offers some resistance then the required p is higher then would
otherwise be expected. Thus if we have a stuffy nose, it takes more effort to breath
through our nose and we probably would start breathing through our mouths where the
air can flow more easily. Many things can add to the resistance experienced by the air.
Small diameter, long tubes tend to have much more resistance than short, fat tubes.
Obstructions or filtering elements also contribute to the resistance. Thus the hairs in the
nose, the relatively sticky linings of the nasal passages, there for the productive task of
cleaning the air before being inhaled into the lungs, do tend to increase the resistance to
airflow. So when we need to handle lots of air quickly, we usually breathe through the
mouth and bypass the relatively high resistance pathway through the nose. The price we
pay of course is getting cooler, less filtered air into our lungs. Besides the nasal
congestion caused by many common viruses, there are many effects that occur in the
lungs from a variety of diseases that impede the air in the lungs. Fluid in the lungs would
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certainly be a major factor in reducing airflow. The flexibility of the lungs also can
dramatically increase the difficulty of moving air into or out of the lungs. Smoking in
addition to the very toxic substances in puts into the blood supply, deposits and lines the
interior of the lungs with the black residue of the tobacco combustion process. This not
only makes the air handling more difficult, but also contributes to deterioration of lung
tissue reducing its overall effectiveness in providing oxygen to the blood. As we have
already discussed, the presence of carbon monoxide in the smoke greatly diminishes the
oxygen carrying capability of the hemoglobin.
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