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Chapter 7
Physics of Lungs and Breathing
Dr. Nabaa Naji
Ph.D in Medical Physics Science
Al-Mustansiriya Medical College
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Functions of Respiratory System:
The lungs perform other physiological functions in addition to exchanging O2 and
Co2. The lungs play a significant role in:
1. Keeping the PH (acidity) of blood constant.
2. the lungs play a secondary role in heat exchange and fluid balance of the body by
warming and moisturizing the air we breathe in (inspire)
3. Our breathing mechanism provide a controlled flow of air for talking, sneezing,
coughing, laughing. In addition, blocking the air passage generates increased
pressure for defecating and vomiting.
4. An important function of breathing apparatus is voice production. Voice is
produced by a controlled outflow of air from the lungs.
Breathing Rate:
We breathe about 6 liters of air per minute (this is also about the same volume of blood
the heart pumps each minute). Men breathe about 12 times per minute at rest, women
breathe about 20 times per minute, and infants breathe about 60 times per minute.
The Airways:
The Nose - Usually air will enter the respiratory system through the nostrils. The
nostrils then lead to open spaces in the nose called the nasal passages. The nasal
passages serve as a moistener, a filter, and to warm up the air before it reaches the lungs.
The hairs existing within the nostrils prevent various foreign particles from
entering. Different air passageways and the nasal passages are covered with a mucous
membrane. Many of the cells which produce the cells that make up the membrane
contain cilia. The cilia which are only 0.1mm long, have a waving motion that moves
mucus carrying dust and other small particles up the major airways. Each of the cilia
vibrates 1000 times a minute. The mucus moves 2 cm/min. Others secrete a type a sticky
fluid called mucus. The mucus and cilia collect dust, bacteria, and other particles in the
air. The mucus also helps in moistening the air. Under the mucous membrane there are a
large number of capillaries. The blood within these capillaries helps to warm the air as it
passes through the nose. The nose serves three purposes. It warms, filters, and moistens
the air before it reaches the lungs. You will obviously lose these special advantages if you
breathe through your mouth.
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Air Ways
Pharynx and Larynx - Air travels from the nasal passages to the pharynx, or more
commonly known as the throat. When the air leaves the pharynx it passes into the larynx,
or the voice box. The voice box is constructed mainly of cartilage, which is a flexible
connective tissue. The vocal chords are two pairs of membranes that are stretched across
the inside of the larynx. As the air is expired, the vocal chords vibrate. Humans can
control the vibrations of the vocal chords, which enables us to make sounds. Food and
liquids are blocked from entering the opening of the larynx by the epiglottis to prevent
people from choking during swallowing.
Trachea - The larynx goes directly into the trachea or the windpipe. The trachea is a
tube approximately 12 centimeters in length and 2.5 centimeters wide. The trachea is
kept open by rings of cartilage within its walls. Similar to the nasal passages, the trachea
is covered with a ciliated mucous membrane. Usually the cilia move mucus and trapped
foreign matter to the pharynx.
Bronchi - Around the center of the chest, the trachea divides into two cartilage-ringed
tubes called bronchi. Also, this section of the respiratory system is lined with ciliated
cells. The bronchi enter the lungs and spread into a treelike fashion into smaller tubes
called bronchial tubes.
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Bronchioles – Each bronchus divides and redivides about 15 -times. By doing this
their walls become thinner and have less and less cartilage. Eventually, they become a
tiny group of tubes called bronchioles. The resulting terminal bronchioles supply air to
millions of small sacs called alveoli.
Alveoli – The alveoli look like small interconnected bubbles. Each alveolus is about 0.2
mm in diameter and has wall thickness of only 0.4µm. at birth lungs have about 30
million alveoli, by age of 8 the number of alveoli has increased to about 300 million.
Beyond this age the number stays relatively constant, but the alveoli increase in diameter.
The alveoli play an important role in breathing, they expand and contract during
breathing, they are where the action is in exchanging of O2 and Co2. Each alveolus is
surrounded by blood so that O2 can diffuse from the alveolus into the red blood cells and
Co2 can diffuse from the blood into the air in the alveolus.
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The interaction of blood and lungs:
Blood is pumped from the heart to the lungs under relatively low pressure. The average
peak blood pressure in the main pulmonary artery carrying blood to the lungs is only
about 20 mmHg or about 15% of the pressure in the main body circulation. The lungs
offer little resistance to the flow of blood.
About one fifth of the body’s blood supply is in the lungs, but only about 70 ml of that
blood is in the capillaries of the lungs getting O2 at any one time.
Two general processes are involved in gas exchange in the lungs:
1- Ventilation: getting the air to the alveolar surface
2- Perfusion: getting the blood to the pulmonary capillaries
If either process fails the blood will not properly oxygenated
There are three perfusion-ventilation areas in the lungs:
1- Areas with good perfusion, good ventilation: It accounts over 90% of the total
volume of the normal lungs.
2- Areas with poor perfusion, good ventilation: When blood flow to part of a
lung is blocked by a clot (a pulmonary embolism) that volume will have poor
perfusion.
3- Areas with good perfusion, poor ventilation: If air passages in the lungs are
obstructed as in pneumonia, the involved area will have poor ventilation.
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The Pressure-Airflow-Volume Relationship:
Breathing consists of two phases, inspiration and expiration. During inspiration,
the diaphragm and the intercostal muscles contract. The diaphragm moves
downwards increasing the volume of the thoracic (chest) cavity, and the intercostal
muscles pull the ribs up expanding the rib cage and further increasing this volume.
This increase of volume lowers the air pressure in the alveoli to below atmospheric
pressure. Because air always flows from a region of high pressure to a region of
lower pressure, it rushes in through the respiratory tract and into the alveoli. This is
called negative pressure breathing, changing the pressure inside the lung relative to
the pressure of the outside atmosphere. In contrast to inspiration, during expiration
the diaphragm and intercostal muscles relax. This returns the thoracic cavity to its
original volume, increasing the air pressure in the lungs, and forcing the air out.
The pressure-airflow-volume
relationship
Flow rate (liter/ min)
Pressure
(cmH20)
normal
patient
patient
+5
normal
Time (sec)
Time (sec)
-5
inspiration
expiration
Volume
(liters)
inspiration
patient
normal
Time (sec)
inspiration
expiration
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expiration
The pressure difference needed to cause air to flow into or out of the lungs of a healthy
individual is quite small. Note that the pressure difference is only few centimeters of
water for normal individuals. While a grater pressure difference is required for air to
flow out of a patient’s lungs.
Physics of Alveoli:
The alveoli are physically like millions of small interconnected bubbles, they have a
natural tendency to get smaller due to the surface tension of a unique fluid lining, called
surfactant which is secreted by the cells lining the internal surface of the alveoli. This
fluid is responsible for decreasing surface tension of the alveoli.
The surface tension is the force acting on an imaginary line along the surface of a liquid.
Force (dyne)
γ=
Length (cm)
For a soap bubble, the surface of the bubble contract as much as they can forming a
sphere & generating a pressure that obeys Laplace law.
4γ
P=
R
For a spherical alveolus:
2γ
P=
R
The surface tension of water is 70 dyne/ cm, its constant with change in surface
area (∆A), and independent on it. While the surface tension of the surfactant greatly
changes with the change in surface area. For the alveoli it falls to extremely low values
when the area is small.
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The physiological advantages of surfactant:
PR
1. A low (γ) =
2
The surfactant increases the alveolar compliance and reduces the work of expanding with
each breath.
2. The stability of alveoli is promoted; the cause of instability is the tendency of small
bubbles to blow up to large ones, in the presence of surfactant, the tendency of small
alveoli to empty is greatly reduced.
3. Keep the alveoli dry, the surfactant prevents the transudation of fluid into the alveolar
space from the capillaries by reducing the surface tension.
Mechanism of O2 and Co2 exchange in blood:
The transfer of O2 and Co2 across the alveolar and pulmonary capillary membranes is
controlled by the physical law of Diffusion, which states that:
Molecules of particular type diffuse from a region of higher concentration to a region
of lower concentration until the concentration is uniform.
Principle of Diffusion:
Each molecule collides in a random manner ≈ 10 times/ sec with another molecules.
N
‫ג‬
D
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The most probable distance (D), a molecule will travel from its origin after (N) collisions
is:
[D= ‫ √ ג‬N]
In the lungs diffusion occur both in gas and liquid
‫ =ג‬the average distance between collisions, ‫ג‬air = 10-7
, ‫ג‬tissue = 10-11
No. of collisions is directly proportional with the change of time
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[N α √∆t] , ∆t α D
Diffusion depends on:
a. the speed of molecules
b. the Temp.
Respiration involves the gaseous exchange of O2 and Co2 by diffusion between alveoli
and the pulmonary capillary, this exchange depend on:
a- Volume of alveolar ventilation.
b- Volume of blood flow through the pulmonary capillaries which is in contact with the
ventilated alveoli.
Partial pressures of O2 and Co2
The behavior of gases in the lungs obeys Dalton law of partial pressures: The total
pressure of mixed gases is the sum of the pressure each would exert when it is
alone occupied the container.
Total pressure= sum of partial pressures
Partial Pressure = %(gas) (Atmospheric Pressure – Partial Pressure of water vapor)
In the lungs, at 37ºC and 100% relative humidity, the partial pressure of water vapor=
47mmHg. At atmospheric pressure = 760 mmHg, the alveolar air contain 14% O2, 5.6%
Co2.
To determine the partial pressure of O2:
Po2= 14% (760mmHg - 47mmHg) =100mmHg
Pco2= 5.6 %( 760mmHg - 47mmHg) =40mmHg
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500 cm3 fresh air
The mixture of air in the lungs will result in alveolar air with:
Po2 = 100 mmHg & Pco2= 40 mmHg
The mixture of gases in alveoli is not the same
2000 cm3 stale air in the lungs
mixture in ordinary air. The lungs are not emptied during expiration,
during normal breathing the lungs retain about 30% of its volume at the end of each
expiration. This is called Functional Residual Capacity (FRC), at each breath about 500
cm3 of fresh air (Po2 of 150 mmHg) mixes with about 2000 cm3 of stale air in the lungs to
result in alveolar air with a Po2 of 100 mmHg. The Pco2 in the alveoli is about 40mmHg.
Expired air includes about 150 cm3 of relatively fresh air from the trachea that was not in
contact with alveolar surface, so expired air has a slightly higher Po2 and lower Pco2 than
alveolar air.
The Solubility of O2 & Co2
The amount of gas dissolve in liquid is directly proportional to the partial pressure of the
gas (Henry Law)
The amount of gas dissolved in blood varies greatly from one gas to another. Oxygen is
not very soluble in blood or water. At body temperature 1 liter of blood plasma at Po2 of
100 mmHg will hold about 2.5 cm3 of O2 at normal pressure and temp.
The difference in the solubility of O2 & Co2 in tissue affects the transport of these gases
across the alveolar wall. A molecule of O2 diffuses faster than a molecule of Co2 because
of its smaller mass. However, because of the greater number of Co2 in solution, the
transport of Co2 is more efficient than the transport of O2. adding to that the solubility of
Co2 molecules in blood 25 times more than O2, this also in help in increasing the
efficiency of Co2 transportation through the blood.
Oxygen Binding Capacity of the Blood
Blood can carry very little O2 in solution, most of O2 is carried to the cells
by chemical combination with the hemoglobin (Hb) in the red blood cells. A liter of
blood can carry about 200 cm3 of O2 at normal body temperature and pressure. Since most
of O2 is not in solution, the law of diffusion is altered; the O2 will combine with or
separate from the Hb in a way that depends on the following dissociation curve.
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The Hb leaving the lungs is about 97% saturated with O2 at Po2 of about 100 mmHg. The
Po2 has to drop by about 50% before the O2 load of the blood is noticeably reduced. When
the blood reaches the cells and their low Po2 environment. The O2 is dissociated from the
Hb and diffuses into the cells, not all the O2 leaves the Hb, the amount the amount of O2
dissociation from the Hb depends on:
a. Po2 of the tissue
b. Pco2
c. PH (acidity)
d. Temp.
Under normal (resting conditions) the venous blood return to the heart with about
75% loads of blood. During heavy physical exercise the Po2 in the working muscle
drops rapidly causing more O2 to be dissociated from the Hb to diffuse into the
muscles. The working muscle can obtain 10 times more O2 than they consume at rest.
The Pco2, PH, temp. are all increase, so that the Hb give more of its O2 to the working
muscles.
Co Poisoning:
The Co molecules attach very securely to the Hb occupy places normally used by O2.
They attach 250 times more tightly to Hb than do O2 molecules. The Co molecules
don’t easily dissociate in the tissue. They inhibit the release of O2 from the Hb.
Measurement of lung volumes:
Spiro meter: It is a device used to measure air flow into and out of the lungs and
record it on a graph of volume versus time.
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Tidal volume (TV) is the amount of air inspired or expired with each breath
Inspiratory Reserve Volume (IRV) is the maximum amount of additional air that can
be inspired beyond a normal inspiration.
Expiratory Reserve Volume (ERV) is the maximum amount of additional air that can
be expired beyond a normal expiration.
Residual Volume (RV) is the amount of air remaining in the lungs after an extended or
complete expiration. RV cannot be measured with a Spiro meter. RV is decreased by
restrictive lung diseases like pulmonary fibrosis, lung cancer, and pneumonia, and its
increased by obstructive lung diseases such as Chronic Obstructive Pulmonary Disease
(COPD), emphysema, asthma.
Total Lung Capacity (TLC) is the amount of air in the lungs at the end of an extended
or complete inspiration. TLC is the sum of all four lung volumes discussed above. (RV +
IRV + TV + ERV = TLC).
Vital Capacity (VC) is the maximum amount of air that can be forcefully expelled from
the lungs after an extended or complete inspiration. VC is the sum of IRV, TV, and ERV.
(IRV + TV + ERV = VC)
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Both obstructive and restrictive lung diseases can cause a reduction in VC because
sufficient amounts of air cannot be inhaled or exhaled from the lungs. VC is also affected
by the body's position. Lying in a prone position (flat on the back), decreases the VC
because the pulmonary blood volume increases and the diaphragm is pushed downwards.
Functional Residual Capacity (FRC) is the amount of air remaining in the lungs at the
end of a normal expiration. FRC is the sum of RV and ERV. (RV + ERV = FRC).
Inspiratory Capacity (IC) is the maximum amount of air that can be inspired after a
normal expiration. IC is the sum of TV and IRV. (TV + IRV = IC)
Body plethysmography: It is a modern device used to get a better understanding of
how the lungs are functioning, pulmonary function tests, where Lung volume
measurements give an indication as to whether a lung disease is present, and if so, which
lung disease.
Anatomical and physiological dead spaces
They are spaces in the respiratory system at which air does not provide O2 to the body.
Anatomical dead space: In the conducting airways (nose, mouth, pharynx, larynx,
trachea, bronchi and bronchioles) there is no significant exchange of O2 & CO2 between
gas and blood, the internal volume of the airways is called the anatomic dead space. The
volume of air in the anatomical dead space= 150 cm3.
Physiological dead space: In some diseases, some air reach the alveoli are poorly
perfused by the blood capillary, result in poor ventilation-perfusion relationship
increasing the physiological dead space. The volume of air in the physiological dead
space= 350 cm3 .
.
Compliance:
It is the change in lung volume produced by a small change in pressure
▲V
C=
▲P
Liter/ cmH2o
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In normal adults, the range of compliance= 0.18-0.27 liter/ cmH2O. Elderies over age 60
have about 25% greater compliance than younger men, in women there is a little change
in compliance for elderies. A fibrotic (stiff) lung has low compliance, while a flabby lung
has large compliance
Airways Resistance
Airway resistance is a concept used in respiratory physiology to describe mechanical
factors which limit the access of inspired air to the pulmonary alveoli, and thus determine
airflow. It is the amount of pressure required to deliver a given flow of gas and is
expressed in terms of a change in pressure divided by flow. During inspiration the forces
on the airways tend to open them further, during expiration the forces tend to close the
airways and thus restrict airflow. For a given lung volume, the expiratory flow rate
reaches a maximum and remains constant; it might even decrease slightly with increased
respiratory force. Resistance is greatest at the bronchi of intermediate size, in between the
fourth and eighth bifurcation.
Air way resistance can be calculated by using Ohm’s law:
P mouth – P alveoli
Pressure Difference
Ra =
=
Rate of Air Flow
▲P
(cmH20 )
V•
(Liter/ sec)
Rate of Air Flow
Ra =
V• = ▲V/ ▲t (liter/ sec)
Ra depends on:
a. The dimensions of the airway
b. The viscosity of the gas
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For typical adult Ra= 3.3 cmH2O/ (liter/ sec). It accounts 50% in the nasal area,
20% in the trachea, 10% in the bronchi & alveoli, and 20% of the airway resistance is
related to the viscosity of the gas we breathe.
The Time Constant
When the respiratory system is subjected to P, time is needed until V occurs, and the
time necessary to inflate 63% of its volume is called the time constant. The time constant
of the lung is related to the airway resistance & the compliance
TC (sec) = Ra x C
The time constant (T) of the lung is complicated, since many parts of the lungs are
interconnected. If one part has large (T) than others parts, it will not get its share of the
air and that part of the lung will be poor ventilated.
Work of Breathing
The amount of work done in normal breathing accounts for a small fraction of the total
energy consumed by the body (2% at rest)
Components of Work
1- Elastic work - work to overcome:
 lung elastic recoil
 thoracic cage displacement
 abdominal organ displacement
2-
Frictional work - work to overcome:
 air-flow resistance (major)
 viscous resistance (lobe friction, minor)
3- Inertial work - work to overcome:
 acceleration and deceleration of air (negligible due to low mass of air)
 acceleration and deceleration of chest wall and lungs (negligible due to
damping)
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over
The primary work of breathing can be thought of as the work done in stretching the
springs representing the lung-chest wall diaphragm system. The resistance of the gas flow
produce heat; these can be represented as ( R ). The springiness of the lung-chest is
represented by the spring C, the inertia ( I ) of the mass of the lungs and chest wall must
also be overcome; at normal breathing rates, the inertia can be neglected, but at maximum
breathing rates (over 100 breath/ min) it is a significant factor.
During normal breathing, no work is done during expiration; the muscles relax and spring
“snap back “to expel the air. The energy dissipating in tissue resistance (R). While during
during exercise. The muscles are used to expel the air and the work may equal about 25%
the total energy consumption
Physics of some lung diseases
Emphysema
Emphysema is a long-term, progressive disease of the lung that primarily causes
shortness of breath. In people with emphysema, the lung tissues necessary to support the
physical shape and function of the lung are destroyed. It is included in a group of diseases
called chronic obstructive pulmonary disease or COPD. Emphysema is called an
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obstructive lung disease because the destruction of lung tissue around smaller airways,
called bronchioles, makes these airways unable to hold their shape properly when you
exhale.
- The no. of lung springs has been greatly reduced, the divisions between the
alveoli broken producing large lung spaces. The lung become flabby and expands.
The tension reduced allows the chest wall to expand to the resting volume.
- In emphysema, the lungs become more compliant, The tissue don’t pull very
hard on the airways, permitting the narrowed airways to collapse easily during
expiration (i.e. increased Ra)
- The increased size of the lungs increases the FRC & the RV.
Asthma
It is a chronic disease involving the respiratory system in which the airways occasionally
constrict, become inflamed, and are lined with excessive amounts of mucus, often in
response to one or more triggers. These episodes may be triggered by such things as
exposure to an environmental stimulant such as an allergen, environmental tobacco
smoke, cold or warm air, perfume, pet dander, moist air, exercise or exertion, or
emotional stress. In children, the most common triggers are viral illnesses such as those
that cause the common cold. This airway narrowing causes symptoms such as wheezing,
shortness of breath, chest tightness, and coughing.
The basic problem is the expiratory difficulty due to increase (Ra). Increasing (Ra) is
due to:
- Swelling and mucus in the smaller airways.
- Contraction of the smooth muscle around the large airways.
In asthma the lung compliance doesn’t change (over the normal value). The FRC is
higher than normal, because the patient often start to inspire before completing a
normal expiration
Fibrosis of the lungs
Pulmonary fibrosis describes a group of diseases which produce interstitial lung damage
and ultimately fibrosis and loss of the elasticity of the lungs. The membrane between the
alveoli thickens. It is a chronic condition characterized by shortness of breath,
This has two marked effects:
-
The compliance of the lung decreases
The diffusion of O2 into the pulmonary capillaries decrease.
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