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Kalsoom Muhammad Saleem
Respiratory Physiology
I. Answer to the following questions.
Q1. Explain briefly the mechanics of ventilation.
Ans. Ventilation:
Air is alternatively moved into and out of lungs so that air can be exchanged between the
atmosphere (external environment) and air sacs (alveoli) of lungs. This exchange is accomplished by
mechanical act of Breathing or Ventilation. This is achieved by body’s metabolic need for oxygen
uptake and carbondioxide removal.1
Explanation:
Ventilation involves lungs hence also known as Pulmonary Ventilation. It includes two
processes which function consecutively referred as inhalation and exhalation.
Mechanics of Pulmonary Ventilation:
The lungs can be expanded and contracted in two ways,
1. By downward and upward movement of diaphragm to lengthen or shorten the chest
cavity and
2. By elevation and depression of ribs to increase and decrease the anteroposterior
diameter of chest cavity.
Inhalation (inspiration):
Stages involved during inhalation are:
1. External intercostal muscles contract
2. Internal intercostal muscles relax
3. Rib cage moves upward and forward
4. Diaphragm contracts and flattens
5. Intrapulmonary pressure decreases
6. Air pushes in
1. External intercostal muscles contract:
The external intercostals muscles are responsible for ~25% of air entrance during normal quite
breathing. The external intercostals assist in deep inspiration by increasing the anterioposterior
diameter of the chest.2
They do it so by contracting, as they contract they elevate the ribs and
sternum which in turn increases the front-to-back dimension of thoracic cavity and air moves in.1

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Muscles other than external intercostals do cause elevation of ribcage such as sternocledomastoid,
anterior serrate and scalene.2
2. Internal intercostals muscles relax:
The relaxed form of internal intercostal muscles does not have any effect on inspiration as
they don’t change their posture to cause inspiration.1
3. Ribcage moves upward and forward:
Aided by movements of ribs and diaphragm the ribs are moved upward as well as forward.2
4. Diaphragm contracts and flattens:
Diaphragm is an important structure for ventilation. During normal quite breathing ~75%
of air enters lungs by movement of diaphragm1
. It contracts during inhalation and enlarges the
thoracic cavity and creating suction that draws air into lungs.2
5. Intra-pulmonary pressure decreases:
The pressure inside lungs is decreased by two means:
i. Contraction of external intercostals muscles causes expansion of lungs hence decreasing
the pulmonary pressure
ii. Contraction of diaphragm also decreases the pulmonary pressure by expansion of lungs,
therefore, air moves in.3
6. Air moves in:
All theses stages of inhalation aid the passage of air form atmosphere into the lungs down
the concentration gradient. Pressure of air in atmosphere is greater as compared to pressure
inside the lungs, therefore, air moves from higher concentration gradient to lower concentration
gradient.3
Exhalation (expiration):
Stages involved during exhalation are:
1. External intercostal muscles relax
2. Internal intercostal muscles contract
3. Rib cage moves downward and backward
4. Diaphragm relaxes
5. Intrapulmonary pressure increases
6. Air moves out
1. External intercostals muscles relax:
As these muscles contract, they return the diaphragm and ribs to resting position.2

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Kalsoom Muhammad Saleem
2. Internal intercostals muscles contract:
These muscles contract and assist in expiration by pulling the ribcage down4
. Depression of
ribs decreases the transverse dimension of thoracic cavity.5
3. Ribcage moves downward and backward:
Aided by the movement of intercostals muscles and diaphragm, the ribcage moves
downward and backward that restores the thoracic cavity to preinspiratory volume.
4. Diaphragm relaxes:
During expiration it simply relaxes that brings the elastic recoil of lungs, chest wall and
abdominal structures that compresses the lungs and expels the air.2
5. Intrapulmonary pressure increases:
Intrapulmonary pressure is decreased by two means
i. The contraction of internal intercostals muscles causes compression of lungs that decreases
the volume of lungs. In turn, the pressure within the lungs increases relative to atmospheric
pressure and, hence, air is expelled out.
ii. The relaxation of diaphragm further causes compression of lungs that increases the pressure
within the lungs and air is expelled out.
6. Air moves out:
All theses stages of exhalation aid the passage of air form lungs into the atmosphere down
the concentration gradient. Pressure of air in lungs is greater as compared to pressure of
atmosphere, therefore, air moves from higher concentration gradient to lower concentration
gradient i.e. out in atmosphere.3
Q2. Write the composition of O2 and CO2 in atmosphere, alveoli and blood.
Ans. Table 1.11, 2
Composition Atmospheric Air Alveolar Air Blood
O2 21% 13.6% Physically
dissolved
1.5%(1)
Bound to
Hb
98.5%(1)
CO2 0.3% 5.3% Physically
dissolved
10%(1)
Bound to
Hb
30% (1)
As
bicarbonate
60% (1)
There are nearly four reasons four changed concentration of O2 and CO2 in atmosphere, alveoli and
blood. First, the alveolar air is only partially replaced by atmospheric air in each breath. Secondly,

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Kalsoom Muhammad Saleem
from alveoli oxygen is constantly being absorbed into blood. Thirdly, carbondioxide diffuses from
blood into alveoli. And fourth, before dry air reaching the alveoli it is being humidified.2
Q3. Draw oxygen dissociation curve and explain Bohr’s curve.
Ans.
Bohr’s curve:
The influence of CO2 and acid on release of O2 is known as Bohr’s effect. (1). Two types of
shifts are observed in Bohr’s effect i) Right shift and ii) Left shift.
Right shift:
As blood passes through the tissues, CO2 diffuses from tissue cells into blood hence
increasing partial pressure of CO2. More the concentration of CO2 in blood, more formation of
H2CO3 and hydrogen ions. Thos effect shifts the oxygen-hemoglobin dissociation curve downward
and right. This means now hemoglobin has less affinity for O2 , therefore O2 is forced away from
hemoglobin as a result increased amount of O2 is delivered to tissues.
Left shift:
The effect occurring antagonistic to right shift causes left shift. CO2 diffuses from blood into
the alveoli in lungs. This reduces the partial pressure of CO2 and decreases the hydrogen
concentration therefore shifting the curve to left and upward. This means now hemoglobin has
greater affinity for O2, therefore more oxygen can bind to hemoglobin which allows greater O2
transport to tissues.2
Q4. Write short note on O2 and CO2 transport.
Ans. Transport of O2:
Oxygen is transported mostly i) in physically dissolved form and ii) bound to hemoglobin.

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i. Physically dissolved form:
Very little O2 physically dissolves in plasma water because o2 is poorly soluble in body fluids.
The amount dissolved is directly proportional to the PO2 of blood. The higher the PO2, the more O2
dissolved. At normal arterial PO2 of 100 mm Hg, only 3 ml of O2 can dissolve in 1 liter of blood. Thus,
only 15 ml of O2/min can dissolve in the normal pulmonary blood flow of 5 liters. Physically dissolved
form of O2 contributes to only 1.5% of O2 transportation.1
ii. O2 bound to hemoglobin:
The normal blood contains 15 grams of hemoglobin in each 100 milliliters of blood and each
gram of hemoglobin can combine with 1.34 milliliters of O2.
Hb + O2 → HbO2
Therefore, on average, 15 grams of hemoglobin can combine with 20.1 milliliters of O2. On
passing through the tissue capillaries, this amount is reduced to 14.4 milliliters. Thus, under normal
conditions about 5 milliliters of O2 are transported from the lungs to the tissues by each 100
milliliters of blood.2
About 98.5% of O2 is transported bound to hemoglobin.1
Transport of CO2:
CO2 is transported mainly in three form i) Physically dissolved, ii) Bound to hemoglobin and
iii) As bicarbonate ions.
i. Physically dissolved form:
Only about 3 milliliters of CO2 is transported in dissolved form by each 100 milliliters of
blood flow.2
This is about 10% of all the CO2 transported to venous blood.
ii. Bound to hemoglobin:
Another 30% of CO2 combines with hemoglobin to form carbaminohemoglobin.
Hb + CO2 → HbCO2
This is the amount of CO2 that can be carried form the peripheral tissues to the lungs in form
of HbCO2 i.e. 1.5milliliters of CO2 in each 100 milliliters of blood.2
iii. As bicarbonate ions:
By far the most important means of CO2 transport is as bicarbonate ions.1
The dissolved
form of CO2 in the blood reacts with water to form carbonic acid. The carbonic acid formed in the
red blood cells dissociates into hydrogen and bicarbonate ions.2
CO2 + H2O → H2CO3 → H+
+ HCO3-
In turn, many of bicarbonate ions diffuse from the red blood cells into the plasma. This
accounts to the 70% of CO2 transported into the lungs from the tissues.1
Q5. Explain briefly the factors affecting diffusion of respiratory gases.
Ans. Factors that affect the rate of transportation of gases through respiratory membranes are

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i. The thickness of membrane ii. The surface area of membrane
iii. The diffusion coefficient of gases in iv. The partial pressure difference of gases
substance of membrane
i. The thickness of membrane:
Rate of diffusion is inversely proportional to respiratory membrane i.e. rate of transfer
decreases as thickness increases. Thickness usually remains normal but certain pathological factors
do increase the thickness of membrane more than two –three times normal which interfere
significantly with normal respiratory exchange of gases (table 1.2).
ii. The surface area of membrane:
Rate of transfer increase as surface area increases. Increasing of surface area is observed
during exercise, as more pulmonary capillaries open up when the cardiac output increases which
expands alveoli to increase breathing rate.2
When total surface area is decreased to about one-third
to one-fourth normal, gas exchange through the membrane is decreased (table 1.2).
iii. The diffusion coefficient:
Rate of transfer increases as diffusion coefficient increases.1
Diffusion coefficient is directly
proportional to gas’s solubility and inversely proportional to square root of gas’s molecular weight.
Therefore, CO2 diffuses 20 times rapidly as O2 and O2 diffuses twice as rapidly as N2 (table 1.2).2
iv. The partial pressure difference of gases:
Pressure difference across the respiratory membrane is the difference between the partial
pressure of gas in alveoli and blood. When the pressure of gas in alveoli is greater than pressure of
gas in blood, the gas moves down the gradient of higher pressure to lower pressure i.e. from alveoli
into blood as in case of O2. The partial pressure of gas is greater in blood than of gas in alveoli, the
gas moves from blood into the alveoli as in case of CO2.2
Table 1.21
Factors Affect on rate of gas transport Comments
Thickness of barrier separating
the air and blood across the
alveolar membrane
Rate of transfer as thickness Thickness normally remains
constant.
Thickness with pathological
conditions such as pulmonary
edema (fluid accumulation in
interstitial spaces), pulmonary
fibrosis (lungs tissues replaces
by scar-forming tissues) and
pneumonia (fluid and blood
accumulation in alveoli)
Surface area of alveolar
membrane
Rate of transfer as surface area Surface area remains normal
under resting conditions.
It during exercise leading rapid
diffusion.

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Kalsoom Muhammad Saleem
It with pathological conditions
such as emphysema(breakdown
of alveolar walls) and lung
collapse.
Diffusion coefficient Rate of transfer as diffusion
coefficient
Diffusion constant for CO2 is 20
times that of O2 offsetting
smaller partial pressure
gradient for CO2; therefore
equal amount of CO2 and O2 are
transported.
Partial pressure of gradients of
O2 and CO2
Rate of transfer as partial
pressure gradient
Major determinant of rate of
transfer.
References
1. Sherwood, Lauralee. (2010): The Respiratory System, Human Physiology from Cells to
System, 7th
ed. Pg 461-497.
2. Guyton, Arthur.C, Hall, John.E. (2011): Respiration, Textbook of Medical Physiology, 12th
ed. Pg 465-518.
3. BarrettKE, Barman SM, Boitano S, Brroks H: Pulmonary Function, Review of Medical
Physiology, 23rd
ed.
4. Philip Tate: Internal Intercostal Muscles, Seeley’s Principles of Anatomy and Physiology.
5. Saladin, Kenneth: The Unity of form and function of Intercostal Muscles, Anatomy and
Physiology, 5th ed.