respiratory physiology

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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 relax3. Rib cage moves upward and forward4. Diaphragm contracts and flattens5. Intrapulmonary pressure decreases6. 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 Muscles

Kalsoom Muhammad Saleem

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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 pressureii. 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 contract3. Rib cage moves downward and backward4. Diaphragm relaxes5. Intrapulmonary pressure increases6. 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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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 BloodO2 21% 13.6% Physicall

y 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)


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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, 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.


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Ans. Transport of O2:Oxygen is transported mostly i) in physically dissolved form and ii) bound to

hemoglobin.

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-


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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

i. The thickness of membrane ii. The surface area of membraneiii. 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 Comments


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transportThickness 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.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.


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5. Saladin, Kenneth: The Unity of form and function of Intercostal Muscles, Anatomy and Physiology, 5th ed.


respiratory physiology

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