physiology For the lecture of this topic, visit BISHAL CHAUHAN channel on YouTube

1. Exchange of Gases
Dr Sushil Kharel

2. • Respiration includes two processes:
1. External respiration: the absorption of
O2 and removal of CO2 from the body.
2. Internal respiration: the utilization of
O2 and production of CO2 by cells and
the gaseous exchanges between the cells
and their fluid medium.

3. Diffusion
Gases diffuse from an area of high
concentration to an area of low
concentration.

4. Gas Pressure
• Gas pressure is caused by the molecules
colliding with the surface.
• In the lungs, the gas molecules are colliding
with the surfaces of the respiratory passages
and alveoli.
• Higher concentrations of gas will produce
more collisions and cause a higher pressure.

5. Partial Pressure
• The total gas pressure is the pressure caused by all the gas
molecules colliding with the surface.
• The partial gas pressure is the pressure exerted by 1 gas
species alone. Written as PO2 (partial pressure of O2), PCO2
(partial pressure of CO2).
• The rate of diffusion of a gas molecule is directly
proportional to its partial pressure.
Nitrogen 597 mm Hg 78.62 %
Oxygen 159 mm Hg 20.84 %
Carbon Dioxide 0.3 mm Hg 0.04 %
Water 3.7 mm Hg 0.5 %
Total 760 mm Hg 100 %

6. Gas exchange in
lungs and tissues
• Exchange of gases between lungs and
blood and gas movement at the tissue
level progress passively by diffusion,
depending on their pressure gradients.

8. Concentration and Partial Pressure
of Gases
• Partial pressure = Percentage of
concentration of specific gas ×
Total pressure of a gas

10. Any gas in a gas mixture exerts a pressure
proportional to its own concentration
independently of the other gases present in the
mixture.
The pressure of the individual component of the
mixture is called partial pressure.

12. • O2 = 20.93% = ~ 159 mm Hg PO2
• CO2 = 0.03% = ~ 0.23 mm Hg
PCO2
• N2 = 79.04% = ~ 600 mm Hg PN2

13. Tracheal Air
• Water vapor reduces the PO2
in the trachea about 10 mm
Hg to 149 mm Hg.

14. Alveolar Air
• Alveolar air is altered by
entry of CO2.
• Average alveolar PO2 = 104
mm Hg

16. Alveolocapillary membrane
• Alveolocapillary membrane is thin and
made up of
–the alveolar epithelium &
–capillary endothelium, and
–their fused basement membranes
• Blood takes 0.75 second to pass along these
membranes in pulmonary capillaries at rest.

17. Respiratory Membrane

19. Diffusion
Driven by concentration gradients:
differences in partial pressure of the individual gases.
Movement of O2 and CO2 between the level of the respiratory bronchiole and that of
the alveolar space depends only on diffusion.
The distances are small, so diffusion here is fast.
• Driven by concentration gradients:
– ferences in partial pressure of the individual gases.
• Movement of O2 and CO2 between the level of
the respiratory bronchiole and that of the
alveolar space depends only on diffusion.
• The distances are small, so diffusion here is
fast.
Diffusion of Oxygen Across the
Alveolar Wall
Pulmonary Surfactant
Alveolar Epithelium
Epithelial and capillary basement membrane
Capillary Endothelium
Plasma
Red Blood Cell
Hemoglobin
Binds

20. Diffusing capacity
Diffusing capacity is defined as the
volume of gas that diffuses through the
respiratory membrane each minute for a
pressure gradient of 1 mm Hg.
• Oxygen under resting conditions
21 ml/min/mmHg
CO2: 400 ml/min/mmHg

21. Solubility Coefficient
• The higher the solubility, the high is the
solubility coefficient.

22. Factors Affecting Diffusing Capacity
1. Pressure gradient
2. Solubility
• CO2 is about 20 times more soluble than O2.
• CO2 and O2 are both more soluble than N2.
3.Total surface area of respiratory membrane
4. Molecular weight of the gas
5. Thickness of respiratory membrane

23. Fick’s Law of Diffusion
The amount of substance crossing a given
area is directly proportional to the area
available for diffusion, the concentration
gradient and a constant known as
diffusion coefficient.
Thus,
J= - D A dc/dx
Where,

24. J= amount of substrate diffused
D= diffusion coefficient
A= area through which diffusion occurs
Dc/dx= concentration gradient

26. • A pulmonary shunt is a physiological
condition which results when
the alveoli of the lungs are perfused with
blood as normal, but ventilation (the
supply of air) fails to supply the perfused
region.
• A pulmonary shunt often occurs when the
alveoli fill with fluid, causing parts of the
lung to be unventilated although they are
still perfused

27. • Physiological shunt
1. Flow of deoxygenated blood from
bronchial circulation into pulmonary
veins without being oxygenated makes up
part of normal physiological shunt
2. Flow of deoxygenated blood from
thebesian veins into cardiac chambers
directly

28. Alveolocapillary block.
• Diffusion of gases across the respiratory
membrane stops due to collapse of the
capillaries
• Affected alveoli become a part of the
physiologic dead space.

29. 1. The partial pressure of Oxygen in
the alveoli is
A.159 mmHg
B. 149 mmHg
C.104 mmHg
D.95 mmHg

30. 2. On the summit of Mt. Everest, where
the barometric pressure is about 250
mm Hg, the partial pressure of O2 is
about
A) 25 mm Hg.
B) 52 mm Hg.
C) 104 mm Hg.
D) 159 mm Hg.

31. 3.Which of the following is
responsible for the movement of
O2 from the alveoli into the blood
in the pulmonary capillaries?
A) Active transport
B) Filtration
C) Facilitated diffusion
D) Passive diffusion

32. 4. Surfactant lining the alveoli
A)helps in preventing alveolar
collapse.
B) is produced in alveolar type I cells
and secreted into the alveolus.
C) is increased in the lungs of heavy
smokers.
D) is a glycolipid complex.

33. 5. Gaseous exchange in
respiratory membrane is
completed in………at rest.
A.0.25 sec
B.0.75 sec
C.1.75 sec
D.2.75 sec

34. • https://forms.gle/Vav5Jui887crihtz9

35. Imp KU questions
• Write short notes on:
(1)Gaseous exchange
(2)Ventilation/ Perfusion ratio