gas exchange & gas transfer dr taha sadig ahmed physiology department, college of medicine, king...
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Gas Exchange & Gas Transfer
Dr Taha Sadig Ahmed Physiology Department , College of Medicine , King Saud University ,
Riyadh
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• Objectives1- Explain what is meant by diffusion .2. Define partial pressure of a gas 3- Understand that gases in a liquid ( e.g.,water) diffuse from higher partial pressure tolower partial pressure .4. State the partial pressures of oxygen andcarbon dioxide in the atmosphere, alveolus,pulmonary capillary & systemic capillary .4- Describe the factors that determine
diffusion and the concentration of a gas in a liquid .
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Definitions• Diffusion is a process leading to equalization ofoxygen and carbon dioxide concentrations betweentwo compartments ( alveolus and blood and pulmonaryblood capillary .• Factors that determine the concentration of gas in aliquid phase (alveolus or capillary) . • Partial presure of a gas is the pressure of a
gaspresent in a mixture of gases . It is independent of thepressure exerted by the other gases (Dalton's Law) It depends upon :(1) Partial pressure differences of the gas ( O2 or CO2)between the two compartments .(2) Diffusivity or Diffusion Coefficient The higher thediffusivity of of a gas ( O2 or CO2) , the faster is thespeed of its diffusion . Diffusivity depends on the (a) molecular weight (MW) ( the smaller the MW , of a substance the faster is itsrate of diffusion ) , & and (b) its solubility ( in water , which lines our alveoli & occupies the interstitial space )• O2 has lower molecular weight than CO2 , and this , theoretically should make it more diffusible than CO2.However, in spite of that , CO2 is 24 times more soluble
inwater than O2 the net result is that CO2 diffusion is20 times faster than O2 diffusion
(3) Diffusion distance across the alveolar-capillary membrane, which
consists of (i) blood capillary endothelium , (& its basement membrane , and (ii) alveolar wall epithelium
(4) Surface area available for diffusion .
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Composition of Inhaledand Exhaled Air
• Partial Pressures of O2 and CO2• Oxygen concentration in the
atmosphere is 21% • Atmospheric pressure = 760
mmHg • Hence oxygen partial pressure
( PO2 ) in atmosphere = 760 mmHg x 21 % = 160 mmHg. • This mixes with “old” air already present in alveolus to arrive at PO2of 104 mmHg in alveoli. • Carbon dioxide concentration in
the atmosphere is 0.04% • Therefore , PCO2 in atmosphere
=760 mmHg x 0.04% = 0.3 mm Hg
• This mixes with high CO2 levels from residual volume in the alveoli to arrive at PCO2 of 40 mmHg in the alveoli.
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Oxygen Carbon Dioxide
Atmospheric air
160 mm Hg (21%?)
0.3 mmHg (0.04 %)
Alveolus 100 -104 mm Hg
40 mmHg
Pulmonary Capillary
PO2=104 40 mmHg
Pulmonary Artery
95 mmHg 40 mmHg
Pulmonary Vein 40 mm
Hg45 mmHg
Tissue capillary
PO2 = 95 mmHg
PCO2=40 mmHg
Interstitial Space
PO2 = 40 mmHg
PCO2=45 mmHg
Tissues PO2 = Less than 40 ( around 20 mmHg)
PCO2=46 mmHg
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Figure 14-3a
Gas exchange in the Lung and in the Tissues:
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40 mmHg
Est means estimated
45mmHg
Pulmonary
Artery 95
mmHg
Alveolus 104 mmHg
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• Oxygen and CO2 Concentration in alveoli • At resting condition 250 ml of oxygen enter the pulmonary
capillaries/min at ventilatory rate of 4.2 L/min. • During exercise 1000 ml of oxygen is absorbed by the pulmonary
capillaries per minute, the rate of alveolar ventilation must increase 4 times to maintain the alveolar PO2 at the normal value of 104 mmHg.
• Normal rate of CO2 excretion is 200 ml/min, at normal rate of
alveolar ventilation of 4.2 L/min.
• How can you explain the fact that the PO2 in the expired air is higher than PO2 in the alveolar air?
• Diffusion of CO2 through the conducting air ways.
• Humidification of expired air with water vapor.• Mixing with the dead space air during expiration.(T)• Uptake of CO2 by pulmonary capillary blood during expiration.