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Exchange and Transport of
Respiratory Gases
YOUSAF KHAN
RENAL DIALYSIS LECTURE
EXCHANGE OF RESPIRATORY
GASES IN LUNGS
 Alveoli of lungs and the blood.
 Oxygen enters the blood from alveoli and carbon dioxide is
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expelled out of blood into alveoli.
Bulk flow diffusion.
Respiratory membrane
Epithelium of respiratory unit and endothelium of pulmonary
capillary.
This facilitates gaseous exchange between air and blood.
Respiratory membrane is formed by different layers of
structures belonging to the alveoli and capillaries.
Layers of Respiratory Membrane
 Average thickness of 0.5 μ of respiratory membrane.
 Total surface area of the respiratory membrane in both the
lungs is about 70 square meter.
 Average diameter of pulmonary capillary is 8 μ, which means
that the RBCs with a diameter of 7.4 μ actually squeeze through
the capillaries.
Layers of Respiratory Membrane
Diffusing Capacity
 The volume of gas that diffuses through the respiratory
membrane each minute for a pressure gradient of 1 mm Hg.
 Oxygen is 21 mL/minute/1 mm Hg.
 Carbon dioxide is 400 mL/minute/1 mm Hg.
 Diffusing capacity for carbon dioxide is about 20 times more
than that of oxygen.
Factors Affecting Diffusing Capacity
1.
Pressure gradient
 Diffusing capacity is directly proportional to pressure gradient
2. Solubility of gas in fluid medium
 Diffusing capacity is directly proportional to solubility of the gas.
3. Total surface area of respiratory membrane
 Diffusing capacity is directly proportional to surface area of
respiratory membrane.
4. Molecular weight of the gas
 Diffusing capacity is inversely proportional to molecular weight of the
gas.
5. Thickness of respiratory membrane
 Diffusion is inversely proportional to the thickness of respiratory
membrane.
DIFFUSION OF OXYGEN
 Diffusion of Oxygen from Atmospheric Air into
Alveoli
 Oxygen in the atmospheric air 159 mm Hg
 In the alveoli, it is 104 mm Hg.
 Diffusion of Oxygen from Alveoli into Blood
 In pulmonary capillary RBC is exposed to oxygen only for
0.75 second.
 At rest only for 0.25 second during severe exercise.
 Partial pressure of oxygen in the pulmonary capillary
 is 40 mm Hg and in the alveoli, it is 104 mm Hg.
DIFFUSION OF CARBON DIOXIDE
 Diffusion of Carbon Dioxide from Blood into Alveoli
 Carbon dioxide in alveoli is 40 mm Hg
 In Blood 46 mm Hg.
 Diffusion of Carbon Dioxide from Alveoli into
Atmospheric Air
 Carbon dioxide in atmospheric air only 0.3 mm Hg.
 In the alveoli 40 mm Hg.
DIFFUSION OF OXYGEN FROM
BLOOD INTO THE TISSUES
DIFFUSION OF CARBON DIOXIDE
FROM TISSUES INTO THE BLOOD
Transport of O2 and CO2 in blood and
body fluid
 Diffusion of O2 from alveolus into pulmonary blood.
 Transport of O2 in arterial blood.
 Diffusion of O2 from capillaries into interstitial fluid.
 Diffusion of O2 from interstitial spaces into cells.
 Diffusion of CO2 from cells into interstitial fluid.
 Diffusion of CO2 from interstitial fluid into capillaries.
 Diffusion of CO2 from pulmonary blood into alveolus.
 Value of oxygen and hemoglobin combination:
 Normal concentration of hemoglobin in blood is – 15 gm /100
ml of blood.
 Normal amount of oxygen carried by 1 gm of Hb is – 1.34 ml
of O2.
 Maximum amount of oxygen that can be carried by 100 ml of
blood 20 ml of O2.
 Transport of CO2 in blood:
 7% of CO2 is transported as dissolved carbon dioxide.
 30% of CO2 is transported in combination with Hb and
Plasma proteins.
 70% of CO2 in form of bicarbonate ions.
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