Gas exchange
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GAS EXCHANGE. Dr. JAWAD NAWAZ. Diffusion. Random movement of molecules of gas by their own kinetic energy Net diffusion from higher conc. to lower conc Molecules try to equilibrate in all empty places. Partial pressure. The pressure exerted by the gas molecules on a surface

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Gas exchange




  • Random movement of molecules of gas by their own kinetic energy

  • Net diffusion from higher conc. to lower conc

  • Molecules try to equilibrate in all empty places

Partial pressure
Partial pressure

  • The pressure exerted by the gas molecules on a surface

    In atmospheric air

  • PO2160mmHg

  • PCO 2 0.3mmHg

  • PN2 600mmHg

Pressure of gases dissolved in water and tissues
Pressure of gases dissolved in water and tissues

  • Partial pressure in fluid develop same way as in air

  • Partial pressure= conc. of dissolved gas/solubility coefficient


  • Solubility coefficients of different gases

    O2=0.024 CO2=O.57 CO=0.018 N2=0.012 H=0.008

  • Water solubility of CO2 20 times more than that of O2

  • Partial pressure of carbon dioxide is less than one twentieth that exerted by oxygen.

Water vapor pressure
Water vapor pressure

  • In airway passage air gets humidified, water vapors mixed up with inspired air

  • At body temp. 370C pH2O =47mm Hg

  • pH2O directly proportional to temperature

  • In fever pH2O is more

Rate of diffusion
Rate of diffusion

D=Δ P×A×S/d×√MW

Δ P=Partial pressure difference A=cross-sectional area S=solubility of gas d= distance

√MW=molecular weight

  • Diffusion coefficient=S/ √MW

  • Two gases at same partial pressure, rate of diffusion proportional to diffusion coefficient

Respiratory unit
Respiratory Unit

  • Respiratory Lobule

  • Respiratory bronchiole

  • Alveolar ducts

  • Atria

  • Alveoli

Gas exchange

Respiratory membrane or pulmonary membrane
Respiratory Membrane or Pulmonary Membrane

Membranes of all the

terminal portions of the


Factors that affect the rate of gas diffusion through the respiratory membrane
Factors That Affect the Rate of Gas Diffusion Through the Respiratory Membrane

  • Thickness of membrane

  • Surface area of membrane

  • Diffusion coefficient

  • Partial pressure difference of the gas

  • Edema & Fibrosis

  • Emphysema

  • Solubility of gas/ √ Mol. Weight

  • partial pressure of gas in the alveoli and partial pressure of the gas in the pulmonary capillary blood

Diffusion capacity
Diffusion Capacity Respiratory Membrane

Volume of a gas that will diffuse through the membrane each minute for a partial pressure difference of 1 mmHg

  • Diffusing capacity for oxygen

    21 ml/min/mm Hg at rest

    65 ml/min/mm Hg during exercise

  • Diffusing capacity for carbon dioxide 20 times more than O2

    400 to 450 ml/min/mm Hg at rest

    1200 to 1300 ml/min/mm Hg during exercise

Measurement of diffusing capacity
Measurement of Diffusing Capacity Respiratory Membrane

  • Alveolar Po2

  • Po2 in the pulmonary capillary blood

  • Rate of oxygen uptake by the blood

    Diffusing capacity(DC) of CO=Volume of CO absorbed


    DC of O2 = DC of CO × 1.23

    = 17× 1.23= 21ml/min/mmHg

Ventilation perfusion ratio
Ventilation – Perfusion Ratio Respiratory Membrane

The imbalance between alveolar ventilation and alveolar blood flow

  • Va Alveolar ventilation

  • Q Blood flow

  • Va/Q

  • When the ventilation(Va) is zero, yet there is still perfusion (Q) of the alveolus, Va/Q is zero

  • When there is adequate ventilation (Va) but zero perfusion (Q),Va/Q is infinity.

Physiological shunt
Physiological Shunt Respiratory Membrane

  • When Va/Q is below normal

  • Shunted blood

  • Bronchial vessels

  • The total quantitative amount of shunted blood per minute is called the physiologic shunt

  • The greater the physiologic shunt, the greater the amount of blood that fails to be oxygenated as it passes through the lungs.

  • Lower part of lung Va/Q is 0.6 times below normal

Physiological dead space
Physiological Dead Space Respiratory Membrane

  • When Va/Q is ∞

  • Alveolar wasted ventilation or alveolar dead space

  • Anatomical dead space

  • The sum of these two types of wasted ventilation is called the physiologic dead space

  • When the physiologic dead space is great, much of the work of ventilation is wasted effort because so much of the ventilating air never reaches the blood

  • Upper part of Lung Va/Q 2.5 times more than normal