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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

In atmospheric air

- PO2160mmHg
- PCO 2 0.3mmHg
- PN2 600mmHg

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

HENRY’S LAW

- 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

- 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

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 Lobule
- Respiratory bronchiole
- Alveolar ducts
- Atria
- Alveoli

- Diameter 0.2 milliliter
- Sheet of flowing blood

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

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

- Alveolar Po2
- Po2 in the pulmonary capillary blood
- Rate of oxygen uptake by the blood

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

pCO

DC of O2 = DC of CO × 1.23

= 17× 1.23= 21ml/min/mmHg

Ventilation – Perfusion Ratio

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

- 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

- 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

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