Learning Objectives
- At the end of the lecture you all should be able to:
- Explain respiratory membrane
- Discuss the factors affecting the diffusion of gases across the respiratory membrane
- Discuss the effect of ventilation-perfusion ratio on alveolar gases
- Explain the physiological dead space and physiological shunt and its relationship with ventilation perfusion ratio
- Discuss the Abnormal ventilation perfusion ratio in chronic obstructive lung disease
Solubility Coefficients
Factor causing diffusion of gases in fluid
Suppose diffusion co-efficient of O2 is 1.
Composition of alveolar air
Replacement of Alveolar Air
Factors affecting Diffusion across Respiratory Membrane
- Thickness of membrane
- Partial pressure difference of gases
- Surface area of the membranes
- Diffusion coefficient of gas
Factor causing diffusion of gases in fluid
Diffusion Capacity
- The volume of the gas that will diffuse through the membrane each minute for a partial pressure difference of 1mmHg.
- O2: 21ml/min/mmHg
- Exercise
Diffusion Capacity
- Carbondioxide: 400ml/min/mmHg
- Carbon monoxide : 17ml/min/mmHg
Affect of Ventilation Perfusion Ratio on Alveolar Gas Concentration
Partial Pressures of Gases in Alveoli
•Oxygen?
Partial Pressures of Gases in Alveoli
- Oxygen: 104 mmHg
- Carbon dioxide?
Partial Pressures of Gases in Alveoli
- Oxygen: 104 mmHg
- Carbon dioxide: 40mmHg
Ventilation- Perfusion ratio (VA/ Q̇ )
- is a ratio used to assess the efficiency and adequacy of the matching of two variables:
- ventilation – the air that reaches the alveoli.
- perfusion – the blood that reaches the alveoli via the capillaries.
Ventilation-Perfusion ratio
- Factors that determine the partial pressure of gases in alveoli:
- (1) The rate of absorption of that gas into the blood and
- (2) The rate of entry of new gas into the lungs by the ventilatory process.
Ventilation Perfusion ratio
•VA/Q
When, Va=0 and Q=Normal
When Va=Normal and Q=0
Physiological Shunt
V/Q ration is less than normal
Physiological Dead space
V/Q ration is more than normal
