Ch. 3 West Flashcards
Fick’s law
States that the rate of transfer of gas through a sheet of tissue is proportional to the tissue area and the difference in gas partial pressure between the two sides, and inversely proportional to the tissue thickness.
The rate of transfer is also dependent upon a diffusion constant which depends on the properties of the tissue and particular gas.
Explain the diffusion constant (D)
D ∝ Solubility/√molecular weight
i.e. the diffusion rate is proportional the solubility of the gas in the tissue and inversely proportional to the square root of the molecular weight
Diffusion limited vs. perfusion limited
Carbon monoxide is well known for its affinity for hemoglobin. When CO passes across the membrane it rapidly binds to hemoglobin and is thus removed from solution. This removal from solution maintains the pressure gradient across the membrane. So with a maintained pressure gradient the major impediment to CO transfer is diffusion across the membrane. Therefore CO transfer is referred to as diffusion limited and dependent on the properties of the membrane.
Nitrous oxide, alternatively, does not bind with hemoglobin at all, so its arterial partial pressure rises rapidly as it stays in solution. So maintaining the pressure gradient is dependent on how quickly the transferred nitrous oxide is washed away by blood flow. Because of this, nitrous oxide is referred to as perfusion limited.
The results for O2 fall much closer to the perfusion limitation (NO) line than the diffusion limitation line (figure 8.2, O2 normal). Oxygen binds to hemoglobin so the arterial PO2 does not rise as quickly as nitrous oxide, but the binding of O2 is so much less than carbon monoxide it actually demonstrates more perfusion, rather than diffusion limitation. The transfer of O2 is also hampered by having to start off at venous partial pressures (40 mmHg), compared to our test gases that start off at zero. Consequently the initial pressure gradient is less. Despite this, the arterial partial pressure of oxygen equilibrates with alveolar pressures within 0.25 seconds (i.e., a third of the distance around the capillary). With the blood still having another 0.5 seconds in the capillary, this provides a large reserve time.
This reserve time is often eaten into in some disease states (figure 8.2, abnormal); if for example a diffusion problem arises, such as thickening of the membrane, then that extra 0.5 seconds, or last 2/3 seconds of the transit time around the alveolus, can still allow alveolar and arterial PO2s to equilibrate. The patient may still show normal oxygen pressures until they exercise, during which the velocity of pulmonary blood flow increases and transit time is reduced; you can see from figure 8.2 if transit time is reduced to 0.5 seconds then arterial PO2 will not equilibrate with alveolar values in the abnormal lung.
While in the normal state the transfer of oxygen is perfusion limited, in lung diseases that affect the surface area or membrane thickness of the gas exchange surface, the transfer of oxygen may become diffusion limited. Being able to measure the transfer of gas into the blood provides a valuable diagnostic tool.
https://pressbooks.lib.vt.edu/pulmonaryphysiology/chapter/perfusion-and-diffusion-limitations-in-gas-exchange/#:~:text=Diffusion%20limitation%20is%20really%20a,pressure%20gradient%20is%20being%20maintained.
The normal value of the diffusing capacity for carbon monoxide at rest
25 mL/min^-1/mmHg^-1
Diffusing capacity (increases/decreases) with exercise
Increases
Tue or false: the time it takes for a compound (O2/CO) to react with hemoglobin contributes to the diffusion resistance
True
Oxygen transfer is normally _____ limited
Perfusion limited
What situations lead to oxygen transfer becoming more diffusion-limited?
Intense exercise, thickening of the blood-gas barrier, and alveolar hypoxia
The finite reaction rate of oxygen with hemoglobin can reduce its transfer rate into the blood, and the effect is similar to that of reducing the ______ rate
Diffusion rate
Diffusion of CO2 is about ____ times faster than that of oxygen due to the higher solubility of CO2
20 x faster
Carbon dioxide transfer across the blood-gas barrier is probably not _____ limited
Diffusion limited
