Gas Exchange Flashcards
What is the formula for henry’s law for concentrations of dissolved gasses?
[x] = PX * SolubilityX
It is important to remember that the concentration of a gas in solution applies only to:
A dissolved gas that is free in solution, and does not include any gas that is present in bound form. (i.e. hemoglobin)
What is the driving force for diffusion of gas?
The partial pressure difference of the gas across a membrane.
NOT the concentration difference
What does the diffusion coefficient of a gas depend upon?
The solubility of a gas and it’s molecular weight.
Lung diffusing capacity DL combines the diffusion coefficient of a gas, surface area of a membrane, thickness of a membrane and the time required for the gas to combine with proteins in pulmonary capillary blood. We can measure DL with CO. Why?
Since CO transfer across the alveolar /pulmonary capillary barrier is limited exlusively by the diffusion process.
What happens to DL in emphysema? Why?
DL decreases due to destruction of alveoli, resulting in a reduction in surface area available for gas exchange.
What happens to DLin pulmonary fibrosis or pulmonary edema?
The fibrosis causes thickening of the membrane across which gas must diffuse. Leading to a reduction in DL
What impact does anemia have on DL?
Since the amount of hemoglobin in the blood is decreased (DL includes the protein binding component of O2 exchange), lung diffusing capacity is decreased.
What impact does exercise have on DL?
Exercise increases DL, because additional capillaries are perfused with blood, which increases the surface area for gas exchange.
What is the total concentration of a gas in solution based on?
The sum of the dissolved gas, bound gas and chemically modified gas present in solution.
What is PaO2?
Measure of dissolved oxygen molecules in the blood. (Not bound to Hb)
What is SaO2?
The percentage of all Hb binding sites that are occupied by oxygen (saturated with)
What is an example of a chemically modified gas in the human respiratory chain?
CO2, which is converted to bicarbonate in RBC’s by carbonic anhydrase. Most CO2 is carried in blood in this form rather than as dissolved or bound CO2
What is a physiological shunt?
When a small fraction of pulmonary blood flow bypasses the alveoli and therefore is not arterialized (oxygenated). This accounts for the small discrepancy between alveolar air and systemic arterial blood)
What are the two sources of physiological shunt?
- Bronchial blood flow
- a small portion of coronary venous blood that drains directly into the left ventricle rather than going to the lungs to be oxygenated
The physiological shunt may be increased in several pathologic conditions. What is this called? What is the outcome?
This is called a ventilation/perfusion defect. When the size of a shunt increases, equilibration between the alveolar gas and pulmonary capillary blood cannot adequately occur, and pulmonary capillary blood is not fully arterialized.
What is the A-a difference? Describe it under physiological and pathological shunting conditions.
This is the difference in PAO2 and PaO2
If the shunt is small “physiologic” then the A-a difference is small or negligable
If the shunt is larger - pathological - then the A-a difference increases to the extent that equilibration fails to occur.
Describe diffusion-limited gas exchange. Under what onditions will diffusion continue?
This means that the total amount of a gas that is transported across an alveolar-capillary barrier is limited by the diffusion process.
As long as the partial pressure gradient for the gas is maintained, diffusion will continue along the length of the capillary.
Describe perfusion-limited gas exchange. In this situation, how can you increase gas transport?
This means that the total amount of gas transported across the alveolar-capillary barrieris limited by blood flow through the pulmonary capillaries.
In perfusion limited exchange, the partial pressure gradient is not maintained, and the only way to increase gas transport is to increase blood flow.
Describe the gas trasport model for Oxygen.
Normally Oxygen is a perfusion limited gas. Under certain circumstances… strenuous exercise, emphysema or fibrosis…
Oxygen becomes a diffusion limited gas.
Normally, O2 equilibration occurs at what distance down the capillary?
1/3 the total distance.
In fibrosis, how is the gas exchange modality for oxygen changed?
The alveolar wall thickens, increasing the diffusion distance and decreasing DL. This slows the rate of diffusion of oxygen and prevents equilibration of oxygen between alveolar air and pulmonary capillary blood.
In this case the partial pressure gradient of oxygen is maintained across the entire length of the capillary, converting it to a diffusion-limited process.
It might seem as though the total amount of oxygen transferred would be greater in a person with fibrosis than in a person with normal lungs, this is not correct. Why?
Although the oxygen partial pressure gradient is maintained across a longer distance of capillary, the total transfer is greatly decreased due to the substantially reduced DL
Why does O2 exchange become diffusion limited during strenuous exercise?
Due to excessively rapid blood flow through the capillaries. Not enough time for O2 to bind.
