Gas diffusion and transport Flashcards
(35 cards)
What is the drive for the diffusion of gases across the alveolar-capillary membrane?
1) Partial pressure gradient
2) Pulmonary capillary blood flow
3) The diffusion properties of the alveolar-capillary membrane
How to calculate the partial pressure of oxygen?
- Dalton’s law:
Po2 (about 21%) = Pb (barometric pressure) * Fo2 (Fractional concentration of O2)
- Pb is the sum of partial pressures of all gases in a mixture
Describe the changes in oxygen tension “PO2” with altitude
As we move to higher altitudes, the atmospheric pressure decreases, while the oxygen concentration stays the same, however, the partial pressure of oxygen (Dalton’s law: PO2 = PB x FO2) decreases.
What happens to the diffusion when we increase our lung volume (inspire)?
Inspiration will increase the lung volume, increasing the area for diffusion and decreasing the thickness of the alveolar-capillary membrane which will ultimately lead to more diffusion
What is the partial pressure of O2 and CO2 in the lung and the systemic circulation?
Oxygen partial pressure in the air is 160 mmHg, while in the alveoli it is 100 mmHg, and in the vein, it is 40 mmHg
- Diffusion gradient across the alveolar-capillary membrane is 60 mmHg
On the other hand, CO2 is 0 mmHg in the air, 40 mmHg in the alveoli, and 46 mmHg in the veins
- The diffusion gradient is 6 mmHg
What is meant by the transit time?
It is the time required for RBC to move through the capillary (0.75 sec)
What are the two factors that limit diffusion?
1) Perfusion-limited
2) Diffusion-limited
What is meant by perfusion-limited?
- Once equilibrium is reached there is no partial pressure difference between the alveoli and the blood
- Limitation to diffusion due to the blood flow, so for example N2O reaches the equilibrium faster than 0.75 seconds (it reaches it by 0.15 seconds) which means that the limit to their perfusion is the pulmonary blood flow (perfusion)
What is meant by diffusion-limited?
- Once equilibrium is reached there is no partial pressure difference between the alveoli and the blood
- How much the lung can let the substance pass (limited by the permeability of the alveoli)
- For example, CO diffuses readily across the alveolar-capillary membrane, and binds to Hb quickly thus equilibrium is almost never reached as only free gases cause a partial pressure and thus it is diffusion-limited
Describe the oxygen diffusion at sea level and high altitude in a normal person
At sea level:
- In the lungs of a normal person oxygen perfusion at rest is perfusion-limited PA02 is constant at 100 mmHg, and PAO2 will reach equilibrium at 1/3 of the time taken for the blood to perfuse
At high altitudes:
- At high altitudes, the barometric pressure is reduced with the same O2 fraction in the inspired air the partial pressure of oxygen in the alveoli will also be reduced, this reduction in oxygen partial pressure means that the diffusion of O2 will be reduced, and more slowly taking 2/3 of the time instead of 1/3
Describe the oxygen diffusion in a person with a fibrosed lung
In fibrosed lungs and during strenuous exercise oxygen diffusion becomes diffusion-limited as
- For example in fibrosis, the alveolar wall thickens increasing the diffusion distance for gases, slowing the rate of O2 diffusion preventing the equilibration between the oxygen in the alveoli and the pulmonary capillary
- In such cases, equilibrium is not met between the alveoli and capillary and thus decreased oxygen pressure in the systemic circulation
What happens to the capacity of diffusion during exercise?
- During exercise, Vt (tidal volume), increases dramatically, increasing the alveolar surface area and decreasing the alveolar-capillary membrane
- Increased pulmonary blood flow due to recruiting closed ones and dilating the ones already open
- Both the alveoli and blood side of the alveolar surface area are increased (aiding diffusion)
- During exercise the uptake of O2 is determined by the amount of blood flow (perfusion limited)
- During rest and exercise CO has the highest diffusing capacity, and O2 and CO have the same capacity
What is Fick’s law of gas diffusion?
Vgas (volume of gas diffusing per minute) = As (membrane surface area) * D (diffusion coefficient of the gas) * partial pressure difference / T (membrane thickness)
- Volume of gas diffusing is directly proportional to the membrane surface area, partial pressure difference, and diffusion coefficient
- And the volume of gas diffusing is indirectly proportional to the thickness of the membrane
What is meant by the diffusion coefficient?
How soluble the thing is the greater its solubility the greater the coefficient, on the other hand, the higher its molecular weight the lower its solubility
If the alveolar surface area is decreased by 50% and the pulmonary thickness is doubled what is the diffusion rate compared to normal?
75% decreased
What is meant by lung diffusion capacity?
- It is a measure of how easy a gas can diffuse across the entire lung
- Lung diffusion capacity is a combination of diffusion coefficient, surface area of the membrane and the thickness of the membrane
Dl (lung diffusion capacity “ml/min/mmHg”) = Vgas / (P1-P2)
What are the different clinical applications of lung diffusing capacity?
1) In emphysema Dl decreases (due to decreased SA as the alveoli are destructed)
2) In fibrosis or pulmonary edema the Dl decreases (due to the increased membrane thickness)
3) During exercise the Dl increases as additional capillaries are perfused, which increases the SA for gas exchange
4) Decreased blood Hb decreases the Dl
5) Capillary blood volume, the lower the lower the Dl
If we want to measure the diffusion capacity of a subject which air molecules would be the most suitable one to use?
CO
- Because it is limited by the diffusion capacity and thus we can determine the DL with a single breath from a container that has CO
What are the two forms by which oxygen is carried in the body?
1) Dissolved (2% of the total O2 content)
2) Bound to Hb (98% of total O2 content)
- Dissolved O2 is the only form of O2 that produces a partial pressure
What is Henry’s law?
It measures the concentration of dissolved gas (ml gas/100 ml blood)
- Cx (concentration of a dissolved gas) = Px (partial pressure of a gas) * Solubility (solubility of a gas in blood)
- O2 has a Cx of 0.3ml/100 ml blood
- Partial pressure of oxygen is a force that is created by the dissolved O2 which acts to keep O2 bounded to Hb
What are the factors that oxygen depends on to remain bounded to Hb?
1) Affinity of Hb binding to O2
2) PO2
What is meant by the positive cooperativity of oxygen to Hb?
Whenever an O2 binds to one of the four Hb molecules in a RBC it makes it easier for the one after it to bind
How to calculate the saturation of Hb with O2 (SO2)?
- Oxygen carrying capacity: maximum amount of O2 that can be carried (20.1 ml O2/100 ml blood “this is the standard”)
- HbO2 content: this is the actual amount of O2 bound to Hb
- SO2 = (HbO2 content / HbO2 capacity) * 100
How to calculate the O2 content?
- O2 content is the actual amount of O2 per volume of blood
- O2 content (ml O2/100 ml blood) = oxygen bound to Hb (oxygen binding capacity “this is the maximum capacity of O2 binding which is usually 15g/100 ml” * Saturation “the actual percent of O2 bound to Hb”) + Dissolved oxygen (usually 0.3)
