4.7 Diffusion Gas Exchange & Gas Transport
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Explain how Fick’s Law describes diffusion from the alveolar space to the blood in the pulmonary capillary.
- Diffusion of a gas through a barrier is proportional to:
- 1. Surface area of the barrier
- 2. Partial pressure difference of the gas from one side of the barrier to the other
- 3. Diffusion coefficient for the gas (proportional to solubility, inversely proportional to the square root of molecular weight)
- Diffusion is inversely proportional to thickness
Compare and contrast diffusion and perfusion limited gases.
- Diffusion limited gas - bound quickly in the blood so that the amount of gas dissolved in the blood increases slowly. Ex. CO.
- Perfusion limited gas - bound weakly in the blood so that the amount of gas dissolved in the blood increases rapidly to the partial pressure in the alveolar space. Ex. N2O (nitrous oxide)
Compare and contrast the time of blood in the pulmonary capillary at rest and during exercise.
Exercise - time is ↓ b/c of the ↑ in cardiac performance
Contrast the ventilation/perfusion ratio at the apex and base of the lungs.
- Apex: 3.0 (Highest)
- Base: 0.6 (Lowest)
Describe the implications of the high and low ventilation/perfusion ratios on the PaO2 and PaCO2 at the apex and base.
- Apex: ↑ V/Q → ↑ PaO2 and ↓ PaCO2
- Base: ↓ V/Q → ↓ PaO2 and ↑ PaCO2
For airway obstruction, what is their effect on the normal ventilation/perfusion ratio, PAO2, PACO2, PaO2, and PaCO2.
- V/Q: 0
- PAO2: --
- PACO2: --
- PaO2: 40
- PaCO2: 46
For pulmonary embolus in terms, what is their effect on the normal ventilation/perfusion ratio, PAO2, PACO2, PaO2, and PaCO2.
- V/Q: ∞
- PAO2: 150
- PACO2: 0
- PaO2: --
- PaCO2: --
Contrast the causes of hypoxemia with the causes of hypoxia.
- Hypoxemia: ↓ in arterial PO2
- Hypoxia: ↓ in delivery of O2 to tissues
- Hypoxemia is one of the major causes of hypoxia
Contrast the effect on the A-a gradient and the benefit of supplemental O2 in the following situations:
- Diffusion defect
- Ventilation/Perfusion defect
- Right-to-left shunt
- High altitude
Describe the binding of O2 and hemoglobin in the red blood cell.
- RBC main job is to protect hemoglobin b/c if free it can be catabolized and excreted w/in minutes
- O2 bound to hemoglobin
Contrast the amounts of O2 dissolved in plasma and bound to hemoglobin
- 98% of O2 in blood bound to hemoglobin
- 2% of O2 dissolved in blood
What will cause a shift to the right for O2 and hemoglobin affinity in terms of CO2, pH, temperature and 2-3-DPG?
- ↑ CO2
- ↓ pH
- ↑ Temperature
- ↑ 2-3 DPG
- ↓ affinity
What will cause a shift to the left for O2 and hemoglobin affinity in terms of CO2, pH, temperature and 2-3-DPG?
- ↓ CO2
- ↑ pH
- ↓ Temperature
- ↓ 2-3 DPG
- ↑ affinity
Relate the change in affinity to the change in the P50 of O2-hemoglobin dissociation curve.
- O2 and hemoglobin affinity ↓ = P50 ↑
- O2 and hemoglobin affinity ↑ = P50 ↓
Describe the transport of CO2 back to the lungs from the peripheral tissues.
- Most of CO2 (90%) combine w/ H20 in the erythrocyte to form first carbonic acid and then H+ and HCO3-
- Rxn is catalyzed in the erythrocyte by the enzyme carbonic anhydrase
- Hemoglobin buffers the H+ to keep the rxn moving toward the production of HCO3- which passes out the erythrocyte into the plasma in exchange w/ Cl- (Chloride Shift)
- Process reverses at lungs so that the CO2 can be liberated and diffuse into the alveolar space
Contrast the amount of CO2 dissolved in plasma, bound as carboaminohemoglobin, and coverted to bicarbonate.
- 5% - Dissolved in blood
- 5% - Bound to hemoglobin forming carbaminohemoglobin
- 90% - Bicarbonate
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