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What are the pertinent physical laws in the determination of factors affecting diffusion at the alveolar-capillary membrane?
Fick's first law
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What are the barriers to diffusion from lungs to the blood?
- 1. Surfacant
- 2. ALVEOLAR MEMBRANE
- 3. INTERSTITIAL SPACE (FLUID)
- 4. CAPILLARY MEMBRANE
- 5. Red blood cells
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What is Fick's first law of diffusion?
- Vgas = A x D / T * (P1-P2)
- A is the cross-sectional area available for diffusion
- D is the diffusion coefficient of the gas
- T is the thickness of the membrane
- (P1-P2) is the partial pressure gradient across the membrane
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If you have a greater surface area, diffusion coefficient, partial pressure gradient and minimal thickness; is the diffuision increased or decreased?
Increased
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If you have less of an area, a lower diffusion coefficient, less of a pressure gradient and a greater thickness or distance; is the diffuision increased or decreased?
Decreased
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What are the partial pressure gradients for O2 and CO2 from mixed venous blood to the alveolus?
- Normal arterial: 40-100
- Normal venous: 46-40
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What is the diffusion coefficient?
Constant of how well a gas will diffuse
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What is the solubility coefficients for O2 and CO2 and explain how this affects diffusion?
0.003 is the solubility coefficient of oxygen at 1 mmHg in 100ml of plasma
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How do time limitations affect diffusion?
Rapid blood flow with a diffusion defect will cause inadequate oxygenation
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What is diffusion capacity?
DL is the diffusing capacity of the lung measured as the amount of gas (ml/min) that diffuses into the blood for each 1 mm Hg difference in the pressure gradient
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What is the normal DLCO?
- 25 ml/min/mm Hg for a steady state test
- for a single breath test is 40 ml/min/mm Hg
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What are systemic diffusion gradients?
- PO2 of 100 mmHg (tissue capillary) --> PO2 <40 mmHg (cells)
- PCO2 >46 mmHg (cells) --> PCO2 of 40 mmHg (capillary blood)
- Venous blood has a PO2 of about 40 mmHg and a PCO2 of about 46 mmHg
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What are the normal variations between ventilation and perfusion in the healthy lung and its effect on local gas tensions?
- 1. Right-to-left shunts in the pulmonary and cardiac circulation
- 2. Regional differences in pulmonary ventilation and blood flow
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What is the difference between alveolar and arterial oxygen tensions?
PaO2 of healthy individuals breathing air at sea level is always about 5 to 10 mmHg less than the calculated PAO2
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What is an anatomic shunt?
perfusion without ventilation
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What is the value for the normal anatomic shunt?
3-5%
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What is the normal respiratory exchange ratio?
0.8
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How can the normal respiratory exchange ratio be altered?
Changes in blood flow
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What is the ventilation to perfusion ratio?
- Comparison of PAO2 to PACO2.
- ideal ratio is 1.0 and indicates that ventilation and perfusion are in perfect balance. (ex. 10/10 = 1)
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What is a high V/Q and a low V/Q?
- high V/Q indicates that ventilation is greater than perfusion. (ex. 10/5 = 2
- Here the PAO2 is higher and the PACO2 is lower than normal
- A low V/Q indicates that perfusion is greater than ventilation. (ex. 5/10 = 0.5)
- Here the PAO2 is lower, and the PACO2 is higher than normal
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What are the effects of alterations of V/Q?
- High V/Q (to the right), R increases, PAO2 increases and PACO2 decreases. Gases in dead space areas are similar to that of inspired air.
- Low V/Q (curve to the left), R decreases, PAO2 decreases, and PACO2 increases. Gases in the shunt range are like that of mixed venous blood.
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What are alveolar shunts and how do they compare to dead space?
- Alveolar shunts are exchange units with a V/Q of 0. They are not normal.
- Dead space has exchange units with a V/Q greater than 1, so if a shunt occurs, there is no ventilation, but normal perfusion
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What are the normal regional differences that occur in the lung?
- Due mainly to gravity and thus are most evident in the upright posture
- Blood flow is 20 times greater at the base than at the apices
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What are the perfusion variations?
both blood flow and ventilation increase from the top to the bottom of the lung, but blood flow increases proportionately more than ventilation
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What are the ventilation variations?
With ventilation, the bases have about four times as much ventilation as the apices due to gravity's effect on pleural pressures
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