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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
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
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
If you have a greater surface area, diffusion coefficient, partial pressure gradient and minimal thickness; is the diffuision increased or decreased?
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?
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
What is the diffusion coefficient?
Constant of how well a gas will diffuse
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
How do time limitations affect diffusion?
Rapid blood flow with a diffusion defect will cause inadequate oxygenation
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
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
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
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
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
What is an anatomic shunt?
perfusion without ventilation
What is the value for the normal anatomic shunt?
What is the normal respiratory exchange ratio?
How can the normal respiratory exchange ratio be altered?
Changes in blood flow
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)
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
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.
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
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
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
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