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What is the difference between the normal gradients in oxygen and carbon dioxide partial pressures between the atmospher and tissues?
- Simple diffusion. For oxygen, it is a decreasing amount of tension from atmospheric to the cells.
- For CO2 it is opposite, from greatest amount at the cell to the least amount in the atmosphere. This aids the lungs in gettings rid of CO2
Why are partial pressure gradients for oxygen and carbon dioxide important within the body?
Because they are what allow for gas flow in and out of the lungs
How do you determine alveolar oxygen and carbon dioxide partial pressures?
- PACO2 = VCO2 x 0.863 / VA
- Normal value is 200 ml/min and VA is alveolar ventilation which is normally 4.3 L, so...
- PACO2 = 200 mL/min x 0.863 / 4.3 L/min = (approx) 40 mmHg
How is PACO2 maintained?
by increasing or decreasing the rate of breathing
How do you apply the air equation in determining PAO2?
- PAO2 = FiO2 x (PB - PH20) - PACO2/R
- IF the FIO2 is 60% or greater the correction for R can be dropped
- FiO2 = fractional concentration of inspired oxygen
- PB = barometric pressure
- PH20 = water vapor tension (47 mmHg at 37 C)
- PACO2 = alveolar partial pressure of carbon dioxide
- R = respiratory exchange ratio (nomarl value is 0.8) which is the ratio of carbon dioxide excretion to oxygen uptake.
- Example: PAO2 = 0.21 (760 - 47) - 40 / 0.8 = 100 mmHg
How do you compute the alveolar-arterial PO2 difference and the a/A ratio?
- Egan's pg. 239
- Normal difference is small, only 5 to 10 torr while breathing air and no more than 65 torr when breathing 100% O2
- P(A-a)O2 = PAO2 - PaO2
- Example: P(A-a)O2 = 449 - 50
- P(A-a)O2 ~ 400 torr
- Abnormal; 400 torr is very high.
- a/A = PaO2/PAO2
- Example: a/A = 50/449
- a/A = 0.11
- Only about 11%
- Abnormal; should be atleast 90%
What is the relationship of oxygen and carbon dioxide in the lungs?
As Oxygen goes up, Carbon dioxide goes down, and vice versa.
How do you calculate alveolar nitrogen partial pressure?
- PAN2 = PB - (PAO2 + PACO2 + PH20)
- PAN2 = 760mmHg - (100 mmHg + 40mmHg + 47mmHg)
- PAN2 - 760 mmHg - 187 mmHg
- PAN2 = 573 mmHg