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19 Cards in this Set
- Front
- Back
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Dalton's Law of partial pressure - atmosphere
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Barometric pressure = 760 mmHg (at sea level)
- P-total = PN2 + PO2 + PCO2 + PA = 760 mmHg |
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Calculating PO2
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Total barometric pressure x fractional concentration of O2
- 760mmHg x 0.21 = 160 mmHg |
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PO2 concentrations by location
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Ambient dry air = 760 x 0.21 = 160 mmHg
- Moist tracheal air - H2O saturation! = 47 mmHg - Thus PO2 = (760-47) x 0.21 = 150 mmHg - Alveloar PO2 = 100 mmHg |
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Minute Ventilation
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Volume of breath per minute = tidal volume x resp. rate
- 500 mL x 15 breaths/min = 7500 mL |
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Alveolar ventilation (VA)
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Volume that makes it to alveoli = Minute ventilation minus deadspace
- (500 mL - 150mL) x 15 breaths/min = 5250mL/min - Clinically determined by PaCO2! |
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Pressure of CO2 in arterial blood (PaCO2)
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- All CO2 in blood is produced by the body!
- Thus, PaCO2 = (production CO2/VA) |
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Hyperventilation vs. hypoventilation
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- Hyperventilation = greater PA (alveolar ventilation), lower PaCO2
- Hypoventilation = less PA, higher PaCO2 |
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Ventilation distribution in lung
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In standing person - ventilation is highest at base!
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Gas exchange values in circulation
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Per earlier, PaCO2 = production CO2/alveolar ventilation
- In trachea - PO2 = 150 mmHg, PCO2 = 0! - In alveoli - PO2 = 100 mmHg, PCO2 = 40 mmHg - Arterial blood = PO2 = 100 mmHg, PCO2 = 40 mmHg - Venous blood = PO2 = 40 mmHg, PCO2 = 46 mmHg |
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Relative constancy of alveolar partial pressure due to....
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Due to FRC!
- FRC = 2500mL, Tidal volume is only 500mL... - New air coming in does little to dilute a much larger volume already present |
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Cause of gradual decrease in PAO2 vs. atmospheric
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- PO2 in atmosphere = 160 mmHg, in trachea = 150, alveoli = 100...
- Saturation with water vapor - Continuous uptake of O2 into blood - constantly decreasing - Dilution with dead space air (from FRC) |
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Alveolar gas equation
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Solving for pp of O2 in alveoli
= PAO2 = PIO2 - (PaCO2/RQ) - RQ - respiratory quotient = VCO2/VO2 = 0.8 - PIO2 = (atmospheric pressure - PH20) x fraction of O2 |
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Fick's Law of Diffusion
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- Determines how much of O2 in alveoli will make it into circulation
- Vgas = A/T(D) - (P1 - P2) - D = diffusion constant related to molecular weight - Less Area (A) - lower diffusion - Thickness of membrane (T) - inversely proportional *** Disease factors = Fibrosis thickens membrane, emphysema decreases area... |
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O2 diffusion times and factors
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- Venous blood PO2 = 40
- RBC's in capillaries for about 0.75 seconds - Exercise reduces RBC time in capillary to 0.25 secs - Normally, within 0.25 seconds, PO2 goes from 40 -> 100 mmHg - Many factors (area, thickness, lower PO2) -> slower diffusion - Thus, PO2 may not reach 100... *** Perfusion limited - amount in blood depends on flow rate - eq. quickly with PAO2 - Higher HR -> greater volume of O2 diffuses into blood |
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CO2 diffusion times and factors
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- Venous blood PCO2 = 45
- CO2 diffusion rate is 20x faster than O2 (more soluble) - CO2 exchange still relatively low despite diffusibility - Lower pressure gradient (P1 - P2) - Time required for HCO3 -> CO2 - Overall, time to eq. = 0.25 seconds also *** Perfusion limited - will eq. quickly with PACO2 - perfusion limited by breathing rate |
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Perfusion-limited gasses
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- Any gas that equilibrates with alveolar partial pressure such that the rate of perfusion
is limited by the HR - Will increase/decrease based on cardiac output/HR - N2O is also perfusion limited |
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Diffusion-limited gasses
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- When gas does not equilibrate across the membrane
- Disease states thicken membrane/lower pressure gradient/lower area - RBC doesn't get to eq. with PAO2 = diffusion limited *** CO = classic example - Binds to Hb quickly, pressures never equilibrate - PCO in circulation is limited by how much CO diffuses while RBC is in capillary! |
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Measurement of diffusion capacity
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- Use CO!
- CO is unique because rapidly binds to Hb -> PCO remains very low in blood - Give 0.03% CO to patient for 10 seconds (1 breath) - Measure [CO] when they exhale - CO not normally in air - whatever comes out must have been given by you - DLCO = diffusion-limited CO DLCO = VCO/PACO - Normally 25 mL/min/mmHg - Decreased DLCO - from disease states! - Emphysema, fibrosis, pneumonia |
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P(A-a)O2 gradient measurement
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- Take blood gas readings (PaO2 and PaCO2) and calculate via alveolar gas equation
- Ideally, no gradient! -> Blood PaO2 has perfectly eq. with PAO2 - Normally, some small gradient...more than 12-15 is bad... - Increased gradient = diffusion impairment, anatomical shunt, V/Q mismatch |
