Flashcards in Lung Ventilation and Gas Exchange Deck (22):
4 major variables that determine O2 delivery from atmosphere to tissue?
Alveolar oxygen content.
Blood oxygen content.
Blood flow & distribution.
One major variable in CO2 elimination?
Partial pressures sum to total pressure.
Partial pressure of X / total pressure = moles of X / total moles.
Good things to be aware of.
Partial pressure of O2 (pO2) in atmosphere at sea level?
(out of the total 760 mmHg atmospheric pressure)
As you increase altitude does pO2 change?
Yes, it decreases.
However, pO2 / total pressure remains constant.
Why are the partial pressures of N2, CO2, and O2 lower in the trachea compared to the atmosphere? (assuming "dry" atmospheric air)
What is the new partial pressure of N2 + CO2 + O2?
The total pressure must stay at 760 mmHg to equilibrate with the atmosphere, but there is additional partial pressure of H2O in the trachea that displaces the other gases.
New partial pressure of N2 + CO2 + O2 = 713.
What's Henry's Law?
Gas content in a liquid = partial pressure * solubility.
PAO2 vs. PaO2?
PAO2 = Alveolar pO2.
PaO2 = arterial pO2.
(but.. it's a really bad idea to use abbreviations that are easily confused...)
V(E) with a dot on top?
Total minute ventilation: Tidal volum * resp. frequency
How can you calculate the proportion of total ventilation that is dead space ventilation?
With alveolar and expired pCO2.
V(D) / V(E) = (Alv. pCO2 - Expired pCO) / Alv pCO2
What can cause increased physiologic dead space ventilation?
What might be a result?
Gas exchange doesn't occur there.
How does increased ventilation affect alveolar pO2?
Increased ventilation raises alveolar pO2 and lowers alveolar pCO2.
How can alveolar pO2 be calculated? (with the alveolar air equation 1)
Alveolar pO2 = Inspired pO2 - (arterial pCO2 / correction factor)
Correction factor = respiratory quotient = 0.8 (under normal circumstances)
(really it's PAO2 = inspired pO2 - pACO2, but ACO2 can be calculated from PaCO2 as show above)
What's the A-a gradient?
pAO2 - paO2
For a young, non-smoker, normal is <10.
But upper limit of normal increases with age.
Final, simplified version of the alveolar air equation to remember?
pAO2 = 150 - 1.2*paCO2
How is the diffusion capacity of the lungs - D(L) - actually measured?
Equation for this?
Using (a small amount of!) carbon monoxide.
D(L) = CO uptake / pACO
3 physiological causes of impaired gas exchange at the alveolus?
R -> L blood flow shunt.
Ventilation/perfusion (V/Q) mismatch.
(but really, they're all V/Q mismatch)
What's the normal, optimum value of V/Q?
(arbitrarily.. this doesn't correspond to normal units if ventilation / blood flow)
What's the V/Q when there's dead space? (i.e. no perfusion)
Dead space: V/Q = (approaches) infinity
What's the V/Q when there's an obstruction in the airway, forming an effective "shunt"?
Why is a blocked airway called a "shunt"?
Deoxygenated blood passes through the alveolar capillary beds and back to the systemic circulation without any gas exchange, which makes this just like a R->L shunt elsewhere in the circulation.