Lung Ventilation and Gas Exchange

Question 0

One major variable in CO2 elimination?

Answer 0

Alveolar ventilation.

Question 1

4 major variables that determine O2 delivery from atmosphere to tissue?

Answer 1

Alveolar oxygen content.
Alveolar-capillary exchange.
Blood oxygen content.
Blood flow & distribution.

Question 2

Partial pressures sum to total pressure.

Partial pressure of X / total pressure = moles of X / total moles.

Answer 2

Good things to be aware of.

Question 3

Partial pressure of O2 (pO2) in atmosphere at sea level?

Answer 3

159 mmHg.

out of the total 760 mmHg atmospheric pressure

Question 4

As you increase altitude does pO2 change?

Answer 4

Yes, it decreases.

However, pO2 / total pressure remains constant.

Question 5

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?

Answer 5

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.

Question 7

What’s Henry’s Law?

Answer 7

Gas content in a liquid = partial pressure * solubility.

Question 8

PAO2 vs. PaO2?

Answer 8

PAO2 = Alveolar pO2.
PaO2 = arterial pO2.
(but.. it’s a really bad idea to use abbreviations that are easily confused…)

Question 9

V(E) with a dot on top?

Answer 9

Total minute ventilation: Tidal volum * resp. frequency

Question 10

How can you calculate the proportion of total ventilation that is dead space ventilation?

Answer 10

With alveolar and expired pCO2.

V(D) / V(E) = (Alv. pCO2 - Expired pCO) / Alv pCO2

Question 11

What can cause increased physiologic dead space ventilation?

What might be a result?

Answer 11

Capillaries damaged.

Gas exchange doesn’t occur there.

Question 12

How does increased ventilation affect alveolar pO2?

pCO2?

Answer 12

Increased ventilation raises alveolar pO2 and lowers alveolar pCO2.

Question 13

How can alveolar pO2 be calculated? (with the alveolar air equation 1)

Answer 13

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)

Question 14

What’s the A-a gradient?

What’s normal?

Answer 14

pAO2 - paO2
For a young, non-smoker, normal is <10.
But upper limit of normal increases with age.

Question 15

Final, simplified version of the alveolar air equation to remember?

Answer 15

pAO2 = 150 - 1.2*paCO2

Question 17

How is the diffusion capacity of the lungs - D(L) - actually measured?
Equation for this?

Answer 17

Using (a small amount of!) carbon monoxide.

D(L) = CO uptake / pACO

Question 18

3 physiological causes of impaired gas exchange at the alveolus?

Answer 18

Decreased diffusion.
R -> L blood flow shunt.
Ventilation/perfusion (V/Q) mismatch.

(but really, they’re all V/Q mismatch)

Question 19

What’s the normal, optimum value of V/Q?

Answer 19

1

arbitrarily.. this doesn’t correspond to normal units if ventilation / blood flow

Question 20

What’s the V/Q when there’s dead space? (i.e. no perfusion)

Answer 20

Dead space: V/Q = (approaches) infinity

Question 21

What’s the V/Q when there’s an obstruction in the airway, forming an effective “shunt”?

Answer 21

0

Question 22

Why is a blocked airway called a “shunt”?

Answer 22

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.

Question 23

What’s Fick’s diffusion equation solved for the diffusion capacity of the lungs, in terms of oxygen?

Answer 23

D(L) = O2 uptake per min / A-a gradient