1.3.3 Mechanism of Gas Exchange

Question 1

What is PvO2?

Answer 1

Partial pressure of venous O2

Question 2

What causes the lag demarcated by the blue lines?

Answer 2

The lag is due to the dead space in the respiratory system where gas exchange doesn’t occur.

Question 3

What must be subtracted from wet air to calculate the partial pressure of oxygen?

Answer 3

The partial pressure of water (PH₂O) which is 47 mm Hg at 37 degrees celcius. This value is independent of the total pressure

Question 4

Are these gases diffusion or perfusion-limited: N₂O and CO.

Answer 4

N₂O = perfusion-limited; Partial pressure of N2O in the capillary raises rapidly because N2O is an “inert” gas, i.e. a gas that does not combine with any element of blood, and because it has very low solubility in blood. N2O crosses the membrane and, since it remains in solution in the blood, it rapidly builds a blood partial pressure which decreases the alveolo-capillary gradient and limits diffusion. As blood PN2O equals alveolar PN2O, net diffusion stops. The only way in which more N2O can be loaded into the blood is if blood without N2O enters the lung, thus creating a gradient again. Poorly soluble, inert gases like N2O are called perfusion limited because transfer depends on providing of adequate supply of fresh blood free of the gas in question.

CO = diffusion-limited; CO is the opposite, since it combines with Hb. Hb affinity for CO is over 200 times that of O2. Molecules of CO entering blood combine with Hb and only a few remain in solution, in this manner PCO of blood does not increase appreciably as blood flows through the capillary. This means that the alveolo-capillary gradient remains essentially unchanged during the entire time the blood remains in the capillary. Gases like CO are called diffusion-limited because the only factor that would limit its transfer is an increased “resistance” of the membrane to diffusion. CO is an excellent candidate to use in measurement of diffusion capacity, as we will see later

Question 5

What is FACO2?

Answer 5

The fraction of CO2 in alveoli

Question 6

What are the two ways to manipulate the partial pressure of a gas?

Answer 6

Changing total pressure (PB) or changing the concentration (FIO₂)

Question 7

What are the respective values for FIO2 and FIN2?

Answer 7

FIO2 - .21

FIN2 - .79

Question 8

What determines the diffusion of O₂?

Answer 8

VO₂ = (PAO₂- PcapilO₂) * DLO₂

Question 9

Doubling and halving ventilation will have what effect on FACO2?

Answer 9

Doubling of alveolar (not total) ventilation will halve PACO2 (and PaCO2); halving alveolar ventilation will double PACO2.

Question 10

What is PaO2?

Answer 10

Partial pressure of arterial oxygen

Question 11

Describe how partial pressure of gases combine to create a total pressure.

Answer 11

Ptotal = Pa + Pb

Question 12

What is VE?

Answer 12

Volume exhaled into the spirometer

Question 13

Find the partial pressures of gases a and b given that the Ptotal is 100 mm Hg; a, n=6 and b, n=14.

Answer 13

Question 14

A person with normal lungs and normal blood gas values, with a tidal volume of 600 ml, breathes through a 200 ml snorkeling tube. Tidal volume and respiratory rate remain constant during the time the person breathes through the tube. Calculate the approximate values that PAO2 and PACO2 will reach once a steady state is achieved.

Answer 14

Snorkeling

Let’s assume that, before snorkeling, dead space was 200 ml and alveolar ventilation 400 ml (1/3 VD, 2/3 VA). If we add 200 ml to VD (snorkel) and VT remains unchanged, then VA goes down to 200 ml (i.e.“true”VD200ml +snorkel200ml+VA200ml= 600ml).This means that VA decreased to 50% of the original value. Other things being equal, this means that PACO2 will double (i.e. from a normal of 40 mm Hg to 80 mm Hg).

The new PAO2 will be:

PAO2 = 147–(80x1.2)=51mmHg

You may get slightly different results depending on the VD/VT value you use; the one I used is on the rather high side of normal but makes calculations easier. The point is that when you add dead space you must increase tidal volume by a proportionate amount, or you effectively end up hypoventilating. This is always taken into consideration in the design of respirators, respiratory equipment and anesthesia machines, which usually add very little dead space.

Question 15

What is the equation for difussion capacity?

Answer 15

Question 16

A patient with alveolar hypoventilation has a PaCO2 of 60 mmHg. Assuming that PaCO2 will not change, calculate how much you would have to increase the inspired PO2 to maintain alveolar PO2 within normal range.

Answer 16

Question 17

How does increasing (PAO₂- P capil) O₂ effect O₂ flux?

Answer 17

Increases flux

Question 18

Diffusing capacity of the lung for CO

Answer 18

Question 19

Answer 19

a. decrease
b. increase. Diffusion capacity is influenced by the amount of Hb contained in the pulmonary capillaries, with DLO2 increasing as Hb increases. Basically, the total amount of Hb in the lungs can change if blood Hb concentration changes, or if capillary blood volume changes.
c. decrease

Question 20

Describe how ventilation and blood flow affect O₂ and CO₂.

Answer 20

O₂: Delivered by ventilation, removed by blood flow (metabolism)

CO₂: delivered by blood flow (metabolism), removed by ventilation

Question 21

What is PAO2?

Answer 21

That partial pressure of alveolar O2

Question 22

If hypoventilation results in a 25% increase PACO2 (from 40 to 50 mm Hg) and PIO2 =147 mm Hg, describe how all of the other gas concentrations will be effected.

Answer 22

Question 23

The air we breathe is a mixture of O2 and N2. In dry air, FO2 is ~ 21%, and N2 is ~ 79%

The total pressure is the barometric pressure, which at sea level it is 760 mmHg. Find the partial pressures of O2 and N2.

Answer 23

PO2 = 760 * 0.21 = 160 mm Hg

PN2 = 760 * 0.79 = 600 mm Hg

Question 24

What are the partial pressures of gases (O₂, CO₂, PAN₂) in alveoli, veins, and arteries?

Answer 24

Question 25

Describe the phases: pre-inspiration, inspiration, end-inspiration, and expiration. What does it mean that V_E = V_A + V_D?

Answer 25

The equation, V_E = V_A + V_D, refers to the fact that expired air is a composition of air that undergoes gas exchange in the alveoli and air that is trapped in dead space (doesn’t undergo exchange).

Question 26

What are some of the factors that determine DLO₂?

Answer 26

Gas properties: O₂ solubility and diffusivity

Lung properties: surface area for diffusion, distance alveolus - red cell

Hb-O₂ characteristics: Time for O₂ to combine with Hb

Question 27

What equation is beneficial for measuring gas exchange? What is the average value in a healthy individual?

Answer 27

VD/VE = (FACO2 - FECO2)/FACO2); 0.3

Question 28

What is the alveolar gas equation?

Answer 28

PAO₂ = PIO₂ - (PaCO₂ * 1.2)

Question 29

Which has a greater capacity for diffusion O₂ or CO₂?

Answer 29

CO₂ has a greater capacity for diffusion. It takes less of a gradient to push CO₂ across the alveolus.

Question 30

The sum of the partial pressures of all gases must add up to what?

Answer 30

P_B

Question 31

Answer 31

Question 32

What happens if DLO₂ is reduced?

Answer 32

A is normal

B is moderate reduction of DLO₂

C is severe reduction in DLO₂

Question 33

At KUMC, PB =747. What is PIO₂? (hint: this is wet air)

Answer 33

PIO2 = (747-47) * 0.21 = 147 mm Hg

Question 34

What is PO2?

Answer 34

Partial pressure of O2 in a system

Question 35

Indicate in which direction the following will change when DLO2 is abnormally low:

a. Difference between alveolar and arterial PO2
b. Arterial PO2 in exercise, compared to resting PaO2
c. Resting arterial PCO2

Answer 35

a. A-aPO2 will increase
b. Arterial PO2 will be lower in exercise than at rest
c. Resting arterial PCO2 will depend on the severity of the problem. If DLO2 is low enough to significantly lower arterial PO2 and induce hyperventilation, then PaCO2 will be low. If PaO2 at rest is normal, PaCO2 will also be normal.

Question 36

What is the equation for the pressure produced by a single gas?

Answer 36

Question 37

What is the equation for the diffusion of a substance across a membrane?

Answer 37

Question 38

What is the normal duration for partial pressures of O2 and CO2 to reach equilibrium?

Answer 38

~0.25 sec

Question 39

What is PIO2?

Answer 39

The partial pressure of O2 in inspirated air

Question 40

If hyperventilation results in a 25% decrease PACO2 (from 40 to 30 mm Hg) and PIO₂ =147 mm Hg, describe how all of the other gas concentrations will be effected.

Answer 40

Question 41

A young person with normal lungs has a resting tidal volume of 600 ml and a respiratory frequency of 12 breaths/min. What would happen to the alveolar PO2 and PCO2 if this person would double the frequency and lower the tidal volume to one half for a few minutes?

Answer 41

In normal conditions during breathing at rest about one fourth to one third of the air entering the lungs remains in the airways (VD). In this case, this would be about 200 ml

So, when the person breathes normally, alveolar ventilation is VA = (600-200) x 12 = 4800 ml/min

when the person halves the tidal volume, the air volume that occupy the airways does not change much, so in this case

VA = (300- 200) x 24= 2400 ml/min.
Alveolar ventilation has halved, so PaCO2 should rise and PaO2 should fall.

Question 42

Describe what hyper- and hypoventilation will do PAO₂ and PACO₂.

Answer 42

Hyperventilation: High PAO₂, Low PACO₂

Hypoventilation: Low PAO₂, High PACO₂

Question 43

During expiration, what portion of gas can provide a good reflection of PAO2 and PACO2?

Answer 43

End tidal gas (air at the end of expiration)