Pulmonary Physiology: Gas Exchange and Transport

Question 1

What is Fick’s law of diffusion?

Answer 1

The flow of a gas (V) is proportional to area/thickness X diffusion coefficient (D) X (P1-P2)

V ∝ A/T x D(P1 - P2)

Question 2

What is Henry’s law?

Answer 2

A gas will dissolve in a liquid in proportion to its partial pressure over the liquid. The actual amount that dissolves in a liquid = solubility coefficient x partial pressure

Question 3

What are the solubility coefficients of O2 and CO2, respectively?

Answer 3

O2 = 0.003 mlO2/dl/mmHg
CO2 = 0.06 mlCO2/dl/mmHg

Question 4

What factors are the diffusion coefficient dependent upon?

Answer 4

solubility and molecular weight

D ∝ solubility / √MW

Question 5

What is responsible for the fact that O2 and CO2 diffuse through the air-blood barrier at roughly equal rates despite CO2 being much more soluble in aqueous solutions?

Answer 5

The pressure difference between alveolar air and venous blood is much higher for O2 than for CO2 so there is a stronger driving force for O2:
PAO2 - PaO2 = 60mmHg
vs.
PaCO2 - PACO2 = 6mmHg

Question 6

How long does it take for PO2 to equilibrate as it passes through the lung capillaries?

Answer 6

.25 sec (first third of the capillary)

Question 7

What is meant by “diffusion limited” and “perfusion limited?”

Answer 7

“Diffusion limited” means that equilibration of the gas is limited by how fast the gas can diffuse through the barrier.

“Perfusion limited” means that the gas equilibrates quickly and the speed at which the gas can diffuse doesn’t matter.

Question 8

In health, what is the normal alveolar O2 to arterial O2 difference (A-a DO2)? What happens as people age and why?

Answer 8

4mmHg. When people age the difference increases due to alveoli becoming dysfunctional. At ages over 30, A-a DO2 is roughly equal to age x .3

Question 9

When O2 is carried by Hb, does it contribute to the partial pressure of dissolved gas?

Answer 9

Nope

Question 10

Describe positive cooperative binding.

Answer 10

As Hb binds O2 molecules, it increases its affinity to bind more O2

Question 11

At arterial PO2 (~100mmHg), how many O2 molecules are bound to Hb? What about at venous PO2 (40mmHg)?

Answer 11

At arterial PO2, 4 O2 molecules are bound to Hb. At venous PO2, 3 molecules are bound to Hb.

Question 12

How much O2 does 1 gram of Hb bind? How much Hb is in one dL of blood?

Answer 12

1 gram of Hb binds 1.34 mL of O2. There are 15 grams of Hb in one dL of blood.

Question 13

Which four factors affect Hb O2 binding?

Answer 13

1. Temp
2. pH
3. pCO2
4. 2,3-BPG

Question 14

Explain how temperature affects Hb O2 binding, including how the Hb saturation curve is changed.

Answer 14

Increased temperature decreases Hb’s affinity for O2 (shifts Hb saturation curve to the right).

Question 15

Explain how pH affects Hb O2 binding, including how the Hb saturation curve is changed.

Answer 15

Decreased pH decreases Hb’s affinity for O2 (shifts curve to the right).

Question 16

Explain how pCO2 affects Hb O2 binding, including how the Hb saturation curve is changed.

Answer 16

Increased pCO2 decreases Hb’s affinity for O2 (shifts curve to the right).

Question 17

Explain how 2,3-BPG affects Hb O2 binding, including how the Hb saturation curve is changed.

Answer 17

Increased 2,3-BPG decreases Hb’s affinity for O2 (shifts curve to the right).

Question 18

Explain how CO is toxic.

Answer 18

It competitively binds to heme with an affinity ~250 times greater than O2.

Question 19

Fetal Hb has a ________ (higher or lower) affinity for O2 than adult (maternal) Hb. Therefore, the Hb O2 affinity curve of fetal Hb lies to the _______ (right or left) of adult Hb.

Answer 19

higher, so the fetal Hb curve lies to the left of the adult Hb curve.

Question 20

What is methemoglobin? How does the body get rid of it?

Answer 20

It is when Fe2+ gets oxidized to Fe3+ in heme, which makes heme unable to bind O2. RBCs have an enzyme called methemoglobin reductase to reduce methemoglobin back to Hb so only 1.5% of Hb is in the metHb state.

Question 21

What are the three forms for transport of CO2 and what are their respective proportions?

Answer 21

1. Dissolved CO2 (6%)
2. Bicarbonate (70%)
3. Carbamino compounds (24%): CO2 + protein-NH2 –> protein-NH-COO- + H+. Mostly happens on Hb.

Question 22

Where exactly does bicarbonate form? What enzyme is responsible? What is the chemical equation?

Answer 22

It forms in RBCs with the enzyme carbonic anhydrase.

CO2 + H2O –> H2CO3 –> H+ + HCO3-

Question 23

What happens to bicarbonate once it is formed in RBCs?

Answer 23

It diffuses out of the cell into the blood.

Question 24

What is the Haldane effect? What are the physiological benefits of this phenomenon?

Answer 24

Oxygenation of Hb LOWERS its affinity for carrying CO2. This allows Hb to efficiently take up CO2 at the tissue level and release it in the lung capillaries.

Question 25

Increased PCO2 results in a ___________ increase in CO2 in ALL forms.

Answer 25

proportional

Question 26

O2 and CO2 can be measured in blood using ________. For CO2, the measurement includes dissolved CO2 + _________ and ________.

Answer 26

using electrodes. For CO2 it includes dissolved CO2 plus bicarbonate and carbonic acid.

Question 27

What is capnography?

Answer 27

Measurement of exhaled CO2 to get a sense of PaCO2. Note that end tidal CO2 is usually less than PaCO2 due to dilution of the CO2 content from the air in the conducting airways.