Gas Exchange

Question 1

Define the partial pressure of a gas in mathematical terms and explain the the stepwise decrease in PO2 in the air we breathe as it enters the the conducting airways and reaches the alveoli.

Answer 1

Pgas= pressure it would exert if it alone occupied a given volume
atm/barometric pressure (Pb)=total pressure exerted by mix of gases (760 mmHg at sea level)
(air=21% O2, pO2=160 mmHg)

Pgas=Pb x Figas, PO2= 760 x 0.21 = 160 mmHg
but it’s humidified/saturated with water vapour (PH20=47)

so in conducting airways: PO2 = [760-47] x 0.21 = 150 mmHg

once it reaches alveoli, pO2 drops further bc it’s mixed in a reservoir of air (more Co2, H2o same). PO2=100 mmHg

Question 2

Compare the PO2 and PCO2 in the pulmonary artery (mixed venous blood) entering the pulmonary capillaries to those in the pulmonary vein (arterial blood) leaving the respiratory zone.

Answer 2

PO2 and PCO2 at end pulmonary capillaries (c’) reflect alveolar (A) counterparts and determine pp of these gases in arterial blood (a)

deoxygenated blood from pulmonary artery
oxygenated blood leaving respiratory zone (arterial, PO=100, PCO2=40)

Question 3

 Specify the driving pressures for the exchange of O2 and CO2 at the lungs and at the tissues.

Answer 3

across pulmonary capillaries: PO2 gradient alveoli to blood 60 mmHg. PCO2 gradient blood to alveoli 6 mmHg.

across tissue capillaries: PO2 gradient blood to tissue 60 mmHg. PCO2 gradient tissue to blood 6 mmHg.

Question 4

Describe the factors that impact the rate of gas exchange across the alveolar-capillary membrane using Fick’s Law of Diffusion. Illustrate their impact in exercise, emphysema, and pulmonary fibrosis.

Answer 4

VGAS α ((ΔP) x A x D)/T = rate of gas diffusion
D= diffusivity, = solubility/√MW
***both reach eq btw air in lungs in blood in pulmonary capillary in similar time (1/4 s even tho O2 has greater P), bc CO2 has greater solubility

Factors:

1. exercise- increases SA by increasing CO and tidal volume
2. emphysema- decrease SA by destruction walls, increased compliance, less area for gas exchange
3. PF- increase collagen deposition, decrease compliance, increase thickness= gas exchange complication (also in pneumonia, edema)

Question 5

Define pulmonary transit time and explain its impact on gas exchange in 1) healthy individuals at rest and during exercise 2) pulmonary fibrosis 4) elite athletes.

Answer 5

1) PTT (how long blood is in capillary, window for exchange) at rest: 3/4 s
2) exercise > 1/4 s even with increased CO
3) PF: reduced gas exchange with exercise. additional time spent in capillary at rest is sufficient
4) elite athletes: PTT<1/4 s during intense exercise with lots CO –> inadequate oxygen exchange –> hypoxemia, low arterial PO2