Gas Exchange

Question 1

What do partial pressures of O2 and CO2 at end pulmonary capillaries (c’) reflect?

Answer 1

their alveolar (A) counterparts, and determine partial pressure of these gases in arterial blood (a)

Question 2

What does gas exchange result in?

Answer 2

end capillary blood gases (Pc’O2 and Pc’CO2) that are in equilibrium with partial pressure of gases in alveoli (PAO2 and PACO2)

Question 3

Where does arterial blood go after gas exchange?

Answer 3

courses through pulmonary vein and is pumped to left side of heart, where it is then pumped to tissues where gas exchange can take place at level of tissue

Question 4

What will end capillary blood ultimately be composed of?

Answer 4

partial pressure of gases in alveoli

Question 5

Ventilation/Perfusion Match

Ideal Lung (V/Q = 1)

Answer 5

matching rate at which alveolus gets ventilated and rate by which blood goes past it, ends up with same PO2 and PCO2 as air in alveoli

Question 6

Ventilation/Perfusion Match

Shunt-like (V/Q = 0)

Answer 6

(obstruction in airway to alveoli)

partial pressures in incoming gas is mixed venous in nature from tissues

• no ventilation, but blood passes
• mixed venous blood will equilibrate with gas in alveoli with time
• since nothing fresh is coming in, essentially it will leave in the same nature, and whatever air is in alveoli will equilibrate with end capillary

Question 7

Ventilation/Perfusion Match

Dead Space (V/Q = infinity)

Answer 7

(complete obstruction in capillary) – if only partially blocked, V/Q value would vary

• no perfusion, but very ventilated alveoli with relatively high O2 and low CO2 compared to normal
• blood does not see it because blood can’t pass (blocked)

Question 8

What is air composed of?

Answer 8

mixture of gases – each gas exerts partial pressure (Pgas), which is pressure it would exert if it alone occupied a given volume

total atmospheric pressure = 760 mmHg

nitrogen: 79%, PN2 = 600 mmHg
oxygen: 21%, PO2 = 160 mmHg
argon: 0.93%
carbon dioxide: 0.03%

neon 
methane 
helium
krypton 
hydrogen 
xenon

Question 9

What is atmospheric/barometric pressure?

Answer 9

total pressure exerted by this mixture of gases = 760 mmHg at sea level

Question 10

How do you determine the partial pressure of a gas?

Answer 10

Pgas = PB x FIgas

where B = barometric
where I = inspired

Question 11

Determining Partial Pressure of Gas in Mixture of Gases and Water Vapour

Answer 11

see notes

Question 12

What does partial pressure reflect?

Answer 12

% of air that is not water vapour

Question 13

What happens to partial pressure of oxygen as inspired air courses through airways?

Answer 13

decreases

conducting airways

• heated and humidified
• PO2 decreases due to subtraction of water vapour introduced

alveoli

• mixed in reservoir of air at FRC
• PO2 decreases further
• PCO2 is introduced – alveolus is seeing blood, and is being influenced by it, and CO2 from blood and enters alveolus

Question 14

At equilibrium, what is the tension of gas in solution (equal pressure in solution)?

Answer 14

= partial pressure of gas in air pocket above it (equal pressure in gaseous phase)

Question 15

What do gases dissolve, diffuse, and react according to?

Answer 15

their partial pressure – NOT concentration

• concentration of gas in solution is not same as partial pressure
• solubility of gas affects its concentration – gases dissolve differently in different solutions

Question 16

What happens to the partial pressure gradient of O2 as O2 moves into blood from alveoli?

Answer 16

gradient gets smaller until PO2 and partial pressure in alveoli are the same

Question 17

Fick’s Law of Diffusion

What are the factors that impact diffusion of gases across barriers?

Answer 17

Vgas = flow of glass across barrier

D = ‘diffusivity’ of ‘diffusion coefficient’

• individualistic to that gas
• proportional to (solubility / √ MW)

ΔP = pressure gradient
- more diffusion if larger

A = surface area of barrier
- more diffusion if larger

T = thickness of barrier
- more diffusion of thinner

Question 18

Fick’s Law of Diffusion

How is both O2 and CO2 able to reach equilibrium between air in lungs and blood in pulmonary capillary (complete gas exchange) in a similar time frame?

Answer 18

CO2 has greater diffusivity, but less driving pressure

O2 has lower diffusivity, but greater driving pressure

Question 19

What is pulmonary fibrosis?

Answer 19

scar formation, which decreases lung compliance and increases thickness

Question 20

What is pneumonia?

Answer 20

inflammatory fluid/pus accumulation in and around alveoli that causes increase in thickness

Question 21

What conditions can result in decrease in surface area?

Answer 21

• emphysema

- atelectasis in NRDS/ARDS

Question 22

What conditions can result in increase in surface area?

Answer 22

exercise

Question 23

What conditions can result in increase in thickness?

Answer 23

• pulmonary fibrosis
• pneumonia
• pulmonary edema

Question 24

What is pulmonary edema?

Answer 24

abnormal buildup of fluid in interstitial or alveolar space

result of direct damage to lung tissue, or inability of heart to pump blood (left heart failure)

Question 25

How long does gas exchange take to complete?

Answer 25

within ¼ second

Question 26

What is the normal pulmonary transit time (PTT) during a normal quiet breath?

Answer 26

under 1 second

Question 27

Gas Exchange Across Pulmonary Capillary

At Rest

Answer 27

¾ PTT > ¼ second required to complete gas exchange

plenty of extra time

Question 28

Gas Exchange Across Pulmonary Capillary

In Exercise

Answer 28

PTT > ¼ sec required to complete gas exchange

• despite ↑ cardiac output (faster blood)
• PTT can be faster, as long as it stays for minimum ¼ sec for equilibrium to occur

Question 29

Gas Exchange Across Pulmonary Capillary

In Pulmonary Fibrosis

Answer 29

reduced gas exchange can occur with exercise

• additional time spent in capillary at rest is often sufficient to compensate for thickened barrier
• although they have enough time for gas exchange to happen, if thickness of their alveoli was normal + add additional thickness, when they exercise, the extra time available is not sufficient due to thickened barrier and faster blood while exercising

Question 30

Gas Exchange Across Pulmonary Capillary

In Elite Athletes

Answer 30

PTT < ¼ sec during intense exercise

• due to large ↑ cardiac output
• ## inadequate O2 exchange at lungs → hypoxemia, low arterial PO2

athletes trained well to reach high CO to get blood to tissues that need it and get O2, BUT CO is so fast that blood that needs to exchange at level of lungs to bring in O2 is not doing adequate job

• desaturation of O2
• only occurring during serious exercise, not when at rest