Lecture 28 : Gas Exchange

Question 1

During gas exchange, describe what happens to the volume of O2:

Answer 1

At a steady state, the volume of O2 consumed by the cells = the volume of O2 entering the pulmonary capillaries from the alveoli

Question 2

During gas exchange, describe what happens to the rate of CO2 production:

Answer 2

The rate of CO2 production by the tissue cells = the rate at which CO2 enters the alveoli and is expired

Question 3

Assuming VA = 4L/min, how much O2 is entering the alveoli?

Answer 3

Only 21% of atmospheric air is O2, so the volume of O2 entering the alveoli is:
21% of 4000 mL/min = 840 mL O2 entering the alveoli per minute

Question 4

Describe the amount of O2 that enters and exits the alveoli/blood:

Answer 4

• 840 mL/min enters alveoli
  250 mL/min into blood
  590 mL/min expired
• 250 mL/min increases blood to 1000 mL O2
• 250 mL/min used by tissues
• Blood returns to lungs at 750 mL/min

Question 5

How does CO relate to O2 transportation?

Answer 5

• Cardiac output (5 L/min) matches O2 transportation (1000 mL/min)
• So the concentration of O2
  in arterial blood is ~200 mL/L (1000ml/5L)

Question 6

Describe the amount of CO2 that enters and exits the blood/lungs:

Answer 6

• 200 ml/min CO2 made
• Venous blood increases to 2800 ml/min
• Exchange 200 ml/min at lungs
• Returns to tissue at 2600
  ml/min

Question 7

What is the concentration of CO2 in arterial blood?

Answer 7

~520 mL/L (2600ml/5L)

Question 8

What is the respiratory quotient (RQ)?

Answer 8

The volume of CO2 produced
by cells does not exactly equal the volume of O2 utilised
RQ is the ratio of CO2 produced to O2
consumed:
RQ = VCO2 / VO2

Question 9

What information does the RQ provide?

Answer 9

Cellular metabolism

Question 10

What is the RQ for a normal ‘mixed’ diet?

Answer 10

RQ = 200 mL/250mL = 0.8

Question 11

When a diet has more carbs what happens to RQ?

Answer 11

Increases to around 1
i.e. C6H12O6 + 6O2 <=> 6H2O + 6CO2 + energy

Question 12

When a diet has more fat what happens to RQ?

Answer 12

Decreases to around 0.7

Question 13

When a diet has more protein what happens to RQ?

Answer 13

Remains around the same

Question 14

What is brownian motion?

Answer 14

Gas molecules undergo continuous random motion

Question 15

What is the result of movement of gas molecules and what is the magnitude of this result dependent on?

Answer 15

• Movement exerts a pressure - force from molecules bouncing against wall
• The magnitude depends on the concentration of the gas and temperature

Question 16

What is Boyle’s law?

Answer 16

When the volume changes, the pressure changes as well
P1V1 = P2V2

Question 17

Do gas molecules normally interfere with eachother and why?

Answer 17

No, because they are so far apart

Question 18

What is a partial pressure?

Answer 18

Each individual gas exerts its own individual pressure e.g. PO2, PCO2

Question 19

Describe the relationship between partial pressure and concentration:

Answer 19

Partial pressure of a gas is directly proportional to its concentration or fractional content

Question 20

How does net diffusion of gas occur?

Answer 20

From a region where its partial pressure is high to a region where it is low

Question 20

What is Dalton’s law?

Answer 20

In a mixture of gases, the total pressure exerted is the sum of the partial pressures
PT = Px + Py

Question 21

What is atmospheric air made up of?

Answer 21

1. Nitrogen ~ 79%
2. Oxygen ~21%
3. Water vapour
4. Small amounts of carbon dioxide (0.04%), inert gases

Question 22

What is atmospheric pressure, Patm or barometric pressure, PB?

Answer 22

The sum of the partial pressures of atmospheric air

Question 22

What is the atmospheric pressure at sea level?

Answer 22

760mmHg

Question 23

How is the partial pressure of a gas in a mixture calculated?

Answer 23

Fractional concentration x total pressure Px = Fx x PT

Question 24

What is the PO2 of atmospheric air at sea level?

Answer 24

~ 160 mmHg 0.21 x 760 mmHg = 160 mmHg

Question 25

What effect does water vapour have on individual gases in the alveoli?

Answer 25

Reduces the pressure of individual gases

Question 26

What pressure does water vapour exert at 37°C?

Answer 26

~47mmHg

Question 27

How is the partial pressure of an inspired gas calculated? Use O2 as an example.

Answer 27

Pgas = Fgas x (Patm - 47) mmHg PO2 = 0.21 x (760 - 47) = 149.73 mmHg

Question 27

What 3 factors affect partial alveolar pressure for O2 (PAO2)?

Answer 27

1. PIo2 – how much O2 are we breathing in 2. VA – how much fresh air is getting to the alveoli 3. VO2 – how much O2 is being extracted/used by the body

Question 27

Is alveolar PCO2 higher or lower than in inspired air?

Answer 27

Much higher (40mmHg) compared to close to 0mmHg inspired air

Question 27

Is alveolar PO2 higher or lower than in inspired air? Why?

Answer 27

Lower (105 mmHg) because some of the O2 entering the alveoli is taken up by the pulmonary capillaries

Question 28

Describe the relationship between PIO2 and PAO2:

Answer 28

Increase PIO2 = Increase PAO2 Keeping VO2 constant

Question 28

What is the alveolar gas equation for O2?

Answer 28

PAO2 = PIO2 - (VO2/VA x 863)

Question 29

Describe the relationship between VA and PAO2:

Answer 29

Increase VA = Increase PAO2 Keeping VO2 constant

Question 30

Describe the relationship between VO2 and PAO2:

Answer 30

Increase VO2 = Decrease PAO2 Keeping PIO2 constant i.e. at sea level

Question 30

Describe the relationship between VCO2 and PACO2:

Answer 30

Increase VCO2 = Increase PACO2 Keeping PICO2 constant i.e. at sea level

Question 31

What 3 factors affect partial alveolar pressure for CO2 (PACO2)?

Answer 31

1. PIco2 – almost always zero 2. VA – how much fresh air is getting to the alveoli 3. VCO2 – how much CO2 is being produced by the body

Question 31

What is the alveolar gas equation for CO2?

Answer 31

PACO2 = VCO2/VA x 863

Question 32

What is hypoventilation?

Answer 32

Decreased alveolar ventilation * PAO2 decrease and PACO2 increase

Question 33

What is hyperventilation?

Answer 33

Increased alveolar ventilation * PAO2 increase and PACO2 decrease

Question 34

How does PO2 change as it moves through the circulatory system?

Answer 34

Atmosphere: 160 mmHg Ventilation Alveoli: 105 mmHg Gas exchange Arterial blood: 100 mmHg Venous blood: 40 mmHg

Question 34

What is the driving force for gas exchange in the lungs?

Answer 34

Differences in partial pressures for both O2 and CO2 in the alveoli and the blood in pulmonary capillaries * Entering blood low PO2 and high PCO2 * Rapid diffusion * PaCO2 = PACO2 (diffusion stops) * PaO2 almost = PAO2

Question 34

What is Henry's law?

Answer 34

The number of O2 molecules entering the liquid (blood) ∝ to the Po2 in the gas (alveoli)

Question 34

What happens when a liquid is exposed to gas e.g. O2?

Answer 34

Molecules of O2 enter the liquid by diffusion and dissolve in it

Question 35

What 4 factors does diffusion of O2 and CO2 rely on?

Answer 35

1. The diffusion properties of O2 and CO2 - Dgas 2. The thin walls of the alveoli - distance ~1 µm air→blood 3. The large alveolar surface area over which gas diffusion can occur - A ~120 m2 4. Partial pressure difference (∆P) for O2 and CO2

Question 35

Describe the diffusion of O2 in a healthy person at the pulmonary capillaries:

Answer 35

1. Initial partial pressure gradient is ~60mmHg (100-40) 2. Diffusion stops when PO2 of arterial blood = alveoli PO2 3. Diffusion complete in first 30% of capillary length The exchange of CO2 from pulmonary capillary blood to the alveoli is also extremely rapid

Question 35

How does PCO2 change as it moves through the circulatory system?

Answer 35

Atmosphere: 0.3 mmHg Ventilation Alveoli: 40 mmHg Gas exchange Arterial blood: 40 mmHg Venous blood: 46 mmHg

Question 36

What is Fick's law of diffusion?

Answer 36

Gases move by diffusion down gradients of partial pressure Diffusion = Dgas x A x ΔPgas/ distance

Question 36

What is the diffusing capacity/rate of diffusion for O2?

Answer 36

Diffusion of O2 = DO2 x A x ΔPO2/ distance

Question 37

Is the diffusing constant greater for CO2 or O2 and what does this mean?

Answer 37

* Diffusion constant for CO2 (DCO2) is 3-5 times greater than for O2 (DO2), partially because CO2 is much more soluble (23x) in plasma than O2 * Rate of CO2 diffusion has the potential to be much faster than O2 diffusion between the alveoli and the pulmonary capillaries

Question 38

What 3 factors reduce O2 diffusion into the lungs?

Answer 38

1. Pulmonary oedema (diffusion distance ↑) - If fluid leaks out of the pulmonary capillaries into the interstitial space, it will reduce the rate of O2 diffusion 2. Interstitial fibrosis (diffusion distance ↑) - Thickening of the alveolar wall reduces the rate of O2 diffusion 3. Emphysema (Destruction of alveolar walls) - SA for O2 diffusion ↓ - Number of pulmonary capillaries ↓

Question 39

What is ventilation-perfusion matching?

Answer 39

* The lungs should have optimal airflow (ventilation) and optimal blood flow (perfusion) to each alveolus * Some alveoli are not perfused as they may not be oxygenated * Prevents sending blood to physiological dead space

Question 40

What lung diseases can cause ventilation-perfusion mismatching?

Answer 40

1. Emphysema – blood keeps flowing to areas of lung that can't undergo gas exchange * Bronchitis/asthma – mucus plug blocking airflow to a particular area of the lungs

Question 41

How can ventilation-perfusion matching be assessed?

Answer 41

RQ > 0.8 or < 0.8 indicates ventilation-perfusion mismatch

Question 42

How is ventilation-perfusion mismatching minimised by vasculature?

Answer 42

1. Decreased airflow to region of lung 2. Pulmonary blood PO2 decrease 3. Vasoconstriction of pulmonary vessels 4. Decreased blood flow -> Local perfusion decreased to match a local decrease in ventilation -> Diversion of blood flow and airflow away from local area of disease to healthy areas of the lung

Question 43

How is ventilation-perfusion mismatching minimised by the airways?

Answer 43

1. Decreased blood flow to region of lung 2. Alveoli PCO2 decrease 3. Bronchoconstriction 4. Decreased airflow -> Local ventilation decreased to match a local decrease in perfusion -> Diversion of blood flow and airflow away from local area of disease to healthy areas of the lung