PBL 3 - ventilation and perfusion balance

Question 1

at equilibrium, what is the partial pressure of a particular gas in solution equal to?

Answer 1

the partial pressure of the same gas in the gas phase

Question 2

what is the partial pressure of gas in solution proportional to?

Answer 2

the concentration of that gas i solution (ie. not bound to any carrier)

Question 3

what is the amount of gas transferred in alveolar-capillary gas exchange proportional to?

Answer 3

• pressure gradient
• SA available for diffusion
• 1/length of the pathway
• solubility
• 1 / [square root of molecular weight]

Question 4

is O2 or CO2 more soluble?

Answer 4

CO2

Question 5

what does a lower molecular weight correlate to?

Answer 5

faster diffusion - co2

Question 6

what determines the concentration of O2 and CO2 leaving the pulmonary capillary?

Answer 6

the partial pressure of gas in alveolus

Question 7

what is a typical value of arterial PO2 (PaO2) in a healthy adult?

Answer 7

13kPa (100mmHg)

Question 8

what is the typical value of PaCO2 in a healthy adult?

Answer 8

5.3kPa (40mmHg)

Question 9

what is the typical value of venous PO2 in a healthy adult?

Answer 9

5kPa (37mmHg)

Question 10

what is the typical value of venous PCO2 in a healthy adult?

Answer 10

6kPa (45mmHg)

Question 11

is O2 soluble in plasma?

Answer 11

not very soluble

Question 12

how many molecules of O2 can Hb bind?

Answer 12

4

Question 13

what is the O2 content of arterial blood compared to in plasma? (ml/l)

Answer 13

200ml/l in arterial blood vs 3ml/l in plasma

Question 14

at what PO2 is the % saturation of Hb with O2 approx 97%?

Answer 14

13kPa

Question 15

what is bicarbonate removed from RBC into blood in exchange for?

Answer 15

Cl-

Question 16

what 3 things can CO2 be present as in blood?

Answer 16

• free CO2
• HCO3-
• HbNHCOO-

Question 17

what are the bicarbonate, buffer and carbamino equations?

Answer 17

Question 18

what type of relationship is it between PCO2 and content in arterial blood?

Answer 18

linear — no carrier so therefore doesn’t saturate

if you 2x PCO2, you 2x the content of CO2 carried in the blood

Question 19

what is the P/Q ratio?

Answer 19

= ventilation/perfusion

= the amount of air that reaches your lungs divided by the amount of blood flow in the capillaries in your lungs

Question 20

describe a low V/Q for CO2

Answer 20

• low ventilation — increase in PCO2

- therefore increased content of CO2 in blood

Question 21

describe a high V/Q for CO2

Answer 21

• high ventilation — decrease in PCO2

- therefore decreases content of CO2 in blood

Question 22

describe a low V/Q for O2

Answer 22

• low PO2

- therefore decreased O2 content in blood

Question 23

describe a high V/Q for O2

Answer 23

• high PO2 but as oxyhemoglobin was already almost fully saturated, not much more O2 is loaded into blood

Question 24

what is blood leaving an area of low V/Q like compared to normal?

Answer 24

• high CO2 content

- low O2 content

Question 25

what is blood leading an area of high V/Q like compared to normal?

Answer 25

• low CO2 content

- normal O2 content

Question 26

what are 2 ventilatory related causes of V/Q inequalities?

Answer 26

1. respiratory disease — airways obstruction (not evenly distributed across lungs, regional inequalities in ventilation)
2. pneumothorax — an extreme form of ventilatory inequality

Question 27

what are 3 perfusion related causes of V/Q inequalities?

Answer 27

1. gravity — both V and Q are greater at the base than the apex
2. respiratory disease — hypoxic pulmonary vasoconstriction (right heart failure)
3. pulmonary embolus (blood clots) — DVT, covid-19 — V/Q scan, CT imaging

Question 28

describe the local adaptions to V/Q inequalities

Answer 28