Gas Exchange in the Lungs

Question 1

where does gas exchange take place?

Answer 1

within the alveoli forming air-blood barrier

Question 2

specialisation of gas exchange system

Answer 2

large surface area
thin outer structure
richly innervated with capillaries

Question 3

what is the pathway of oxygen into blood?

Answer 3

atmosphere –> alveolar airspace –> alveolar lining fluid –> through epithelium, BM, endothelial cells –> blood plasma –> Hb

Question 4

how long for RBC to pass through pulmonary capillary

Answer 4

0. 75 seconds

0. 25 in exercise

Question 5

what determines rate of diffusion?

Answer 5

1- partial pressure gradient between two areas
2- size of diffusion distance
3- SA

Question 6

factors for max diffusion?

Answer 6

• high pp gradient
• high SA
• low distance

Question 7

what decreases SA

Answer 7

emphysema

Question 8

what increases diffusion distance

Answer 8

fibrosis increases thickness of membrane and pulmonary oedema increases thickness of fluid

Question 9

what decreases PA

Answer 9

hypoventilation (type II resp failure)

Question 10

what decreases PC?

Answer 10

hypoperfusion (type I respiratory failure)

Question 11

what is hypoventilation?

Answer 11

insufficient levels of breathing = excessive CO2 (pa < 4.9 kPa) and reduced O2

Question 12

what is hyperventilation?

Answer 12

excessive levels of breathing = reduced CO2 (pa < 6.0 pKa) and increased O2

Question 13

what does increasing rate of ventilation do?

Answer 13

increases alveolar oxygen partial pressure and decreases alveolar carbon dioxide pp

Question 14

What affect does hypoventilation and hyperventilation have on PaCO2?

Answer 14

• Ventilation increase will decrease PACO2
• So hyperventilation will decrease it  because constantly exchanging it with fresh atmospheric so less CO2 left in alveoli
• Hypoventilation will increase it

Question 15

what is perfusion?

Answer 15

supply of blood

Question 16

what should the ventilation/perfusion ratio be?

Answer 16

close to 1

Question 17

what happens if V/Q > 1 (perfusion reduced relative to ventilation)?

Answer 17

inspired oxygen is wasted and not participating in gas exchange as blood supply to lungs reduced

Question 18

what is physiologic dead space?

Answer 18

parts of lungs not participating in gas exchange despite presence of O2

Question 19

what is a pulmonary embolism?

Answer 19

block of artery in lungs

Question 20

what happens in pulmonary embolism?

Answer 20

overall perfusion of lungs may not decrease as blood diverted to other pulmonary arteries and capillaries

Question 21

how is hypoxaemia and hypercapnia avoided?

Answer 21

ventilation of perfused alveoli increases

Question 22

What is the V/Q ratio?

Answer 22

Describes the relationship between perfusion (Q) and alveolar ventilation (V) which both needs to match to enable efficient gas exchange –> as there’s a max amount of 02 each unit of blood can carry

Question 23

V/Q less than 1

Answer 23

hypoperfusion (dead space effect)

Question 24

V/Q more than 1

Answer 24

hypoventilation (shunt)

Question 25

How is V/Q coupling maintained?

Answer 25

where it’s well ventilated there will be more perfusion/blood flow, if not then it will be diverted away –>this is to match ventilation and perfusion (so V/Q is close to 1)

Question 26

more depth on V/Q maintenance

Answer 26

• By hypoxic vasoconstriction of capillaries (this is a homeostatic mechanism) –> this diverts blood flow from poor to well ventilated alveoli
• If ventilation of an alveoli decreases –> PACO2 will rise and PAO2 will fall –> this will reduce level of oxygen in pulmonary capillary as pressure gradient less steep–> starts to cause vasoconstriction because smooth muscle and tissues sensitive to lack of oxygen –>constriction increases resistance –>divert blood (because one capillary will have more

Question 27

what is pulmonary shunt?

Answer 27

reduced ventilation of alveoli so decreased V/Q. Deoxygenated blood returns to left side of heart from right without gas exchange happening

Question 28

what conditions are associated with pulmonary shunt?

Answer 28

cardiac shunts, pneumonia, acute lung injury, RDS and atelectasis

Question 29

how does V/Q change with the shunt effect?

Answer 29

• V/Q ratio will decrease because Q remains same and V decreases –> response to oxygen therapy is poor because you can’t compensate for decrease of oxygen within regions affected by shunt by increasing ventilation to the regions that weren’t –> as blood can only carry a certain amount of oxygen (the extra oxygen you will add to regions not affected by shunt will have blood with slightly more oxygen but when mixed with the blood of the shunted under perfused alveoli blood –> it is not enough to compensate

Question 30

what is hypoxic vasoconstriction?

Answer 30

ventilation to alveoli reduced, decreased ventilation results in rising CO2 and falling O2 so vascular smooth muscle contracts

Question 31

what does the constriction of the blood vessels do?

Answer 31

diverts blood to other capillaries that innervate better ventilated alveoli

Question 32

what does hypoxic vasoconstriction do?

Answer 32

protective mechanism to ensure efficient V-Q coupling

Question 33

When does hypoxic vasoconstriction become pathological?

Answer 33

COPD, prolonged widespread pulmonary vasoconstriction. Increases resistance in pulmonary vasculature leading to pulmonary hypertension, right heart hypertrophy and right heart failure