Physiology of Gas Exchange and Mechanisms

Question 1

What is the effect of gravity on the lungs?

Answer 1

1. the alveoli at the apical end (the top) of the lungs are suspended and stretched by the gravitational force which forces them open.
2. The alveoli at the lower end of the lungs are pressed on by the alveoli on top and this compresses them and closes them.

Question 2

What is the effect of gravity on the expansion of alveoli in the apical and bottom ends of the lungs?

Answer 2

1. Gravity reduces further expansion of the alveoli in the apical end as it already forces it open
2. the alveoli at the bottom of the lungs are liable to greater expansion than those at the top

Question 3

what is the effect of gravity on the partial pleural pressure?

Answer 3

1. due to alveoli at top pulling down on pleura, pressure in pleura at the top is more negative (in comparison to atmospheric pressure)
2. due to alveoli at bottom of lungs pushing outwards onto the pleura, the pleura pressure becomes more positive (closer to zero
3. So there is an intrapleural pressure gradient.

Question 4

What part of the lungs is ventilation the greatest in normal lung volumes?

Answer 4

the bottom of the lungs

Question 5

at normal lung volumes, what are the implications for a change in volume for a given pressure change?

Answer 5

a change in pressure would result in a large change in volume as on the steep part of the curve (slide 5)

Question 6

At residual lung volumes, the partial pleural pressure is less negative, what does this mean for the elastic recoil of the lungs?

Answer 6

elastic recoil will be reduced

Question 7

At residual lung volume, what is the partial pleural pressure(Ppl) at the bottom of the lungs?

Answer 7

positive

Question 8

what is the effect of a positive Ppl?

Answer 8

it means thePpl is greater than atmospheric pressure and that alveoli are overly compressed and won’t inflate until the Ppl is below atmospheric pressure.

Question 9

With residual lung volume, what part of the lungs ventilate the best?

Answer 9

the top part

Question 10

What happens to the size of the alveoli and Ppl when lung volume is almost at its total lung capacity?

Answer 10

alveoli is bigger

Ppl is more negative so there’s greater elastic recoil.

Question 11

What are the three layers that gas needs to travel across from alveoli to blood?

Answer 11

1. alveolar epithelium
2. capillary endothelium
3. interstitium (not always present though)

Question 12

two factors that permit rapid gas exchange are…?

Answer 12

1. large SA (50-100m^2)

2. short diffusion pathway (0.2-0.3 micrometers)

Question 13

what is exchanged at the thin and thick portions of the barriers?

Answer 13

thin: gasses
thick: fluids

Question 14

what makes the barrier thicker?

Answer 14

collagen fibres

Question 15

What is an ideal gas mixture?

Answer 15

a mixture in which:

1. each cmponent behaves independently (brownian motion)
2. each component exerts a pressure (partial pressure)
3. total pressure is the sum of the partial pressures

Question 16

What is Dalton’s law of partial pressures for dry mixtures?

Answer 16

partial pressure = (total pressure X Fractional concentration in mixture)

Question 17

What is Dalton’s law of partial pressure for wet mixtures?

Answer 17

Partial pressure(wet) = (total pressure - pressure of liquid e.g. H2O) X functional concentration in mixture(dry)

Question 18

What is Henry’s law of partial pressure?

Answer 18

gas concentration = K X partial pressure of gas
(where K = a constant)

Henry’s law states that gas concentration is directly proportional to it’s partial pressure.

Question 19

What does fick’s law of diffusion state?

Answer 19

Vgas (rate of diffusion) is proportional to the surface area(A), partial pressure gradient (P) and diffusion coefficient. And that it’s inversely proportional to the thee barrier thickness (T).

Vgas = k X (A/T) X D X (P1-P2)

Question 20

Which has a greater diffusion rate O2 or CO2 and why?

Answer 20

CO2 diffuses more rapidly than O2 as it’s got a higher solubility

Question 21

how long does it take for blood to move through the pulmonary capillary?

Answer 21

0.75s

Question 22

what is the effect of strenuous exercise on cardiac output hence transit time of red blood cells?

Answer 22

increases cardiac output and increases rate at which red blood cells move through pulmonary capillary (increased transit time) so not enough time for complete equilibrium to occur.

Question 23

two conditions that can increase the diffusion pathway/barrier for O2 and CO2 exchaneg?

Answer 23

pulmonary oedema and fibrosis

Question 24

two factors that can limit the amount of gas we can get into the blood?

Answer 24

diffusion and perfussion

Question 25

What limits the amount of gas we can get into the blood when at rest?

Answer 25

perfusion

Question 26

What limits the amount of gas we can get into the blood when carrying out strenuous exercise?

Answer 26

diffusion

Question 27

how is the uptake of CO diffusion limited?

Answer 27

1. gas binds to Hb so no longer in solution
2. gas can only exert a partial pressure if it’s free, but once bound to Hb, no longer free
3. no partial pressure gradient formed b/w capillary and alveoli as no CO pp in capillary since it’s bound to Hb.
4. so the amount of gas transferred is limited by the diffusion properties of the membrane. aka diffusion limited!!

Question 28

how does perfusion limit nitrous oxide (N2O) uptake?

Answer 28

1. alveoli and capillary nitrous oxide partial pressures equilibrate rapidly.
2. so diffusion is stopped before the blood reaches the end of the capillary.
3. so the amount of gas transferred is limited by flow rate (perfusion).

Question 29

what limits the uptake of O2?

Answer 29

pefusion but can be diffusion if person has a disease.

Question 30

how to carry out a pulmonary function test?

Answer 30

1. patient breathes in a dilute mixture of a gas that is diffusion limited (e.g. CO)
2. measure amount of CO breathed in and out and determine rate of absorption

Question 31

What affects the diffusion capacity of lungs?

Answer 31

1. area
2. thickness of barrier
3. uneven ventilation
4. volume of blood

Question 32

what is the effect of a higher altitude on oxygen’s partial pressure?

Answer 32

increases your body’s internal oxygen partial pressure making ventilation harder.