Gas Exchange in the lungs

Question 1

What is the percentage of O2 present in air?

Answer 1

21%

Question 2

What is the percentge of CO2 and NO2 in air?

Answer 2

0. 4%

0. 5%

Question 3

What is the partial pressure of oxygen and carbon dioxide in alveolar gas?

Answer 3

13. 3 Kpa - O2

5. 3 Kpa CO2

Question 4

What is alveolar pO2 determined by?

Answer 4

A balance between the rate of removal of O2 by the blood and the rate of replenishment of O2 by alveolar ventilation

Question 5

What is alveolar pCO2 determined by

Answer 5

The rate at which CO2 enters the alveoli from the blood and the rate at which it is removed from alveolar gas by ventilation

Question 6

What 3 factors is the rate of GE determined by?

Answer 6

1. Area available for GE
2. resistance to diffusion
3. Gradient of partial pressure

Question 7

What are the partial pressures of O2 and CO2 in the pulmonary capillary entering the alveoli?

Answer 7

6Kpa

6Kpa

Question 8

What is the normal area of GE in the lung?

Answer 8

alveolar surface 70m2

Question 9

What 5 things must the O2 must diffuse through to reach the RBC?

Answer 9

1. The alveolar epithelial cell
2. interstitial fluid
3. Capillary endothelial cell
4. Plasma
5. RBC membrane

Question 10

What is the overall distance the O2 has to diffuse across?

Answer 10

0.6um

Question 11

What factors affect diffusion of gas in a fluid? (6)

Answer 11

1. Pressure change
2. Solubility of gas in solution
3. Cross sectional area of fluid
4. Distance the molecules must diffuse across
5. MW of gas
6. Temp of fluid

Question 12

Why does carbon diffuse faster than oxygen?

Answer 12

CO2 is more soluble in water than oxygen (21X)

Question 13

What is the pp of O2 and CO2 in arterial blood?

Answer 13

In a normal subject the pp of O2 and CO2 should be the same in arterial blood as in alveolar gas (13.3 Kpa and 5.3Kpa)

Question 14

What affect does interstitial lung disease have on diffusion and why? (fibrotic lung disease)

Answer 14

It increases the diffusion distance as there is excessive collagen deposition in the interstitial space, with thickening of alveolar walls, therefore pO2 in blood is low (pCO2 is normal).

Question 15

what will the pO2 be in interstitial lung disease and why?

Answer 15

Increased in the diffusion distance as there is excessive collagen deposition in the interstitial space, with thickening of alveolar walls, therefore pO2 in blood is low (pCO2 is normal).

Question 16

What does diffusion resistance depend on?

Answer 16

The nature of the barrier

The nature of the gas

Question 17

What is the equation to work out permeability of a gas across a barrier?

Answer 17

The diffusion coefficient of molecules in the membrane X their solubility in the lipid bi-layer/ thickness of the membrane.

Question 18

What is the H2O pressure in tracheal air?`

Answer 18

6.28Kpa

Question 19

What does SVP (saturated vapour pressure) depend on?

Answer 19

Temperature independent of total pressure

Question 20

Wha is the SVP of H2O at 37 degrees?

Answer 20

6.28 Kpa

Question 21

How is CO2 carried in the blood?

Answer 21

8% is dissolved in the plasma
12% is carried as carboxy Hb on proteins
80% is HCO3- in plasma

Question 22

Whats the pressure of O2 and CO2 in ambient air?

Answer 22

O2 = 20.8 Kpa 
CO2= 0.04 Kpa

Question 23

What are the difference in pressures for O2 and CO2 between the alveolar gas and the venous blood?

Answer 23

The pO2 is < pO2 in alveolar gas, so O2 diffuses into the blood
The pCO2 is > the pCO2 of alveolar gas, so CO2 diffuses into the alveoli and is expelled

Question 24

What happens during in exercise to the pCO2?

Answer 24

The pCO2 in the pulmonary capillary increases, there pressure gradient is steeper and CO2 is diffused across twice as fast.
Normally blood spends 3X as long in PC as needed so in exercise exchange is not limited

Question 25

Why is the pressure difference required for CO2 less than O2

Answer 25

Because CO2 is more soluble (20X faster)

Question 26

How long does it take for GE of O2?

Answer 26

within 0.5s of RBC arriving (RBC spend around 1s in capillary)

Question 27

What affect does pulmonary oedma have on diffusion resistance?

Answer 27

It increases the length of the diffusion pathway as fluid in the interstitial space and alveolus increases

Question 28

How does Emphysema affect the diffusion resistance?

Answer 28

Emphysema causes destruction of the alveolar walls resulting in large air spaces and reduction in total surface area available for GE (pO2 is low)

Question 29

How is diffusion resistance measured?

Answer 29

Carbon monoxide transfer factor Measure the CO uptake following a single maximal breath of a gas mixture containing air 14% helium and 0.1% CO. Inhaled CO is used because of its high affinity for Hb Amount of CO transferred from alveoli to the blood is an estimate of the diffusion resistance

Question 30

What is meant by anatomical dead space?

Answer 30

The conducting portion of the respiratory tract up till the terminal bronchioles

Question 31

What is the normal volume for the anatomical (or serial) dead space?

Answer 31

150 ml

Question 32

How would you measure the volume of anatomical dead space?

Answer 32

Nitrogen washout test

Question 33

What is alveolar dead space or distributive dead space? Why might it occur?

Answer 33

The volume of air in the alveoli not taking part in GE This could be due to insufficent blood suppy, damage by accident or disease - so even air that does reach there will be a proportion that is not exchanged.

Question 34

What is physiological dead space?

Answer 34

The sum of anatomical dead space + alveolar dead space

Question 35

How can the physiological dead space be determined?

Answer 35

Measure the pCO2 (or pO2) of expired and alveolar air. Alveolar air is diluted by dead space air to form the expired air and the degree of dilution is a measurement of the physiological dead space.

Question 36

What is the pulmonary ventilation rate composed of?

Answer 36

The dead space ventilation and the alveolar ventilation

Question 37

What is the ventilation perfusion ratio?

Answer 37

The imblance between alvolar ventilation and alveolar blood flow VA/Q (BF)