2 - Ventilation

Question 1

What is the main function of the lungs?

Answer 1

Gas exchange across the alveolar membrane.

Question 2

How does alveolar air differ from atmospheric air?

Answer 2

Less O2 - 13.3kPa, More CO2 - 5.3kPa.

Question 3

What is the composition of mixed venous blood?

Answer 3

O2 - 6kPa, CO2 - 6.5kPa although it varies with metabolism.

Question 4

What does diffusion depend on?

Answer 4

Surface area - large Gradient (pressure between alveolar air and mixed venous blood) - large Diffusion resistance (dependent on nature of barrier and gas)

Question 5

What is the diffusion barrier?

Answer 5

An O2 molecule must diffuse through gas to reach the alveolar wall. It then diffuses through the epithelial cell of the alveolus, tissue fluid, endothelial cell of the capillary, plasma and finally the RBC membrane. It is 0.6um thick.

Question 6

For gases diffusing through gases, how are rate of diffusion and molecular weight related?

Answer 6

They are inversely proportional i.e. the larger the molecule, the slower it moves. Therefore CO2 diffuses slower than O2.

Question 7

For gases diffusing though liquids how are rate of diffusion and solubility related?

Answer 7

They are proportional. CO2 is much more soluble than O2 so diffuses 21 times faster.

Question 8

Overall in the system (the body) which of O2 or CO2 will diffuse faster?

Answer 8

CO2 - this means gas exchange is limited by O2.

Question 9

In the lung how long does gas exchange take? How long do RBCs spend in the capillary?

Answer 9

O2 exchange is complete with 0.5s. Blood cell is in the capillary for 1s.

Question 10

What are the pressures of pO2 and pCO2 in the normal lung?

Answer 10

The same as that of alveolar air. Blood leaving the alveolar capillaries is in equilibrium with alveolar air.

Question 11

Why is the composition of alveolar air so important?

Answer 11

The composition of alveolar air determines the gas composition of arterial blood determining oxygen supply to tissues.

Question 12

Why is ventilation important?

Answer 12

Exchange between alveolar gas and mixed venous blood will lower the pO2 and raise the pCO2 of alveolar air. Ventilation allows atmospheric air (with an increased pO2 and decreased pCO2) to be brought next to the alveoli.

Question 13

During ventilation what happens to the lungs?

Answer 13

They expand, increasing the volume of respiratory bronchioles and alveolar ducts so that air flows down airways to them.

Question 14

How can ventilation be measured?

Answer 14

Through spirometry.

Question 15

What is tidal volume?

Answer 15

The volume in and out with every breath.

Question 16

What is the inspiratory reserve volume?

Answer 16

The volume in and out when forced inspiration is carried out.

Question 17

What is the expiratory reserve volume?

Answer 17

The volume in and out when forced expiration is carried out.

Question 18

What is the residual volume?

Answer 18

The volume left in the lungs even after max expiration.
It cannot be measured by spirometer - a helium dilution must be used.

Question 19

What is the vital capacity?

Answer 19

Measured from max inspiration to max expiration - the biggest breath that can be taken in. ~5L but often changes in disease states.

Question 20

What is the inspiratory capacity?

Answer 20

The max inspiration from resting expiratory level - i.e. biggest breath possible from rest, usually ~3L.

Question 21

What is the functional residual capacity?

Answer 21

Volume of air in lungs at resting expiratory level.
It is the expiratory reserve volume + residual volume, normally ~2L.

Question 22

What are the typical values of… Tidal volume, Inspiratory reserve, Expiratory reserve, Residual volume, Functional residual capacity, Inspiratory capacity, Vital capacity, Total lung capacity?!

Answer 22

Tidal volume - 0.5L
Inspiratory reserve - 2.5L
Expiratory reserve - 1.5L
Residual volume- 0.8L
Functional residual capacity - 2.3L
Inspiratory capacity - 3L
Vital capacity - 5L
Total lung capacity - 5.8L

Question 23

What is ventilation rate?

Answer 23

The amount of air moved in and out of a space in a minute.

Question 24

What is a typical pulmonary ventilation rate at rest and during exercise?

Answer 24

8L / min at rest.
>80L / min during exercise.

Question 25

Are pulmonary and alveolar ventilation rate the same?

Answer 25

No. Air enters and leaves the lungs through the same airways - not all of it is exchanged. This means some of it never reaches the alveoli and is wasted due to these ‘dead spaces’.

Question 26

What are the two dead spaces? Together what are they known as? Give some typical ~ values.

Answer 26

Serial dead space - the volume of the airways that gas cannot be exchanged in (i.e. the mouth, trachea) - ~0.15L
Can be measured by nitrogen washout.

Distributive dead space - some of the alveoli may be dead or damaged or have poor perfusion - ~0.02L

Physiological dead space is the total - ~0.17L

Question 27

How could you calculate alveolar ventilation rate?

Answer 27

TV * RR = PVR

DS * RR = DSVR

PVR - DSVR = AVR

Question 28

What effects does the physiological dead space being 0.17L have on fast, shallow breathing, breathing at rest and slow, deep breathing?

Answer 28

Fast, shallow breathing - a high proportion of air that moves in and out is not used - this will decrease the AVR.

Breathing at rest - around a 1/3 of all air inhaled is wasted.
Most energy efficient so we adopt this.

Slow, deep breathing - not very much air is wasted.