Mechanism of breathing (pulmonary volumes and capacities)

Question 1

What is meant by the breathing cycle and what are the involved muscles?

Answer 1

The breathing cycle is simply inspiration and expiration

Inspirations muscles:

1) Diaphragm (innervated by two phrenic nerves)

• At increased work for inspiration
  2) External intercostal
  3) Sternocleidomastoid
  4) Scalene muscle

Expiration muscles:

• Normally passive due to elastic recoil, when exercising:

1) Internal intercostal
2) External oblique muscle
3) Rectus abdominas
4) Transversus abdominis
5) Internal oblique

Question 2

What is boyle’s law?

Answer 2

• Boyles states that at a constant temperature, the pressure of gas is inversely proportional to the volume of gas

P1V1 = P2V2

Question 3

What is the sequence of events during inspiration in normal breathing?

Answer 3

1) Inspiratory muscles contract

2) The thoracic cavity expands

3) Pleural pressure becomes more negative

4) Transpulmonary pressure increases

5) Lungs inflate

6) Alveolar pressure becomes subatmospheric

7) Air flows into the lungs until the alveolar pressure equals the atmospheric

Question 4

What is meant by transmural pressure?

Answer 4

It is the pressure difference across the airway/lung wall, the pressure inside the wall minus the pressure outside the wall (in - out)

Question 5

What are the different transmural pressures?

Answer 5

1) Transpulmonary pressure (PL) the pressure difference between the lung wall

PL = Pa (alveolar pressure) - Ppl (pleural pressure)

2) TransAirway Pressure (Pta) the pressure difference between the airway

Pta = Paw (airway pressure) - Ppl (pleural pressure)

Question 6

Why is the intrapleural pressure always negative?

Answer 6

1) The elastic recoil of the lungs & chest wall, a stretched lung tends to recoil inwards and the chest wall tends to recoil outwards in an equal and opposite direction, leading to a negative pleural pressure which helps to keep the lungs inflated

• If the transpulmonary pressure is equal to zero it means that there is pneumothorax leading to the collapse of the lungs

Question 7

What are the pressure changes that happen during a normal breathing cycle?

Answer 7

1) Intrapleural pressure

• Inspiration: -8cmH2O
• Expiration: -5cmH2O

2) Alveolar pressure

• Inspiration: -1cmH2O
• Expiration: +1cmH2O

Question 8

What are the different lung volumes?

Answer 8

1) Tidal Volume

2) Inspiratory Reserve Volume

3) Expiratory Reserve Volume

4) Residual Volume

Question 9

What is the tidal volume?

Answer 9

It is the volume of air entering and leaving the lung during a single normal breath approx 500ml at rest

Question 10

What is the inspiratory reserve volume?

Answer 10

The extra volume of air that can be maximally inspired above the typical resting tidal volume approx 3000 ml

Question 11

What is meant by the expiratory reserve volume?

Answer 11

It is the extra volume of air that can be actively expired at the end of a typical resting tidal volume approximately 1000 ml

Question 12

What is meant by residual volume?

Answer 12

It is the minimum volume of air remaining in the lungs after maximum expiration approx 1200 ml

Question 13

What is meant by the functional residual Capacity?

Answer 13

• Capacity is when you add two or more volumes together

FRC is the volume of air in the lungs at the end of a normal passive expiration (ERV + RV) APPROX 2200 ml

Question 14

What is meant by the total lung capacity?

Answer 14

The maximum volume of air that the lungs can hold approx 5700 ml

Question 15

What is meant by the vital capacity?

Answer 15

The maximum volume of air that can be exhaled after a maximum inspiration (ERV + TV+ IRV) APPROX 3600

Question 16

What is meant by the inspiratory capacity?

Answer 16

Inspiratory Capacity = Tidal Volume + Inspiratory Reserve Volume approx 2600

Question 17

What is the RV/TLC and what does it indicate?

Answer 17

It is the volume left in the lungs after forced expiration divided by the total lung that enters the lungs which is a measurement of the efficiency of the recoil of the lungs as in older individuals and emphysema patients this number increases indicating a higher RV

Question 18

What are the different capacities of the lungs?

Answer 18

1) Funcional Residual Capacity

2) Total Lung Capacity

3) Inspiratory Capacity

4) Vital Capacity

Question 19

What techniques are used to measure the RV and thus the FRC and TLC?

Answer 19

1) The Helium-dilution technique (RV anf FRC)

2) Body plethysmograph to measure FRC

Question 20

What is the helium dilution technique?

Answer 20

• Used to measure residual volume

Basically, you put 10% helium in an oxygen container which is the concentration of helium C1 in a known volume of a container V1, ask the person to inhale it, then C1V1 should equal = the concentration of the container after inhaling C2 * (V1 (volume that was in the container) + V2 (the unknown volume in the lungs)), you can calculate the total lung volume V2 by V2 = V1 (C1-C2) / C2

• If you measure it after normal tidal volume then you are measuring the functional residual capacity
• If you measure it after the end of forced vital capacity then you are measuring the residual volume
• A limitation of the test is that it gives false FRC in some diseases due to the poor ventilation of the lungs

Question 21

What is the plethysmograph technique?

Question 22

What is the FEV1/FVC ratio & what does it indicate?

Answer 22

It is the volume of forced exhalation in one second given the total forced vital capacity which indicates how much of the FVC can be exhaled in one second normally it is 80%/0.8 which is useful in diagnosing certain pathologies

Question 23

How do we measure the FEV1/FVR ratio?

Answer 23

The individual inspires maximally and then exhales into a spirometer forcefully and rapidly, plotting on the graph after 1 second and dividing it by the FVC

Question 24

What are the different values of the FEV1/FVC ratio in different pathologies?

Answer 24

• Normal 80%/0.8
• In case of pathologies both the FVC and FEV are decreased:

1) Obstructive diseases like asthma or COPD:

• 42% reduced as it takes more time to exhale the air

2) Restrictive diseases like fibrosis

• 90% as the elasticity is lost not much air will enter and it will leave quickly