West's 9th ed - Chapter 7 - Mechanics of Breathing (1)

Question 1

Name the accessory muscles of inspiration, and their specific function.

Answer 1

Scalene muscles (these elevate the first two ribs)
Sternocleidomastoids (these raise the sternum)
Pectoralis major can act as an accessory muscle of respiration when the arm is in fixed abduction (think of swimming, when you finish a lap and you hang off the end of the pool by your elbows, panting)

Question 2

What happens when the external intercostal muscles contract?

Answer 2

The ribs are pulled upward and forward, causing an increase in both the AP and the lateral diameters of the thorax. (like a bucket handle)

Question 3

What happens when the internal intercostal muscles contract?

Answer 3

The ribs are pulled downward and in, reducing the AP diameter and the lateral diameter of the thorax.

Question 4

Is expiration an active process or a passive process?

Answer 4

During quiet breathing, expiration is a passive process. The elasticity of the lung and chest wall allow them to return passively to equilibrium position when the muscles are relaxed. During exercise and voluntary hyperventilation, expiration becomes active.

Question 5

Which muscles are used in active expiration?

Answer 5

Muscles of the abdominal wall: Rectus abdominis, Internal and External oblique, and Transverse abdominis. These muscles contract to raise the intra-abdominal pressure, and push the diaphragm up. The internal intercostal muscles are also used during active expiration.

Question 6

What is meant by the phenomenon of “hysteresis”?

Answer 6

This is the phenomenon that the lung volume at any given pressure is higher during expiration than it is during inspiration at the same pressure.

Question 7

What is meant by transpulmonary pressure?

Answer 7

This refers to the pressure difference between the pressure around the lung (intrapleural pressure) and atmospheric pressure, assuming that the alveolar pressure is equal to atmospheric pressure. During inspiration, the transpulmonary pressure is negative.

Question 8

How is compliance discussed, in terms of units, or on a graph?

Answer 8

Compliance is the volume change per unit pressure change. It is the SLOPE of the pressure-volume curve (where volume is on the y-axis).

Question 9

What is the normal value of compliance in a human, in the middle of inspiration?

Answer 9

200mL per cm H2O.

Question 10

Name some factors that will INCREASE compliance.

Answer 10

Emphysema. Asthma. Aging.

Question 11

Name some factors that will DECREASE compliance.

Answer 11

Fibrosis. Oedema. Atelectasis. Surfactant deficiency.

Question 12

Write the equation for Laplace’s Law.

Answer 12

Pressure = (4 x surface tension) / radius.

Question 13

What kind of a molecule is surfactant?

Answer 13

A phospholipid.

Question 14

What is the biochemical / physiological effect of surfactant?

Answer 14

It reduces surface tension, due to the fact that the molecules are hydrophobic at one end and hydrophilic at the other. This means that there are intermolecular repulsive forces.

Question 15

Name three roles of surfactant.

Answer 15

1. Reduces surface tension and thus increases compliance
2. Prevents atelectasis
3. Keeps the alveoli dry

Question 16

What is the difference between the intrapleural pressure at the base of the lung, and that of the apex of the lung?

Answer 16

The intrapleural pressure is less negative at the base than it is at the apex.

Question 17

Which lung is easier to inflate: the lung with lower volume or higher volume?

Answer 17

Lower volume. At higher volumes, the lung becomes stiffer and less compliant.

Question 18

What is the intrapleural pressure at the base of the lung?

Answer 18

-2.5 cm H2O (assuming atmospheric pressure of 0)

Question 19

What is an equation that describes the airway resistance?

Answer 19

This is the pressure in the mouth minus the pressure in the alveoli, all divided by flow rate.

Question 20

In which region of the lung is the airway resistance greatest?

Answer 20

In the first 7 generations of airways.

Question 21

In surfactant deficiency, what happens in the lungs apart from atelectasis and poor compliance?

Answer 21

The high surface tension sucks fluid from the capillaries into the alveoli, and pulmonary oedema develops.

Question 22

What happens to the lung bases if you continue to exhale after the end of a tidal exhalation?

Answer 22

The intrapleural pressure at the base of the lung becomes less negative, in fact in becomes positive (higher than atmospheric pressure). This means that there is airway closure in the bronchioles, gas trapping in the alveoli, and reduced compliance.

Question 23

At the end of a tidal expiration (at FRC), which two forces balance each other?

Answer 23

The inward pull of the lung is balanced by the outward spring of the chest wall.

Question 24

In obstructive airways disease, what happens to the ratio of FEV1/FVC ?

Answer 24

It reduces. (although both individual values decrease)

Question 25

In restrictive airways disease, what happens to the ratio of FEV1/FVC ?

Answer 25

It increases. (although both individual values decrease)

Question 26

What happens to airway resistance when you halve the radius of the airway in question?

Answer 26

The resistance increases 16-fold, because the numerator in Poisuille’s equation involves radius to the power 4.

Question 27

What factors will increase airway resistance?

Answer 27

• Decrease in lung volume
• Decrease in radius of airway (e.g. nasal breathing, bronchospasm)
• The first 7 generations of airways
• Breathing a dense gas (e.g. diving)