Chp. 7: Mechanics of Breathing

Question 1

Most important muscle of inspiration

Answer 1

Diaphragm

Question 2

What happens when the diaphragm contracts?

Answer 2

Abdominal contents are forced downward and forward and vertical dimension of chest cavity is increased. Rib margins are lifted and moved out.

Question 3

What happens when one side of the diaphragm is paralyzed?

Answer 3

It moves up rather than down with inspiration because intrathoracic pressure falls –> paradoxical movement

Question 4

External Intercostal Muscles

Answer 4

When they contract, ribs are pulled upward and forward, causing an increase in both the lateral and anteroposterior diameters of thorax

Question 5

Accessory muscle of inspiration

Answer 5

Scalene muscles (elevate first two ribs)
Sternocleidomastoids (elevate sternum)

Question 6

Expiration

Answer 6

Passive during quiet breathing

Abdominal wall muscles most important: rectus, internal and external oblique, transversus

Internal intercostals assist active expiration by pulling ribs downward and inward

Question 7

Lung volume during inflation vs. deflation

Answer 7

Lung volume at any given pressure during deflation is larger than is that during inflation

Question 8

Airway closure

Answer 8

Occurs at higher lung volumes with increasing age and also in some types of lung disease, including emphysema

Question 9

Transpulmonary pressure

Answer 9

Difference in pressure between inside and outside of lung

Numerically equal to pressure around the lung when the alveolar pressure is atmospheric

Question 10

Reduced compliance

Answer 10

Increase in fibrous tissue in lung
Alveolar edema
Lung unventilated for long period, esp. at low volume (atelectasis and inc. surface tension)
When pulm. venous pressure increased

Question 11

Increased compliance

Answer 11

Emphysema
Normal aging

Question 12

Surface tension

Answer 12

Arises because attractive forces between adjacent molecules of liquid are much stronger than those between liquid and gas

Question 13

Surfactant

Answer 13

Phospholipid
Main constituent is dipalmityoyl phosphatidylcholine (DPPC)

May be depleted if blood flow to a region of lung is abolished

Reduction of surface tension greater when film is compressed because DPPC molecules are closer together and repel each other more

Question 14

Advantages of surfactant

Answer 14

Increases lung compliance
Stability of alveoli is promoted
Keeps alveoli dry (curved alveolar surface reduces hydrostatic pressure in tissue outside capillaries)

Question 15

Interdependence

Answer 15

Support offered to lung units by those surrounding them

Question 16

Intrapleural pressure

Answer 16

Higher at base of lung (less negative) than apex to support weight of lung

Question 17

Is the lung easier to inflate at low or high volumes?

Answer 17

Low

Question 18

Is the base or apex of lung better ventilated?

Answer 18

Base

Question 19

At low lung volumes when a small inspiration is made from residual volume, what happens to the distribution of ventilation?

Answer 19

Normal distribution is inverted and upper regions ventilate better than lower zones

Question 20

Airway closure

Answer 20

Occurs at very low lung volumes in young healthy subjects

Occurs at higher volumes in the lowermost regions of the lungs in elderly people and may be present at FRC

Question 21

Relationship between lung and chest wall

Answer 21

Chest wall is pulled inward while lung is pulled outward

Question 22

Lung and chest wall at FRC***

Answer 22

FRC is the equilibrium volume when the elastic recoil of the lung is balanced by the normal tendency for the chest wall to spring out. At volumes above this, pressure is positive, and at smaller volumes, pressure is subatmospheric

Question 23

When does the chest wall find its equilibrium position?

Answer 23

When volume is increased to about 75% of vital capacity and relaxation pressure is atmospheric

ie, chest wall tends to expand at volumes up to 75% of vital capacity

Question 24

Poiseuille equation

Answer 24

V (volume flow rate) = P x pi x r^4 / 8nl

Question 25

What happens to gas at the center of a tube during laminar flow?

Answer 25

Gas there moves twice as fast as the average velocity

Question 26

With turbulent flow, pressure is proportional to what?

Answer 26

The square of flow rate

Question 27

Where in the lungs is flow laminar?

Answer 27

Fully developed laminar flow probably only occurs in very small airways where Reynolds numbers are very low

Question 28

What is the characteristic of flow in most of the bronchial tree?

Answer 28

Flow is transitional True turbulence may occur in the trachea, especially on exercise

Question 29

Airway resistance

Answer 29

Pressure difference between alveoli and mouth divided by flow rate

Question 30

Why does intrapleural pressure fall during inspiration?

Answer 30

1) As lung expands, elastic recoil increases 2) Reduction in alveolar pressure causes a further fall in intrapleural pressure

Question 31

Major site of airway resistance in lungs

Answer 31

Medium-sized bronchi

Question 32

Dynamic compression of airways

Answer 32

- Limits air flow in healthy subjects during a forced expiration - May occur in diseased lungs at relatively low expiratory flow rate, reducing exercise ability - During dynamic compression, flow is determined by alveolar pressure minus intrapleural pressure (not mouth pressure) and is independent of effort - Is exaggerated in diseases such as emphysema due to reduced lung elastic recoil and loss of radial traction on airways

Question 33

What factors exaggerate dynamic airway compression?

Answer 33

- Low lung volume because that reduces driving pressure - Reduction of recoil pressure, as in emphysema - Decreased radial traction on airways (emphysema)

Question 34

Forced Expiration Test

Answer 34

Subject inspires maximally and then exhales as hard and completely as possible Forced Expiratory Volume (FEV1.0): Volume exhaled in first second Forced Vital Capacity (FVC): Total volume exhaled In RESTRICTIVE diseases, primary problem is expanding on inhalation, both FEV1.0 and FVC are reduced but the ratio is normal or increased In OBSTRUCTIVE diseases, primary problem is obstruction to airflow on exhalation, and FEV1.0 is reduced more than FVC, giving a low ratio

Question 35

Long time constant unit

Answer 35

Shorter the time available for inspiration (fast RR), the smaller the inspired volume Time constant value given by compliance times resistance

Question 36

WOB

Answer 36

Pressure x Volume

Question 37

KEY CONCEPTS for mechanics or breathing

Answer 37

1) Inspiration is active but expiration during rest is passive. The most important muscle of inspiration is the diaphragm. 2) P-V curve of the lung is nonlinear and shows hysteresis. Recoil pressure of lung is attributable to both its elastic tissue and the surface tension of the alveolar lining layer. 3) Pulmonary surfactant is a phospholipid produced by Type II alveolar epithelial cells. Lack of surfactant leads to low compliance or respiratory failure. 4) The chest wall is elastic like the lung but normally tends to expand. At FRC, inward recoil or lung and outward recoil of chest wall are balanced. 5) In laminar flow, resistance is inversely proportional to fourth power of radius. 6) Lung airway resistance is reduced by increasing lung volume and by stimulation of beta2 receptors 7) Dynamic compression of airways during a forced respiration results in flow that is effort independent. The driving pressure is then alveolar minus intrapleural pressure. In patients with COPD, dynamic compression can occur during mild exercise, causing severe disability.