Respiratory Mechanisms

Question 1

Flow equation

Answer 1

F = change in P/R

Flow is proportional to the pressure (P) difference between two points and inversely proportional to resistance (R).

Question 2

Boyle’s law

Answer 2

P1V1=P2V2

The pressure of a gas is inversely related with its volume.

Question 3

Muscles of normal quiet inspiration

Answer 3

Diaphragm

External intercostal muscles (move sternum superiorly, ribs swing outwards - bucket handle).

Question 4

Muscles of deep/forced inspiration

Answer 4

Scalenes
Sternocleidomastoid
Pectoralis major and minor

Question 5

Muscles of normal quiet expiration

Answer 5

This is a passive process which depends more on lung elasticity than muscle contraction. The inspiratory muscles relax and resume resting length, rib cage descends and lungs recoil - thoracic and intrapulmonary volume decrease.

Question 6

Muscles of forced expiration

Answer 6

Abdominal wall muscles - contract to increase abdominal pressure forcing abdominal organs superiorly against the diaphragm.

Internal intercostal muscles depress the rib cage.

Question 7

Compliance equation.

Answer 7

C = change in volume / change in pressure

Question 8

What is lung compliance affected by?

Answer 8

1. Elastic properties of the lung tissue
2. Surface tension of alveolar fluid
3. Elastic properties of the thoracic cage

Question 9

Law of LaPlace

Answer 9

P = 2T/r
Pressure = 2 x surface tension / radius

Question 10

Poiseuille’s law

Answer 10

R = 8ln/pi r^4

R = 8 x length x gas viscosity / pi x radius^4

Radius has the biggest influence on resistance of an airway.

Question 11

Bernoulli principle

Answer 11

The sum of kinetic and potential energy must remain constant. When airflow enters a constriction, linear velocity increases and therefore pressure must decrease (and vice versa).

P decreases as we move towards the mouth.

Question 12

What are the main determinants of the cross sectional area (and therefore resistance) of airways?

Answer 12

1. Lung volume
2. Elastic recoil of the lung
3. Bronchial smooth muscle tone

Question 13

What is the relationship between lung volume and airway resistance?

Answer 13

Resistance decreases as lung volume increases.

Small airways distend during inspiration and compress during expiration (i.e. also respond to changes in intrapleural pressure).

Small airways also attached to alveoli, therefore as alveoli expand, so do small airways (tethering effect of elastic tissues).

Question 14

When does dynamic airway compression occur?

Answer 14

When the intrapleural pressure is greater than the pressure in the airways.

Question 15

Work of breathing equation

Answer 15

Work = change in V / change in P

Question 16

Work of breathing component

Answer 16

1. Elastic resistance (65%)
2. Airflow resistance (28%)
3. Work against viscous/tissue resistance (7%)

Question 17

Breathing pattern in increased elastic resistance (e.g. pulmonary fibrosis).

Answer 17

WOB is minimised by rapid, shallow breaths as elastic resistance increases with increased depth (VT) and decreased frequency.

Question 18

Breathing pattern in airflow resistance (e.g. emphysema).

Answer 18

WOB is minimised by slower, deeper breathing as airflow resistance increases as frequency increases and depth decreases

Question 19

Dead space

Answer 19

Anatomical dead space = air in the conducting pathway.

Alveolar dead space = alveoli that do not participate in gas exchange because of insufficient blood supply.

Physiological dead space = anatomical + alveolar dead space.