Chest wall anatomy and Breathing mechanics

Question 1

State Boyle’s Law

Answer 1

Pressure exerted by a gas is inversely proportional to its volume: P alpha 1/V

Question 2

In what pressure direction do gases move?

Answer 2

Areas of high pressure to areas of low pressure

Question 3

State Dalton’s Law

Answer 3

The total pressure of a gas mixture is the sum of the pressures of the individual gases

Question 4

State Charles Law and how it is clinically relevant

Answer 4

The volume occupied by a gas is directly related to the absolute temperature: v alpha T
This is explained by as you increase the temperature, gas particles move about more, therefore the volume increases.
Clinical relevance: measuring volume of expired air

Question 5

State Henry’s Law

Answer 5

The amount of gas dissolved in a liquid is determined by the pressure of the gas and its solubility in a liquid.

Question 6

Explain the anatomy of the pleural sac

Answer 6

Each lung is surrounded by a pleural cavity.
The pleural cavity of each lung is completely independent of the other.
The lungs and the interior of the thorax are covered by the pleural membranes (this is really just one membrane per lung which doubles back).

Question 7

State the function of the pleura

Answer 7

Attaches lungs to the ribs and diaphragm.

Due to the pleural membranes the lungs are effectively stuck to the ribs and diaphragm.

Question 8

State the difference between the visceral and parietal pleura

Answer 8

The visceral pleura is the membrane in contact with the lungs.
The parietal pleura is in contact with the ribs or diaphragm.

Question 9

State the volume and function of the pleural fluid

Answer 9

3ml of pleural fluid is between the parietal and visceral pleural membranes.
Function: lubrication for the lungs to glide up over the ribs as lungs inflate and down as they deflate.

Question 10

Define pleurisy

Answer 10

Inflammation of the pleura where membranes are inflamed and there is no pleural fluid.

Question 11

Explain the importance of the relationship between the pleural membranes and intrapleural pressure

Answer 11

When the normal lung is at rest the intrapleural pressure is -3mmHg as the elastic recoil of the chest tries to pull the chest outwards and the elastic recoil of the lungs tries to push the chest inwards creating an equilibrium.
Intrapleural pressure in normal healthy lungs at rest should always be less than atmospheric pressure.

Question 12

Explain how breathing can occur based on Boyle’s law.

Answer 12

When volume increases, pressure decreases
When volume decreases, pressure increases
Gases move from high pressure to low pressure
Therefore when the volume of the chest cavity increases, intracellular pressure becomes more sub atmospheric, and the lungs expand. Alveolar pressure decreases and air is blown into the lungs during inspiration from high pressure outside the lungs to low pressure inside the lungs.
When the volume of the chest cavity decreases as the inspiratory muscles cease contracting, pressure increases allowing the chest wall and lungs to return to their original size, compressing the alveoli. This increases alveolar pressure above atmospheric pressure so air is blown out of the lungs from high pressure to low pressure during expiration

Question 13

State the muscles of inspiration

Answer 13

External intercostals 
Diaphragm 
Sternoclenomastoid 
Scalene 
(Sternoclenomastoid and scalene act on upper 2 ribs to elevate)

Question 14

State the muscles of expiration

Answer 14

Passive at rest

Under severe respiratory load:
internal intercostals
abdominal muscles

Question 15

Explain the mechanics of the diaphragm

Answer 15

During inspiration, the diaphragm contracts, increasing thoracic volume and reducing pressure, thereby drawing air into the lungs.
During expiration, the diaphragm relaxes, decreasing thoracic volume and increasing pressure, thereby drawing air out of the lungs

Question 16

Explain the mechanics of the external intercostal muscles

Answer 16

The external intercostals contract during inspiration in two motions:
The pump handle motion increases the chest volume (thereby reducing pressure) anteriorly and posteriorly.
The bucket handle motion increases chest volume laterally.

Question 17

Explain the pathophysiology of asthma

Answer 17

The overactive constriction of smooth muscle in the airways increases resistance to airflow and therefore makes it more difficult to remove air from the lungs.

Question 18

Explain the pathophysiology of a pneumothorax

Answer 18

When atmospheric air enters the pleural cavity, the negative intrapleural pressure is lost. This means that the chest wall recoils outwards and the lungs recoil inwards. The two units become independent of each other.

Question 19

Define intra-thoracic (alveolar) pressure (Palv)

Answer 19

Pressure inside the thorax (essentially inside the lungs)

May be positive or negative.

Question 20

Define intra pleural pressure (Pip)

Answer 20

Pressure inside the pleural cavity.

Always negative in healthy lungs.

Question 21

Define transpulmonary pressure (Pt)

Answer 21

Palv minus Pip. Should always be positive since Pip should always be negative.

Question 22

Describe the pressure changes that occur during inspiration and expiration

Answer 22

Palv decreases during inspiration and increases during expiration
Pip decreases from -3mmHg to -6mmHg at the end of inspiration before rising back to -3mmHg at the end of expiration (it becomes less negative during expiration i.e. it increases)

Question 23

How is bulk flow of air between the atmosphere and alveoli proportional to atospheric and alveolar pressure and airway resistance?

Answer 23

Proportional to difference between atmospheric and alveolar pressures.
Inversely proprtional to resistance
F=(Patm-Palv)/R

Question 24

Describe how a transpulmonary pressure that opposes the effects of elastic recoil is created

Answer 24

Between breaths at the end of an unforced expiration, Palv=Patm.
No air flowing
Dimensions of lungs and thoracic cage stable due to opposing elastic forces.
Creates subatmospheric intrapleural pressure and thus a transpulmonary pressure.

Question 25

What determines how much air flows into the lungs?

Answer 25

Airway resistance, which in itself is determined by the radii of airways.

Question 26

Describe the difference between passive and forced expiration.

Answer 26

During passive expiration, phrenic and intercostal nerve activity is ceases and the inspiratory muscles cease to contract.
During forced expiration, the internal intercostals and abdominal muscles actively decrease thoracic dimensions, allowing more breaths/min by speeding up the breathing cycle.