Anatomy of the Chest Wall and Mechanics of Breathing

Question 1

What is Boyle’s Law?

Answer 1

The pressure exerted by a gas is inversely proportional to its volume (P=1/V)

Gases (singly or in mixtures) move from areas of high pressure to areas of low pressure.

Question 2

What is the importance of the pleural cavity?

Answer 2

Allows air to move in and out of the lungs, allowing the mechanics of breathing.

Parietal pleural membrane and visceral membrane which are continuous with each other.

• Visceral membrane is attached to the outer surface of the lungs.
• Parietal pleural membrane covers the inner surface of the ribs.

Question 3

In between these two membranes is…

what does this allow?

Answer 3

pleural fluid

In between the parietal and visceral pleura lies pleural fluid which sticks the two membranes together and holds the lungd against the ribs and diaphragm.
The ribs and lungs are constantly trying to pull apart but are prevented because of the parietal fluid.

Question 4

what is pleurisy?

Answer 4

inflammation of the pleura

Question 5

What are the mechanics of breathing?

Answer 5

Occurs because the thoracic cavity changes volume.
It is based on Boyles Law (The pressure exerted by a gas is inversely proportional to its volume):
o Increase volume= decrease pressure
o Decrease volume= increase pressure

Question 6

What are the relvant pressures associated with the lungs?

Answer 6

Intra-thoracic pressure (Alveolar) Pressure (Pa):
- Pressure inside the thoracic cavity/lungs. May be negative or positive compared to atmoshpheric pressure.

Intra-pleural Pressure (Pip):
- Pressure inside the pleural cavity, ALWAYS negative (in healthy lungs)

Transpulmonary pressure (Pt):

• Difference between alveolar pressure and intra-pleural pressure. ALWAYS positive (in healthy lungs) because Pip is ALWAYS negative.
• Pt= Pa-Pip
• Determines the volume of the lungs and how stretchable they are

Question 7

What does Inspiration use?

Describe how the thoracic volume changes and its effect on pressure.

Answer 7

During Inspiration the contractions of External intercostal muscles and the diaphragm increase the volume of the thoracic cage. (“bucket handle “acting on the lateral dimension of rib cage and “pump handle” acting on posterior and anterior dimension of rib cage)

• This makes intrapleural pressure more subatmospheric (negative) and causes the lungs to expand.
• This expansion makes alveolar pressure more subatmospheric (negative), and causes the lungs to expand.

Question 8

What does expiration use ?and Describe how the thoracic volume changes and its effect on pressure.

Answer 8

During Expiration, the inspiratory muscles stop contracting, allowing the elastic recoil of the chest wall and lungs to return to their original between breath size. It is passive at rest.
- Uses internal intercostal muscles and abdominal muscles during severe respiratory load. (aided by elastic recoil within the alveoli) It decreases thoracic dimensions, reducing the duration of breathing cycle and thus allows more breaths per minute.

Question 9

What is the pathophysiology of asthma?

Answer 9

Asthma is the OVER REACTIVE CONSTRICTION of bronchial smooth muscle.
Increases resistance, expiration phase is the most affected.

Question 10

What happens to the airways during inspiration and expiration?

Answer 10

During inspiration the airways are pulled open by physical forces of inspiration.

During expiration the airways are compressed by physical forces of expiration.

Question 11

Exemplify the importance of the relationshio between the pleural membranes. (Pneumothorax example.)

Answer 11

A normal lung at rest allows the elastic recoil of the chest wall trying to push the chest wall outward. At the same time there is elastic recoil of the lung creating inward pull of the chest wall.

However in Pneumothorax (collapsed lung) the sealed pleural cavity is opened to the atmosphere and air flows in. the air moves from the higher atmospheric pressure to the lower Pip (intra pleural pressure; pressure inside the pleural cavity), in attempts to equalise the pressure.
The relationship between the two pleural membranes is lost.

Question 12

What are the factors that affect breathing?

Answer 12

• Bulk flow of air between the atmosphere and alveolli= proportional to the difference between the atmospheric and alveolar pressures
• Between breaths at the end of an unforced expiration Patm-PA,
• Airway resistance which determines how much air flows into the lungs

Question 13

Exemplify the factors affecting breathing.

Answer 13

• Bulk flow of air between the atmosphere and alveolli= proportional to the difference between the atmospheric and alveolar pressures and inversely proprtonal to the airway resistance. F= Patm- Pa/R
• Between breaths at the end of an unforced expiration Patm-PA, no air is flowing and the dimensions of the lungs and thoracic cage are stable as the result of opposing elastic forces.
• The lungs are stretched and attempting to recoil, whereas the chest wall Is compressed and attempting to move outward. -> this creaes a subatmospheric intrapleural pressure and hence a transpulmonary pressure that opposes the forces of elastic recoil.
• Airway resistance determines how much air flows into the lungs at any given pressure difference between the atosphere and alveoli.