Anatomy of the chest wall and mechanics of breathing

Question 1

Pleural cavity: Structure

Answer 1

Thin fluid-filled (5ml) potential space between the pulmonary pleurae of each lung

Question 2

Pleura: Structure

Answer 2

Serous membrane which folds back onto itself to form a two-layered membranous pleural sac

• Visceral: adjacent to lung
• Parietal: adjacent to ribs

Question 3

Pleural fluid: Volume and Function

Answer 3

• 5ml
• Acts as a lubricant between lungs and ribs
• Prevents separation of the pleural membranes

Question 4

Inspiratory muscles

Answer 4

• Diaphragm (C3,4,5 innervation): contracts and lowers increasing thoracic cavity volume
• External intercostal muscles: ascends ribs

Question 5

Expiratory muscles

Answer 5

• Passive at rest
• Internal intercostal muscles: descends ribs decreasing thoracic cavity volume
• Abdominal muscles

Question 6

Boyle’s Law: relate to mechanics of breathing

Answer 6

The pressure exerted by a gas is inversely proportional to its volume.
Contraction of inspiratory muscles increases thoracic cavity volume. This decreases thoracic cavity pressure resulting in inspiration (air is sucked into lungs)

Question 7

Respiratory minute volume

Answer 7

Volume of gas inhaled or exhaled from a person’s lungs per minute

Question 8

Mechanical factors affecting respiratory minute volume

Answer 8

Flow of air = atmospheric P - alveolar P/airway resistance

• Difference between atmospheric and alveolar pressures
• Airway resistance: radii of airways

Question 9

Intra-thoracic/alveolar pressure (Pa)

Answer 9

Pressure inside lungs

• At end of normal expiration: Pa is equal to Patm
• Can be positive or negative compared to atmospheric pressure

Question 10

Intra-pleural pressure (Pip)

Answer 10

Pressure inside pleural cavity

• Normal: -3mmHg (compared to atmospheric pressure)
• Always negative due to opposing elastic forces of lungs and ribs

Question 11

Transpulmonary pressure (Pt)

Answer 11

Difference between alveolar pressure and intra-pleural pressure.
- Always positive in health (because intra-pleural pressure is negative)