Anatomy of Chest Wall and Mechanics of Breathing

Question 1

What does Boyle’s Law state?

Answer 1

It states that the pressure exerted by a gas is inversely proportional to its volume (P a 1/V).

Question 2

What does Dalton’s Law state?

Answer 2

It states that the total pressure of a gas mixture is equal to the sum of the pressures of individual gases (Pt = P1 + P2 + P3…)

Question 3

What does Charle’s Law state?

Answer 3

It states that the volume occupied by a gas is directly related to its absolute temperature.

Question 4

What does Henry’s Law state?

Answer 4

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

Question 5

How many lobes are there in each lung?

What are the names?

Answer 5

Right lung: 3 lobes; superior lobe, middle lobe, inferior lobe

Left lung: 2 lobes; superior lobe, inferior lobe

Question 6

How many sacs are there in total in the thorax?

What are they?

Answer 6

3

The pericardium and the two pleural cavities.

Question 7

What is the superficial membrane surrounding the lungs called?

Answer 7

The visceral pleural membrane.

It extends into and lines the lungs fissures.

Question 8

What is the outer membrane around the lungs called?

Answer 8

The parietal pleural membrane.

Question 9

What is pleurisy?

Answer 9

Inflammation of the pleura.

It is painful to breathe in and out in this situation.

Question 10

What is the function of the intrapleural fluid?

Answer 10

• It allows the two pleural membranes to glide across one another and avoids friction.
• It also sticks the two membranes together, ultimately anchoring the lungs to the thoracic wall (rig cage), think of it like to glass slides being held together with water in between them.

Question 11

Ultimately, what are the mechanics behind breathing?

What causes air to move into and out of the lungs.

Answer 11

Change in lung volume.

According to Boyle’s Law:

Increased volume = decreased pressure (high pressure atmospheric air moves into lung)

Decreased volume = increased pressure (high pressure air in lungs is expelled into the atmosphere)

Question 12

Which muscles are used for inspiration?

Answer 12

diaphragm, external intercostal muscles, sternocleidomastoids and scalenes.

Question 13

Which muscles are used for expiration when not at rest?

Answer 13

internal intercostal muscles, abdominal muscles

Question 14

What causes expiration at rest?

Answer 14

It is a passive process, the muscles which cause the inspiration relax which decreases thoracic volume and increases lung pressure, causing air to leave the lungs.

Question 15

What happens to the diaphragm during inspiration and expiration, at rest?

Answer 15

On inspiration, the diaphragm contracts and flattens out (increasing thoracic volume).

On expiration, the diaphragm relaxes (dome shape).

Question 16

In what directions do the external intercostal muscles increase the thoracic cavity?

Answer 16

anterio-posteriorally, medio-laterally

Question 17

Explain the pathophysiology of asthma.

Answer 17

Over-reactive constriction of bronchial smooth muscle. It has more effect on expiration over inspiration. During an attack, the smooth muscles of the bronchioles contract and the diameter decreases, which increases resistance to air exiting during expiration.

During inspiration, diaphragm contracts and thoracic volume increases. Airways are forced open by physical forces of inspiration.

During expiration, diaphragm relaxes and thoracic volume decreases. Airways are compressed by physical forces of expiration.

Question 18

What is intra-thoracic (alveolar) pressure (P subscript A)?

Answer 18

The pressure within the thoracic cavity (essentially within the lungs). It can be both positive or negative compared to atmospheric pressure.

Question 19

What is intra-pleural pressure (P subscript ip)?

Answer 19

The pressure within the pleural cavity. It is ALWAYS negative - in healthy lungs at least.

Question 20

What is the transpulmonary pressure (P subscript T)?

Answer 20

This is the pressure difference between alveolar pressure and intra-pleural pressure. It is ALWAYS positive seen as intra-pleural pressure is always negative and more negative than alveolar pressure - in healthy lungs at least:

Pt = Pa - Pip

Question 21

What is a pneumothorax?

Answer 21

If the thoracic wall/pleural cavity is punctured, high pressure atmospheric air will enter the pleural cavity where there is subatmospheric pressure to equalise the pressure (high pressure to low pressure).

The relationship between the two pleural membranes is lost, the affected lung will not increase in volume with increases in thoracic volume as it will have collapsed.

Question 22

Summary

Answer 22

• The lungs are elastic structures whose volume depends upon the pressure difference across the lungs (the transpulmonary pressure) and how stretchable the lungs are.
• During inspiration, the contractions of the diaphragm and inspiratory (external) intercostal muscles increase the volume of the thoracic cage.
• This makes intrapleural pressure more subatmospheric (negative) and causes the lungs to expand.
• This expansion makes alveolar pressure subatmospheric, which creates the pressure difference between atmosphere and alveoli to drive air flow into the lungs.
• During expiration, the inspiratory muscles cease contracting, allowing the elastic recoil of the chest wall and lungs to return them to their original between-breath size.
• This compresses the alveolar air, raising alveolar pressure above atmospheric pressure and driving air out of the lungs.
• In forced expirations, the contraction of expiratory (internal) intercostal muscles and abdominal muscles actively decreases thoracic dimensions, reducing duration of breathing cycle and allowing more breaths/min.

Question 23

What is the atmospheric pressure at sea level?

Why do the lungs always equalise with this value between inspiration and expiration?

Answer 23

760 mmHg (0 mmHg)

Because the lungs and the atmosphere are connected by the airways and the partial pressure gradients cause them to equalise.

Air moves from high pressure to low pressure, and vice versa.

Question 24

What happens to the intra-pleural pressure during ve ntilation?

Answer 24

It fluctuates, but it always remains negative.

Approx. -4 mmHg throughout the breathing cycle.

Question 25

How does the intra-pleural pressure have a constant negative value?

Answer 25

Competing forces within the thorax cause the formation of the negative intra-pleural pressure.

One of these forces relates to the elasticity of the lungs themselves—elastic tissue pulls the lungs inward, away from the thoracic wall. This inward tension from the lungs is countered by opposing forces from the pleural fluid and thoracic wall.

Surface tension within the pleural cavity pulls the lungs outward. Since the parietal pleura is attached to the thoracic wall, the natural elasticity of the chest wall opposes the inward pull of the lungs.

Ultimately, the outward pull is slightly greater than the inward pull, creating the –4 mm Hg intra-pleural pressure relative to the intra-alveolar pressure.

Question 26

What are the physical factors that effect ventilation?

Answer 26

1. Resistance - The size of the airway is the primary factor affecting resistance:

F = P(atm) - P(A) / R

The major determinant of airway resistance is the radii of the airways.

2. 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 are attempting to recoil, whereas the chest wall is compressed and attempting to move outward. This creates a subatmospheric intrapleural pressure and hence a transpulmonary pressure that opposes the forces of elastic recoil.