Mechanics of breathing

Question 1

What does changes in lung volume induce?

Answer 1

Changes in lung volume induce changes in alveolar pressure, which generate pressure gradients between alveoli & atmosphere, causing air to flow

Question 2

What happens in inspiration?

Answer 2

Diaphragm contracts & thoracic cavity expands. Alveolar pressure decreases

Question 3

What happens in expiration?

Answer 3

Diaphragm relaxes (and lung recoils). Thoracic cavity volume decreases, alveolar pressure increases.

Question 4

Comment on the pressure at the end of expiration

Answer 4

At the end of expiration, Palveoli = Patmosphere, therefore there is no movement of air

Question 5

What happens during inspiration?(In terms of volume and pressure)

Answer 5

The outer surfaces of the lung are pulled outwards (expansion).
↑volume = ↓alveolar pressure.
Palveoli < Patmosphere
Air flows from high (atmosphere) to low (alveoli) pressure.

Question 6

What happens during expiration?(In terms of volume and pressure)

Answer 6

Air within the lung is compressed.
↓volume = ↑alveolar pressure.
Palveoli > Patmosphere
Air flows from high (alveoli) to low (atmosphere) pressure.

Question 7

How are the lungs and chest attached?

Answer 7

The lungs and chest wall are indirectly attached via the pleural cavity

Question 8

What is the pleural cavity?

Answer 8

fluid filled space between the membranes (pleura) that line the chest wall and each lung.

Question 9

What generates negative pressure within the pleural cavity?

Answer 9

The opposing elastic recoil of the chest wall (outward) and lungs (inward) generates negative pressure within the pleural cavity (sealed cavity + increased volume = decreased pressure).

Question 10

What does negative pressure act to do?

Answer 10

Negative pressure acts to pull the two membranes together (more negative pressure = more force pulling pleurae together).

Question 11

What does pleurae provide to aid movement of the lungs?

Answer 11

Pleurae also provide a frictionless surface to aid movement of the lungs.

Question 12

What is the system in at the end of expiration?

Answer 12

System is in equilibrium

Question 13

When will lung volume remain constant?

Answer 13

If the force pulling the visceral pleura inwards (lung recoil) = the force pulling the visceral pleura outwards (negative intrapleural pressure)

Question 14

What happens to parietal pleura during inspiration and what does it decrease?

Answer 14

During inspiration, muscular contraction pulls the parietal pleura outwards, stretching the cavity, decreasing PIP

Question 15

What does a decreased intrapleural pressure create?

Answer 15

The decreased (more negative) intrapleural pressure creates a greater level of force acting to pull the pleurae together (effectively acting to pull the visceral pleura outwards).

Question 16

What happens when the force generated by decreased intrapleural pressure>than inward force generated by lung coil?

Answer 16

When the force generated by negative PIP becomes greater than the inward force generated by lung recoil, the visceral pleura (and the attached lungs) will be pulled outwards, expanding the lung.

Question 17

What happens to the parietal pleura during expiration and what does it increase?

Answer 17

During (passive) expiration, relaxation of previously contracted respiratory muscles, reduces the outward force acting on the parietal pleura, reducing the degree to which the cavity is stretched, increasing PIP

Question 18

What happens to the visceral pleura when the increased (less negative) intrapleural pressure no longer generates sufficient force to overcome the elastic recoil of the lung?

Answer 18

When the increased (less negative) intrapleural pressure no longer generates sufficient force to overcome the elastic recoil of the lung, the visceral pleura will be pulled inward (along with the pleural cavity and parietal pleura) decreasing lung volume.

Question 19

What happens during forced expiration and what happens to the lung volume?

Answer 19

During a forced expiration, contraction of other respiratory muscles (such as the abdominals and internal intercostals) acts to provide further inward force on the parietal pleura, compressing the pleural cavity (further increasing PIP), forcing an increased and more rapid decline in lung volume.

Question 20

Steps involved in inspiration

Answer 20

Respiratory muscles (e.g. diaphragm) contract
↓
Volume of thoracic cavity increases
↓
Intrapleural pressure becomes more negative
↓
Outward force exerted on visceral pleura becomes greater than inward recoil force
↓
Lungs expand, increasing volume
↓
PAlv (alveolar pressure) decreases below PAtm (atmospheric pressure)
↓
Air moves down pressure gradient, through airways into alveoli, expanding the lungs

Question 21

Steps involved in expiration

Answer 21

Respiratory muscles (e.g. diaphragm) relax, lungs recoil due to elastic fibres
↓
Volume of thoracic cavity decreases
↓
Intrapleural pressure increases
↓
Lungs compressed*, volume decreases
↓
PAlv increases above PAtm
↓
Air moves down pressure gradient, into atmosphere, deflating lungs

Question 22

What does piercing of either pleura cause?

Answer 22

cause air to enter the space (pneumothorax) from either the lungs or atmosphere, depending on the injury

Question 23

What does a pneumothorax cause to the negative intrapleural pressure?

Answer 23

Negative intrapleural pressure is lost – as the chest wall and lung are no longer indirectly attached, they will recoil in opposite directions, causing the lungs to collapse (atelectasis)

Question 24

What happens if either pleural membrane is ruptured in terms of pressure?

Answer 24

If either pleural membrane is ruptured, the pressure gradient between the pleural cavity and surrounding environment will cause air to enter (pneumothorax) until intrapleural pressure = atmospheric pressure.

Question 25

What does entry of air into the pneumothorax increase and do?

Answer 25

Entry of air = ↑ pleural cavity volume (at the expense of the lung volume, which decreases). This then acts to reduce intrapleural pressure changes generated during inspiration, preventing the lungs from expanding properly.