breathing mechanics

Question 1

How does the respiratory system achieve movement of air?

Answer 1

Gases naturally move from areas of higher pressure —-> lower pressure, until an equilibrium is re-established

Question 2

Describe the pressure changes during inspiration and expiration and how this relates to volume changes

Answer 2

Inspiration:
	• Diaphragm contracts and flattens
	• Thoracic cavity volume increases 
	• Alveolar pressure decreases; allowing air to move in down it’s pressure gradient 
Expiration: 
	• Diaphragm relaxes and curves back up
	• Thoracic cavity volume decreases
	• Alveolar pressure increases; allowing air to move out down its pressure gradient

Question 3

Why is there no air movement at the end of expiration?

Answer 3

P(alveoli)=P(atmosphere) therefore there is no movement of air

Question 4

Why is there air movement inwards during inspiration in terms of partial pressure?

Answer 4

• Outer surface of the lung are puled outwards (expanded) = increases volume = decreases alveolar pressure
  
  • P(alveoli) < P(atmosphere)
  • Therefore air flows from high atmospheric pressure to low alveolar pressure

Question 5

Why is there air movement outwards during expiration in terms of partial pressures?

Answer 5

• Air within the lung is compressed = lowers volume = increased alveolar pressure
• P(alveoli) > P(atmosphere)
Air flows from high alveolar pressure to low atmospheric pressure

Question 6

What is the pleural cavity and what is its function?

Answer 6

Fluid filled space between the membranes (pleura) that line the chest walls and each lung
These help to provide a frictionless surface to aid movement of lungs

Question 7

What is negative pressure and how is it generated?

Answer 7

Negative pressure acts to pull the 2 membranes together
More negative pressure = more force pulling pleura together
Generated:
The opposing elastic recoil of the chest wall (outward) and lungs (inward) generates negative pressure within the pleural cavity.
This sealed cavity along with the increased volume results in a decreased pressure.

Question 8

Define negative pressure

Answer 8

• Lower number of molecules per volume relative to surrounding

Generates collapsing force (pulls surfaces of contained spaces together)

Question 9

Define positive pressure

Answer 9

• Higher number of molecules per volume relative to surrounding
  
  • Generates expanding force (pushes surfaces of contained space apart)

Question 10

Why does the lung volume remain the same at the end of expiration?

Answer 10

Forces pulling the visceral pleura inwards (lung recoil) = the force pulling the visceral pleura outwards (negative intrapleural pressure)
THIS MEANS LUNG VOLUME WILL REMAIN THE SAME

Question 11

Describe the changes in the pleural cavity during inspiration

Answer 11

As inspiration begins, contraction of respiratory muscles (eg. the diaphragm) generates sufficient force to pull the parietal pleura outwards. This stretched the pleural cavity, decreasing the intrapleural pressure. As the intrapleural pressure becomes more negative, the force pulling the two pleurae together increases. When this force becomes greater than the force generated by the elastic recoil of the lung, the visceral pleura will be pulled outward, expanding the lung.

Question 12

Describe the changes in the pleural cavity during passive expiration

Answer 12

During (passive) expiration, relaxation of respiratory muscles that were previously contracted during inspiration occurs, reducing the outward force acting on the parietal pleura. This reduces the degree to which the cavity is stretched, increasing intrapleural pressure. 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 inwards (along with the pleural cavity and parietal pleura), decreasing the lung volume.

Question 13

Describe the changes in the pleural cavity during forced expiration

Answer 13

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

Question 14

Describe in detail, the process of inspiration

Answer 14

1. Respiratory muscles (diaphragm) contract
   
   2. Thoracic cavity volume increases
   3. Intrapleural pressure becomes more negative
   4. Outward force exerted on visceral pleura becomes greater than inward recoil force
   5. Lungs expand, increasing volume
   6. Alveolar pressure < atmospheric pressure
   7. Air moves down pressure gradient, through airways into the alveoli
2. Lungs expand

Question 15

Describe in detail, the process of expiration

Answer 15

1. Respiratory muscles (diaphragm) relax
   
   2. Lungs recoil due to elastic fibers
   3. Thoracic cavity volume decreases
   4. Intrapleural pressure increases
   5. Lungs compressed and volume decreases
   6. Alveolar pressure > atmospheric pressure
   7. Air moves down pressure gradient, into atmosphere
      Deflating lungs

Question 16

Explain the main difference between passive and forced expiration

Answer 16

Compression of the lungs due to increased intrapleural pressure only occurs during forced expiration. In passive breathing, elastic recoil is sufficient to decrease lung volume

Question 17

What is pneumothorax, and what are its consequences?

Answer 17

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

Entry of air = increased pleural cavity volume = reduces intrapleural pressure during inspiration therefore preventing the lungs from expanding properly
Pneumothorax involves:
1. The entry of air into the pleural cavity
2. Loss of negative intrapleural pressure
3. Collapse of lung tissue