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Flashcards in Mechanics of Breathing I Deck (12)
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RECAP: what is Boyle's Law?

  • Boyle's Law (derived from the ideal gas law) describes the relationship between pressure (P), volume (V) and molar quantity (n, the number of gas molecules present).
  • P ∝ n/v
  • If we want to move gases we can manipulate the pressure of the spaces that they are in.
  • Molecules move from a high to low pressure gradient so by increasing the pressure (or decreasing the volume, same effect) we can move the molecules as needed.


What causes air to flow in and out of the lungs?

Changes in lung volume induce changes in aleolar pressure, which generate pressure gradients between alveoli and atmosphere, causing air to flow.


Describe the structural events that occur that allow inspiration and expiration to take place.

    • The diaphragm and the external intercostal muscles contract (while the internal intercostal muscles relax), causing the thoracic cavity to expand (volume increases).
    • This means that the alveolar pressure decreases (according to Boyle's Law), and now the pressure in the atmosphere is higher than the pressure in the alveoli, so the air is sucked into the lungs.
    • The diaphragm and the external intercostal muscles relax (while the internal intercostal muscles contract), causing the thoracic cavity to condense (volume decreases).
    • This means that the alveolar pressure increases (according to Boyle's Law), and now the pressure in the atmosphere is lower than the pressure in the alveoli, so the air is sucked out of the lungs.


What is the pleural cavity?

  • The pleural cavity is a fluid filled space between the membranes (pleura) that line the chest wall and each lung.
  • They help to provide a frictionless surface to aid movement of the lungs.


Describe the structure of the pleural cavity (the pleural membranes) and how it generates its force.

  • The (inner) visceral pleura lines each lung, whereas the (outer) parietal pleura lines the inner surface of the thoracic cavity.
  • Between the two pleural is the sealed, fluid-filled pleural cavity.
  • As the tissues attached to each pleura recoil in opposite directions (ie. the lungs recoil inwards, the chest wall recoils outwards), the sealed cavity is stretched slightly.
  • This decreases the pressure within the pleural cavity ('intrapleural pressure') below atmospheric pressure ('negative pressure').
  • Negative pressure acts to pull the two pleura together: the greater the level of negative intrapleural pressure, the greater the level of force acting to pull the pleura together.


What is negetive and positive pressure?

  • Differences in pressure between neighbouring spaces are unstable.
  • In the absence of sufficient opposing force, equilibrum will be re-established via either movement of liquid/gas, or collapse/expansion of volume (at the expense of surronding structures).
  • Negetive pressure :-
    • There are lower number of molecules per volume (relating to surroundings).
    • This generates collpasing force (pulls surfaces of contained space together).
  • Positive pressure :-
    • Increased number of molecules per volume (relative to surroundings).
    • This generates expanding force (pushes surfaces of contained space apart).


Why is the system in equilibrim at the end of expiration?

If the force pulling the visceral pleura inwards (lung recoil) is equal to the force pulling the parietal pleura outwards (negetive intrapleural pressure), lung volume will remain constant.


Explain how inspiration, passive expiration and forced expiration  take place.


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

Passive expiration

  1. During (passive) expiration, relaxation of respiratory muscles that were previously contracted during inspiration occurs, reducing the outward force acting on the parietal pleura.
  2. This reduces the degree to which the cavity is stretched, increasing intrapleural pressure.
  3. 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.

Forced expiration

  1. 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.


What are the steps for inspiration?


What are the steps for expiration?


What is pneumothorax, and what are its consequences?

  • Intrapleural pressure is naturally sub-atmospheric due to the opposing recoil of the chest wall and lungs, and is required to generate expansion of the lungs during inspiration.
  • However, if either of the pleural membranes is pierced (for eg. due to trauma, a wound or a cancerous growth), the pressure gradient between the pleural cavity and the atmosphere (or lungs, depending on the injury) will cause air to enter the pleural space, until intrapleural pressure =  atmospheric pressure.
  • This is known as a pneumothorax.
  • Entry of air results in the loss of negative pressure (the intrapleural pressure increases until it equals atmospheric pressure) and expansion of the pleural cavity (at the expense of lung volume).
  • Both of these factors then act to reduce intrapleural pressure changes during inspiration, preventing the lungs from expanding properly as the chest wall moves outwards.
  • As the parietal and visceral pleurae are no longer being pulled together by negative intrapleural pressure, they recoil in opposite directions, causing particular regions of the lungs to collapse depending on the site and extent of the injury.


What are the two different types of pneumothorax?

  1. Open pneumothorax :-
    • In open pneumothorax the chest wall is punctured, and the air enters the pleural cavity from outside as in the cases such as stabbing, gun wounds etc.
  2. Closed pneumothorax :-
    • Closed pneumothorax is when air or gas gets in the pleural space without any outside wound.
    • This sometimes happens when the lung is already injured somehow, like from diseases such as cancer or cystic fibrosis. 

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