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Flashcards in Mechanics of breathing Deck (26)
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Describe the process of inhalation

Inhalation is an active process that occurs when the chest wall expands, creating a negative pressure gradient between the atmospheric air and air within the lungs

  • Diaphragm increases thorax height
  • External intercostals increase anterior-posterior and left-right dimensions

Air flows into the lungs through bulk flow down its pressure gradient. 


Describe the process of exhalation

Passive process (usually).

As inhalation ceases, the lungs revert back to their original size due to the elastic recoil of lung tissue.

This creates a negative pressure gradient between the lungs and atmospheric pressure, causing air to flow via bulk flow out of the lungs. 


What provides resistance to flow?

Airway diameter and contents provide resistance to flow 


What muscles are used to facilitate forced exhalation?

  • Abdominal wall muscles 
  • Extracartilagenous internal intercostals
  • Pectoral girdle muscles


What is the functional residual capacity?

The volume in the lungs remaining at end passive exhalation. Caused by equilibrium between thoracic wall and the lungs. 



Describe quiet respiration

Inhalation requires muscular work

Exhalation is passive process

Functional residual capacity always present in lungs. 


What is the pleural space?

What is its role?

The potential space between the pleural membranes covering the lung (visceral pleura) and the inside of the thoracic wall (parietal pleura). 

  • Provides lubrication during chest wall expansion and retraction.
  • Transfers expansile force from the chest wall to the lung 
  • Lung expands with the chest wall due to the pleural seal which keeps the pleura held tightly together. 
    • Drop in pressure in the pleural space forces lungs to expand with the chest wall. 
    • Suction from the pleural seal between the two pleura forces the lungs to expand with the chest wall 
    • Expansive force of chest wall exceeds elastic recoil of the lung. 



What nerve innervates the diaphragm?

Phrenic nerve: C3, 4 & 5


Which muscles stabilise the upper part of the chest wall (first rib)?


Anterior, middle and posterior scalenes. 



What is the work of breathing?

What are its components?

The energy expended during respiration:

  1. The energy required to expand the lungs against the lung and chest elastic forces - compliance work/ elastic work
  2. The energy required to overcome the viscosity of the lung and chest wall structures- tissue resistance work
  3. The energy required to overcome airway resistance to movement of air into the lungs- airway resistance work


What are the accessory muscles of respiration?

When may they be needed?

  • Scalenes: elevate first and second ribs
  • Sternocleidomastoid: elevates sternum

Needed in extremes of respiratory effort to aid inhalation. 


What lung volume is represented when the forces expanding and recoiling the lungs are equal?

At what phase of the respiratory cycle does this occur?

Functional residual capacity

Occurs at the end of passive exhalation


What is a pneumothorax?

Air into the pleural space causing increase in pleural space pressure. 

  • Removes the suction effect of the pleural seal
    • Allows the lung to collapse away from the chest wall under the unopposed force of its elastic recoil. 
  • Occurs when the chest wall or lung surface are punctured. 


Define compliance

Volume change of the lung per unit force applied

  • A measure of the disposition of the lungs to expand under traction or pressure


Define elastance

A measure of the disposition of the lungs to return to resting position due to their intrinsic elasticity. 


Explain Hooke's Law

How do the lungs differ from this law?

The force required to expand a spring (aka the lungs) by some distance is proportional to that distance.

BUT the lung is stiffer at high and low volumes. 


Explain the hysteresis curve

Caused by variations in compliance at different lung volumes. 

The lung is stiffer at high and low lung volumes:

  • Needs higher pressure change at low volumes to initially expand (due to surface tension in alveoli)
  • Needs higher pressure change at high volumes to complete inhalation


What are the effects of emphysema and fibrosis on lung compliance and FRC?


  • Increased compliance (tissue destruction = less elastic recoil = hyperinflation)
  • Increased FRC (less resistance and elastic recoil of the lung)
  • Reduced elastance


  • Decreased compliance
  • Decreased FRC (increased resistance and elastic recoil of the lung) 
  • Increased elastance 


Explain the effect of surface tension on compliance

Surface tension: at a surface (e.g. alveoli) attraction between water molecules are much higher than between water and air due to hydrogen bonds. 

  • Surface becomes as small as possible, decreasing compliance

Water lines alveoli, therefore a substance is needed to overcome the surface tension to allow alveoli expansion and increase compliance. 



Explain Laplace's Law

P = 4T /r

Pressure = 4x surface tension / radius

The higher the surface tension, the smaller the radius and the higher the pressure in the alveoli. 

All of the air in the smaller alveoli will flow into the larger alveoli due to the pressure gradient. 


How is surface tension reduced?

Surfactant is produced by type II pneumocytes. 

Reduces the surface tension of the fluid in the alveoli. 

Allows the lungs to expand in the early stage of inhalation by reducing surface tension to the point where muscular power can exceed it. 


What is surfactant?

What is it made from and secreted from?

What are its roles?

  • Secreted by type II pneumocytes
  • Main ingredient is phospholipid


  • Increases compliance by reducing surface tension
  • Reduces likelihood of tissue fluid transudation (
  • Bind pathogens- opsonisation (makes pathogens more attractive to immune cells)
  • Activates macrophages and neutrophils



How is resistance to air flow expressed?

Assuming flow is laminar:

Pouiselle's Law R = 8nl/πr4  

For individual airways, resistance rises by the fourth power of the radius. 




Where does the greatest resistance to airflow lie?

In the medium sized bronchi

Not in the bronchioles as they are many in number so have a low combined resistance (arranged in parallel)


Which bedside test is used to assess airway resistance?

Peak Expiratory Flow Rate



How does lung volume affect airway resistance?

As lung volume decreases, airways become smaller. Around 10% will close. This 10% can rise to 40% in older age and disease states