Breathing in

• The breathing-in, or?inspiration, process causes the?volume of the chest to increase?and the?air pressure to decrease?until it is?lower than the atmospheric pressure
  • When gas is in a large-volume container that allows the gas particles to spread out, the pressure exerted by the gas on the walls of the container is low
• As a result, air moves?down the pressure gradient?and rushes into the lungs
  • A gas will always move down a pressure gradient from an area of high pressure to an area of low pressure
• The inspiration process
  • The?diaphragm?contracts?and?flattens, increasing chest volume
  • In addition to the flattening of the diaphragm the?external?intercostal muscles?contract, causing the ribcage to move?upwards?and?outwards; this also increases chest volume

The process of inspiration

Breathing out

• Breathing out, or?expiration, occurs mostly due to the recoil of the lungs after they have been stretched by the inspiration process, and is therefore a mainly passive process
• Volume of the chest decreases?and?pressure increases, causing air to be forced out down its?pressure gradient
  • When gas is in a low-volume container it is compressed, causing the gas particles to exert more pressure on the walls of the container
• The passive expiration process
  • External intercostal muscles?relax,?allowing the ribcage to move?down?and?in
  • Diaphragm?relaxes?and becomes?dome-shaped
  • The?recoil?of?elastic fibres?in the alveoli walls reduces the volume of the lungs
• The expiration process can be active when there is a need to expel excess air from the lungs e.g. when blowing out a candle
• The active expiration process
  • Internal?intercostal muscles?contract?to pull the ribs?down?and?in
  • Abdominal muscles contract?to push organs upwards against the diaphragm, decreasing the volume of the chest cavity
  • This causes forced exhalation

The process of passive expiration

• Muscles only carry out the work of moving the body when they are?contracting, or?pulling;?they cannot push
• As a result of this limitation muscles often operate in?pairs?when movement in two directions in required
• One muscle of the pair pulls in one direction and the other muscle?pulls in the opposite direction
  • This is described as?antagonistic?muscle action
• Examples of antagonistic muscle action in ventilation are
  • Internal?and?external intercostal muscles
    • When the internal intercostal muscles contract, the rib cage moves down and in
    • When the external intercostal muscles contract, the rib cage moves up and out
  • The?diaphragm?and?abdominal muscles
    • When the diaphragm contracts, it flattens and moves downwards
    • When the abdominal muscles contract, the internal organs of the abdomen are compressed and pushed upwards, exerting upward pressure on the diaphragm