6.4.2 Ventilation: Mechanism

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