IB DP Biology: SL复习笔记6.4.2 Ventilation: Mechanism

Inspiration & Expiration

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

Antagonistic Muscle Action

  • 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

Exam Tip

The intercostal muscles work in an antagonistic manner; as one contracts the other relaxes!Note that the internal intercostal muscles only contract to cause forced expiration; expiration is passive the majority of the timeRemember, if you learn one of either inspiration or expiration, the other is almost exactly the opposite.