Ventilation: Mechanism
- Ventilation is essential for the effective exchange of gases in the lungs
- It replaces older air in the lungs with fresh air from the external environment
- This helps to maintain the concentration gradient of oxygen and carbon dioxide between the alveoli and blood
- Ventilation involves inspiration (breathing in) and expiration (breathing out)
Inspiration
- 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
Expiration
- 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