The Lungs

Breathing mechanisms

Breathing mechanisms

The lungs are vital for respiration and are found in many air-breathing animals. Mammals contain two which are located on either side of the heart. Their main function is transport oxygen from the air into the body’s bloodstream and then transport carbon dioxide from the bloodstream and out into the atmosphere.


The lungs have a specific structure in line with their function.

Component Function



Located in the throat this is where the air travels down to reach the bronchi. They’re lined with C-shaped cartilage rings to prevent the tube from collapsing.There are two bronchi the air can travel down: the right bronchus and the left bronchus. They’re lined with ciliated epithelia to help stop microorganisms from entering the lungs.In the lungs, the bronchi then divide into smaller bronchioles.

The bronchioles then form alveoli, then site at which gaseous exchange takes place.

Breathing mechanisms

Breathing or ventilation is, to a large extent, involuntary. However, it is possible for you to control the rate at which you breathe but not for an extended amount of time.

Lungs aren’t able to ventilate themselves because they’re not muscular. Instead the thorax moves, controlled by the diaphragm and the intercostal muscles. The pleural sac which surrounds the lungs transmits these movements to each one: the inner membrane of the sac is attached to the lungs while the outer membrane is attached to the thorax. The pleural fluid, located between the membranes, cannot be compressed so when the thorax moves the lungs are made to move as well. The thorax must remain stretched otherwise the elastic alveoli will collapse.

Muscle contractions change the thorax volume which then changes the lung pressure causing air to move. In humans ventilation is tidal which means that air uses the same route to flow in and out of the body. Air always moves from an area of high pressure to an area of low pressure.

The movements involved are:

  • inhalation
  • exhalation

In inhalation:

  • The outer intercostal muscles contract.
  • This causes the ribs to lift and the diaphragm to lower.
  • The thoracic cavity increases in size which reduces the air pressure in the lungs and the alveoli walls are stretched.
  • Air is sucked in from the outside, moving from a high to a low pressure.

In exhalation:

  • The inner intercostal muscles contract.
  • The ribs are moved back down and the diaphragm raised by the abdominal muscles.
  • The space in the thoracic cavity decreases and the alveoli and bronchioles shrink due to elastic recoil.
  • Air is pushed out of the lungs, moving from a low to high pressure.