The human gas exchange system

Aerobic respiration needs oxygen, and it produces carbon dioxide as a waste product. The human respiratory system contains the organs that allow us to get the oxygen we need and to remove the waste carbon dioxide we do not need. It contains these parts:

  • two lungs
  • tubes leading from the mouth and nose to the lungs
  • various structures in the chest that allow air to move in and out of the lungs
The trachea is below the throat and passes under the ribcage into the lungs. Lungs consist of air sacs, bronchiole and bronchus. Around ribs are intercostal muscles. Below the lungs is the diaphragm.Structures in the respiratory system

Connecting the outside to the inside

Air passes from the mouth into the trachea, often called the windpipe. The trachea divides into two bronchi, with one bronchus for each lung.

Each bronchus divides further in the lungs into smaller tubes called bronchioles. At the end of each bronchiole, there is a group of tiny air sacs. These air sacs have bulges called alveoli to increase their surface area.


Ventilation or breathing involves movements of the ribs, intercostal muscles and diaphragm to move air into and out of the lungs:

  • when we breathe in, we inhale
  • when we breathe out, we exhale

The table describes the changes involved:

DiaphragmContracts and moves downwardsRelaxes and moves upwards
Intercostal musclesContract, moving the ribs upwards and outwardsRelax, letting the ribs move downwards and inwards
Volume of ribcageIncreasesDecreases
Pressure inside the chestDecreases below atmospheric pressureIncreases above atmospheric pressure
Movement of airMoves into the lungsMoves out of the lungs
Air is drawn in to nasal cavity and mouth and down throat. Pressure decreases in lungs, air is pushed in. Diaphragm contracts and pulls downwards. Intercostal muscles contract and expand the ribcage.


When we inhale, changes in the diaphragm and ribcage reduce the pressure in the chest, and air moves into the lungs

A recap of respiration and an explanation of why we need glucose and oxygen