Gaseous exchange in birds

Due to metabolic rate, birds need high supply of oxygen and an efficient gaseous exchange mechanism.
The respiratory system is made up of lungs and air sacs.
During inhalation, air enters through the trachea, bronchus and to the posterior air sac to the lings, then to the anterior air sac and finally to the exterior (atmosphere) through the trachea.


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The respiratory organs in man are lungs and the respiratory surfaces are the sac like structures called alveoli.

The respiratory tract (air passage)
Air enters through the nostrils into the nasal cavity where it is warmed to body temperature.
It begins from the nostrils into the back of the mouth, then into the pharynx from which it goes into the larynx and then to the trachea. From here, it travels through the bronchus, bronchioles and lastly to the alveolus.
The membrane of the nasal cavity is covered with cilia between which are goblet cells, which produce mucus.
Dust and germs inhaled from the atmosphere are trapped in mucus and are carried by the beating action of cilia towards the back of the mouth where they are swallowed.
This helps to prevent dust and germs from entering the lungs. Therefore, by the time air reaches the lungs it is dust and germ free, warm and moist. It is drawn from the nasal cavity into the trachea (wind pipe).

The trachea
This is a tube running from the pharynx to the lungs. It is always kept open by the circular rings of cartilage within it. The cartilage prevents the trachea from collapsing in case there is no air.
Cilia and goblet cells extend into the trachea to draw germs and dust out of trachea into the mouth where they are lost.
At the lower end, the trachea divides into sub tubes called bronchi, which penetrate further into the lungs and divide repeatedly to form small tubes called bronchioles.
The bronchioles divide into many small tubes called alveolar ducts, which end in air sacs called alveoli.
The alveoli are the respiratory surfaces of mammals. There are about 300 million alveoli in a human lung. This increases the surface area over which gaseous exchange takes place.

Location of the lungs in the body
They are located in the thoracic cavity, enclosed by thorax wall and diaphragm.


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The alveoli
An alveolus is a sac-like structure. The outer surface of the alveolus is covered with a network of blood capillaries. The alveolus is moist and thin walled. The oxygen in the alveolus diffuses into blood in the capillaries and it is carried around the body. At the same time, Carbon dioxide diffuses from blood into the alveolus and travels through the alveolar duct to the bronchioles then to the bronchi and trachea and out through the nostrils.

The mammalian lung
These are two elastic spongy-like structures located within the thoracic cavity and protected by the rib cage. Between the ribs are intercostal muscles, which move the rib cage. Below the lungs is a muscular sheet of tissue called the diaphragm.

Breathing mechanism in mammals/ lung ventilation
The breathing mechanism in mammals involves two sub-processes that are inspiration and expiration.


  • This is the process by which air is allowed into the respiratory organs (lungs).
    The external intercostal muscles contract while the internal intercostal ones relax.
  • This makes the rib cage to move outwards and upwards. The diaphragm contracts and flattens.
    This increases the volume of the thoracic cavity and reduces the pressure in it below that of the atmosphere.
    This causes air to enter from the atmosphere through the nostril, trachea, bronchi, and bronchioles until it reaches the alveoli.


  • The internal intercostal muscles contract and the external ones relax.
  • This makes the rib cage to move downwards and inwards and the diaphragm becomes dome-shaped.
  • This reduces the volume of the thoracic cavity and increases its pressure beyond that of the atmosphere.
  • This forces the lungs to contract and release Carbon dioxide through the bronchi, trachea and out through the nostrils.