The function of the human respiratory system is to transport air into the lungs and to facilitate the diffusion of Oxygen into the blood stream. Its also receives waste Carbon Dioxide from the blood and exhales it.

The respiratory system consists of the upper and lower respiratory tracts. The upper respiratory tract includes the mouth, nose, nasal cavity, pharynx (wind pipe and food pipe) and larynx or voice box. Each has a specific function to aid the flow of air into the body.

Upper Respiratory Tract

  • Mouth, nose & nasal cavity: The function of this part of the system is to warm, filter and moisten the incoming air
  • Pharynx: Here the throat divides into the trachea (wind pipe) and oesophagus (food pipe). There is also a small flap of cartilage called the epiglottis which prevents food from entering the trachea
  • Larynx: This is also known as the voice box as it is where sound is generated. It also helps protect the trachea by producing a strong cough reflex if any solid objects pass the epiglottis.

Parts of the Lower Respiratory Tract

  • Trachea: Also known as the windpipe this is the tube which carries air from the throat into the lungs. It ranges from 20-25mm in diameter and 10-16cm in length. The inner membrane of the trachea is covered in tiny hairs called cilia, which catch particles of dust which we can then remove through coughing. The trachea is surrounded by 15-20 C-shaped rings of cartilage at the front and side which help protect the trachea and keep it open. They are not complete circles due to the position of the oesophagus immediately behind the trachea and the need for the trachea to partially collapse to allow the expansion of the oesophagus when swallowing large pieces of food.


  • Bronchi: The trachea divides into two tubes called bronchi, one entering the left and one entering the right lung. The left bronchi is narrower, longer and more horizontal than the right. Irregular rings of cartilage surround the bronchi, whose walls also consist of smooth muscle. Once inside the lung the bronchi split several ways, forming tertiary bronchi.
  • Bronchioles: Tertiary bronchi continue to divide and become bronchioles, very narrow tubes, less than 1 millimeter in diameter. There is no cartilage within the bronchioles and they lead to alveolar sacs.
  • Alveoli: Individual hollow cavities contained within alveolar sacs (or ducts). Alveoli have very thin walls which permit the exchange of gases Oxygen and Carbon Dioxide. They are surrounded by a network of capillaries, into which the inspired gases pass. There are approximately 3 million alveoli within an average adult lung.
  • Diaphragm: The diaphragm is a broad band of muscle which sits underneath the lungs, attaching to the lower ribs, sternum and lumbar spine and forming the base of the thoracic cavity.

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