The Respiratory System

The respiratory system

The function of the human respiratory system is to transport air into the lungs and facilitate the diffusion of oxygen into the bloodstream. It also receives waste Carbon Dioxide from the blood and exhales it. Here we explain the anatomy of the airways and how oxygen gets into the blood.


The respiratory system organs are separated into the upper and lower respiratory tracts. The upper respiratory tract includes the mouth, nose, nasal cavity, pharynx (windpipe 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 organs

  • 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 (windpipe) 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.

Respiratory system diagram

Respiratory system

Lower respiratory tract organs

  • Trachea: Also known as the windpipe this is the tube that 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 that 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 are 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 broadband of muscle that sits underneath the lungs, attaching to the lower ribs, sternum, and lumbar spine and forming the base of the thoracic cavity.

Respiratory system-related quizzes

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