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The Respiratory System



The respiratory system consists of several organs that function as a whole to oxygenate the body by inhaling air and exhaling carbon dioxide. The respiratory tract is divided into three sections, the first being the upper respiratory tract which is comprised of the nose and nasal passages, the paranasal sinuses and the pharynx. The respiratory airways are the middle section and include the larynx, the trachea, the bronchi and the bronchioles. Lastly, the lower section of the respiratory tract are the lungs themselves which are comprised of the respiratory bronchioles, the alveolar ducts, the alveolar sacs and the alveoli. This article will highlight the main anatomical information of each section and finish off with a brief overview of the pathology of the respiratory system.

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Overview of the anatomy of the lungs.

The Upper Respiratory Tract

The upper respiratory tract is lined with respiratory epithelium which is known as ciliated pseudostratified columnar epithelium, save certain parts of the pharynx, which also come into contact with the food bolus from the oral cavity during swallowing. When air is breathed in it passes through the nose into the nasal passages which communicate with the paranasal sinuses. In the roof of the nasal cavity there are olfactory cells which shift through the incoming air and pick up various odours, allowing the brain to register them and provide the human body with a sense of smell. The air then passes posteriorly into the nasopharynx, which is the most posterior part of the nasal cavity that communicates with the throat. Once in the pharynx, it descends past the oropharynx and into the laryngopharynx.

The Respiratory Airways

As with the upper respiratory tract, the respiratory airways and the lungs are all lined with respiratory epithelium. Once the air enters the larynx, it is purely within the respiratory organs. It continues down the larynx and into the trachea which bifurcates into the left and right main bronchi and then further in a tree branch like pattern into bronchioles.

The Lungs

The final section of the respiratory tract is within the lung tissues themselves. The bronchioles further divide into smaller and smaller respiratory bronchioles whose terminal branches contain grape like bunches of alveolar ducts, sacs and the terminal alveoli. It is here that the oxygen in the air cells diffuses into the blood and the opposite occurs for the carbon dioxide when attempts to leave the hemoglobin molecules in the erythrocytes and travel back out of the body via exhalation.

The lungs are made of a light and soft elastic tissue. The right lung is larger than the left lung and is comprised of three lobes, including the superior, middle and inferior. The left lung only has two lobes which are the superior and inferior. Fissures help separate the lobes and each lung has an oblique fissure which divides the upper and lower lobes of the left lung and the middle and lower lobes of the right lung. The right lung also has a horizontal fissure which separates the superior and middle lobes. Each lung also has three surfaces, the costal, the mediastinal and the diaphragmatic, which are of course named after the adjacent anatomical structure which that surface faces. The mediastinal surface connects the lung to the mediastinum via its root. The root of the lung contains the mainstem or lobar bronchi, the pulmonary vessels and bronchi as well as the bronchial vessels, lymphatics and autonomic nerves.


The disorders of the respiratory system are classified into four major categories. The obstructive conditions make breathing difficult and include diseases like emphysema, bronchitis and asthma. Restrictive conditions limit the capacity of a patient's lungs and this group of ailments includes fibrosis, sarcoidosis and pleural effusion as examples. Vascular diseases limit the hematological access to the lung tissue which in turn prevent optimum oxygenation of the tissues and include pulmonary edema, pulmonary embolism and pulmonary hypertension. Lastly, infectious, environmental and other diseases include pneumonia, tuberculosis, asbestosis and particulate pollutants.

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Show references


  • Neil S. Norton, Ph.D. and Frank H. Netter, MD, Netter’s Head and Neck Anatomy for Dentistry, 2nd Edition, Elsevier Saunders, Chapter 22 Introduction to the Upper Limb, Back Thorax and Abdomen, Page 583.
  • Frank H. Netter, MD, Atlas of Human Anatomy, Fifth Edition, Saunders - Elsevier, Chapter 3 Thorax, Subchapter 21. Lungs, Page 105 to 108.
  • Tim Taylor. Respiratory system. InnerBody.


  • Dr. Alexandra Sieroslawska


  • Thorax - Yousun Koh 
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