Structure of respiratory tract


The human respiratory tract is one of the most essential systems of the human body. Oxygen is the primary need help in the survival of mammals, hence a brief cessation of breathing for 3 minutes can result in the death of an individual. When compared to many other systems, lungs are unique in their structure and functions. Anatomically, the respiratory system is divided into the upper respiratory tract ( URT)and the lower respiratory tract(LRT). This article is designed to brief the anatomy and physiology of URT and LRT.


Anatomy of the upper respiratory tract (URT)

The upper respiratory tract comprises the nose, nasal passages, pharynx, tonsils, adenoid tissues, larynx, and trachea. The nose is divided into external and internal parts. The external nose has 2 anterior nares or nostrils through which air enters inside the lungs. The inner portion of the nose has a hollow space providing shelter for mucus membrane and the fine hair-like projections help in filtering the air. The nose also has a rich blood supply and nerves. The throat is generally known as the pharynx. It is a tube-like structure that anchors the nasal, oral and laryngeal portions hence it is divided into the nasopharynx, oropharynx, and laryngopharynx. The nasopharynx is located just beneath the nose and above the soft palate. The oropharynx houses near palatine and tonsil region. The laryngopharynx is the combination of a part of larynx and pharynx. Tonsils and adenoids are the oral glands found in the roof of the mouth assist in the immune function. The pharynx is the common passage for both food and air. The larynx, on the other hand, is known as the voice box made up of cartilaginous epithelium-which produces voice through vocal vibrations. The picture below shows the sequence of the respiratory system through which air flows.


Anatomy of the lower respiratory tract (LRT)

Bronchi, lungs, bronchioles, and alveolus are the major components of LRT. LRT, especially alveolus provides a physiological space for the air exchange. When the lungs start dividing into left and right through left and right bronchi. The bronchi further channelized into fine bronchioles-smaller parts. Each bronchiole becomes the smallest units of lungs called alveoli. Alveoli is the principal area of exchange of carbon dioxide and oxygen. 


Lobe and lobules

Each lung is divided into lobes and lobules. The left lung consists of 2 lobes, the upper and lower lobe, whereas the right lung has 3 lobes- upper, middle, and lower lobes. The lobar areas of bronchi further segmented into multiple lobules. Each lobule further divided into fine bronchioles. Inside the lungs, the lobes, lobules, bronchi, alveoli are supported by mediastinal connective tissues. The inner lining of most of the alveolar space is lined with a mucus membrane having many cilia. Cilia help in producing free movement of lungs, I,e inspiration and expiration. During the respiratory movements, alveoli undergo a constant whipping motion with the help of cilia. The airways contain an additional  150 ML of air that doesn't participate in a gas exchange known as physiologic dead space


Anatomy of the respiratory system


Structure of alveolus

The inner alveolar surface is lined with simple squamous epithelium with an extracellular matrix. Alveoli have a very rich blood supply spread across parenchymal cells( interstitial spaces) . The alveolar surface is also known as the respiratory membrane help in allowing the gases. The respiratory membrane has several layers of piled systematically. The friction and mechanical wear tear is managed by a sticky substance called surfactant. The respiratory membrane is a very thin layer measuring between 0.2 µm at its thinnest part and 0.6 µm at its thickest. In the alveolar walls contain thin and elastic fibres known as collagen fibres. The fibres allow the alveoli to expand during inspiration and contraction as the lungs deflate. Alveolar cells are divided into 3 major types I,e type-1, type-2, and type-3. Type-1 alveolar cells are epithelial cells that form the alveolar walls. Type II alveolar cells are metabolically active and to help in secreting the surfactant. Type-2 cells have a phospholipid that lines the inner surface to prevent alveolar collapse. Type III alveolar cells are the macrophages that participate in a phagocytic activity that ingests foreign matter (eg; bacteria) and act as an important defence mechanism.


Physiology of respiration

Respiration is a combination of inspiration and expiration. During inspiration, the air is taken in and expiration will push the carbon dioxide out. The atmospheric air inhaled contain nitrogen – 78%, Oxygen at  21%, Carbon dioxide – 0.03 – 0.04% with few traces of hydrogen and Noble gases. Alveoli participate in a selective absorption excluding all the gases except the oxygen. The exchange of gases takes place through the blood capillaries and the alveolar surfaces.  The main mechanism of respiration is supported based on the partial pressure gradients at different surfaces of alveolar space.



Related topics

1 .   

Lower respiratory tract


Exchange of gases in lungs


Structure and functions of nose


Human body systems




  1. Name the parts of the upper and lower respiratory system.
  2. What are the 3 segments of the larynx?
  3. Explain the role of larynx apart from respiratory function.
  4. What are alveoli? Describe the structure and functions of alveolus?
  5. Where do gases exchange in lungs ?
  6. Name the  Physiologic mechanism  that help in gas exchange.


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