The respiratory system is formed by organs involved in ventilation - exchanging air between the outer environment and the lungs, and respiration - exchanging gases (oxygen and carbon dioxide) between the outside environment and the tissue and cells of the body.
Before reaching the lungs the air passes through air passages of the respiratory system. The air passages include the nasal cavity, nasopharynx, larynx, trachea and bronchi. Air in the air passages undergoes several transformations, including cleaning, warming and humidifying.
The respiratory system or respiratory tract can be divided into two major parts: the upper and the lower respiratory tracts.
The upper respiratory tract includes the following structures:
The nose consists of its visible (outer) part and the nasal cavity. The nasal cavity has a bony skeleton and is lined with mucous membrane (mucosa), it also houses the olfactory epithelium. In the lower part of the nose the respiratory meatus begins, and it provides mechanical cleansing of the inspired air.
The paranasal sinuses are air-filled cavities in bones of the skull lined with mucosa. Among functions of the paranasal sinuses are additional humidifying and heating of the inhaled air and increasing resonance of the voice.
The larynx is a flexible passageway for air between the oropharynx and trachea, which plays an essential role in voice formation and protects the lower airway against food inspiration.
The trachea is a short, flexible air tube, which extends from the larynx down to the middle of the thorax, where it divides into two main bronchi. It allows the passage of air from the larynx and pharynx to the lungs.
The lower respiratory tract is built from the following structures:
The bronchi are airway passages that conduct air from the trachea to the lungs. The bronchi branch and form the bronchial tree. The first two bronchi arising from the trachea are the right and left main bronchi. Each main bronchus divides into lobar bronchi, the lobar bronchus divides into segmental bronchi, further, into intrasegmental bronchi. Finally, the smallest bronchi divide into bronchioles. Each bronchiole divides until the terminal bronchiole, which continues as the pulmonary alveolus, forming the alveolar tree within the lungs.
The most prominent organs of the respiratory system are the lungs, which are essential for respiration and gas exchange. There are two lungs in the human body, the right and the left lung, conical-shaped organs situated in the thoracic cavity. The right lung is divided into three lobes, while the left lung consists of two lobes. Each lobe further divides into bronchopulmonary segments, which is an anatomical and functional unit of the lung.