The body system which is responsible for the exchange of gases between body fluid and outer environment is called respiratory system.The human respiratory system can be divided into Lower two regions, upper respiratory tract and lower respiratory tract.

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Upper Respiratory Tract

The upper respiratory tract includes nostrils, nasal cavity and pharynx.


The nose is only externally visible part of the respiratory system. Human nose is composed of bones. cartilage and fatty tissues. The external openings of nose are called nostrils and the inner hollow spaces are called nasal cavities. There are two nasal cavities which are partitioned by means of nasal septum (the pan of nasal bone). The anterior pans of nasal cavities near the nostrils are called vestibules which contain a network of hairs. Both the nostrils and nasal cavities are lined by mucous membrane alongwith cilia.

Nose hairs, mucus and cilia serve as a defence mechanism against the harmful pathogens and solid particulate (relating to particles) matter present in the air The mucus and cilia filter the air and prevent the entry of foreign particles such as microorganisms, dust and particulate matter inside the respiratory system. The mucus also helps in moistening the air. Cilia move the trapped substances to the pharynx for their removal. Underneath the mucous membrane, there are blood capillaries that help to warm the air to about 30oC, depending upon the external temperature.


Pharynx is cone-shaped passageway leading from the oral and nasal cavities to the esophagus and larynx. The pharynx is part of the digestive system and also the respiratory system. It is also important in vocalization.The human pharynx is conventionally divided into three sections: the nasopharynx, the oropharynx and the laryngopharynx.

Nasopharynx (Nasal Pharynx)

It is the uppermost section, located between the skull base and the soft palate. The adenoids or pharyngeal tonsils are located on its posterior wall.

Oropharynx (Oral Pharynx)

The part after the nasopharynx, the oropharynx is the region behind the base of the tongue, between the soft palate and the epiglottis.


Also known as the hypopharynx, it is the last section of the pharynx, located between the epiglottis and the cricoid cartilage, continuing into the larynx and esophagus.


The larynx (noise box) is composed of an external skeleton of cartilage plates that prevent collapse of the structure The plates are fastened together by membrances and muscle fibers. The front set of plates, called thyroid cartilage, has a central ridge and elevation commonly known as the Adam’s apple. The plates tend to be replaced by bone cells beginning from about 20 years of age and onwards. Two fibrous bands called vocalcordsare located to the larynx The vocal cords are composed of twininfoldingsof mucous membrane stretched horizentally across theLarynx. Larynx serves for dual function: as an air canal to the lungs and controller of its access, and the organ of phonation (voice production).


The traches or windpipe is a membranous tube. It consists of dense regular tissue and smooth muscle reinforced with 15.20 C shaped pieces of cartilage. The trachea has an inside diameter of 12rnm and a length of 10-12 cm. It descends from the level of the 5th thoracic vertebra.

Bronchi and Bronchioles

The trachea divides to form two smaller tubes called primary bronchi. The primary bronchi divide into secondary bronchi with each lung. There re two secondary bronchi in the left lung and three in the right lung. The secondary bronchi, in turn, give rise to ternary bronchi. The bronchi continues to branch, finally giving rise to bronchioles which ate less than 1mm in diameter. The bronchioles also subdivide several times to become even smaller terminal bronchioles. In cartilages are replaced with cartilage plates but the bronchioles and their terminal branches have no cartilage structures.

Alveolar Ducts and Alveoli

The terminal bronchioles divide to form respiratory bronchioles. The respiratory bronchioles give rise to alveolar ducts. These alveolar ducts contain tiny air filled chambers called alveoli which are the sites of gas exchange between the air and the blood. There are over 700 million alveoli present in the lungs, representing a total surface area of 70-90 m2. The wall of each alveolus is only 0.1 micro meter thick. On its outside is a dense network of blood capillaries. Lining each alveolus is moist squamous epithelium. This consists of very thin, flattened cells, reducing the distance over which diffusion occur. Collagen and elastin proteins are also present in their walls which allow the alveoli to expand and recoil easily during breathing.

External Structure of Lungs

The lungs are the principal organs of respiration. Each lung is conical in shape, with its base resting on the diaphragm and its apex extending superiorly to a point approximately 2.5 cm superior to the clavicle.
The right and left lungs are separated medially by the heart and mediastinum, which is the area between the lungs.
The left lung has two lobes, superior lobe and inferior lobe separated by the oblique fissure. The right lung has three lobes. The hilum is a triangular shaped depression on the concave medial surface of the lungs. A membrane called pleura surrounds each lung. It is a double membrane; consists of an inner and an outer layer. The inner membrane is called visceral pleuron which is firmly attached to the lungs while the outer is called parietal pleuron which lines the chest wall and covers the superior surface of the diaphragm. The space between the visceral and parietal pleuron is called pleural cavity. The cavity is filled by a film of fluid. The fluid enables them to slide over one another. The lungs are spongy due to presence of alveoli. Each alveolar sac is made up of simple squamous epithelium.