The hairs in the nostrils prevent the entry of particles of dust into the nasal chamber.
The inner lining of the nasal chamber contains a fluid called the mucus spread over it. This liquid traps disease causing microorganisms.
The air going inside is moistened as it flows through the nasal chambers on coming in contact with mucus.
The air going inside is warmed in the nasal chamber, as the nasal chamber contains blood vessels through which the temperature of the air can be regulated.
The nose has an additional function of smell. Sensory cells that have the ability to smell are present in the nasal chamber. When we sniff, little wafts of air go high up in the nasal chamber into a special pocket where these sensory cells are located.
Pharynx
The pharynx is a common passageway for food and air, because on one hand the nasal chambers open into it from above, and on the other hand the mouth opens into it from below the nasal chambers. It leads into two tubes, the windpipe, also called as the trachea, and the food pipe, also called as the esophagus.
To prevent food from entering the wind pipe, there is a cartilaginous flap present over the wind pipe, called the epiglottis. The epiglottis acts as a lid for the wind pipe, and keeps it covered while swallowing food. Thus, it prevents the entry of food into the wind pipe.
Larynx
The larynx is a hollow, cartilaginous structure located at the point of meeting of the pharynx and the trachea (wind pipe).
It can be felt with the hands at the front part of the neck. It is not a passageway for air like the pharynx, nasal chamber and the trachea, but it is an essential part of the respiratory system, as we are able to speak only when we respire air in and out of the body.
It is made up of two folds of cartilage, hollow in the middle, through which air passes as it flows in and out of the wind pipe. This is the organ by which gives us the ability to speak: As air passes through the opening in between its two chords, the two chords vibrate producing sound. There are a set of muscles located near the chords, which help us in changing the tension of the two chords. By changing the tension of the two chords and passing air through it, one is able to produce different sounds from the larynx. Thus it is called the voice box of the human body.
Trachea
It is the wind pipe. It is partly covered by the thyroid gland in the neck. The food pipe (esophagus) is present along with the trachea in the neck, but its walls remain collapsed when not eating or drinking food, thus providing space for the trachea to take in and expel out air.
The walls of the trachea are made up of C shaped rings of cartilage. These rings provide strength to the trachea and keep it distended (expanded) permanently.
Bronchi
Close to the lungs, the trachea divides into two tubes, called the bronchi. Each of these tubes enters one lung. The bronchi also have C shaped rings in their walls like the trachea, to keep the air passage permanently and fully distended.
As the two bronchi enter the two lungs respectively, they divide into finer tubes called the secondary bronchi. In the lungs, the secondary bronchi divide further into still finer tubes called tertiary bronchi, which further divide into still finer tubes called the bronchioles. The bronchioles are very fine tubes and their diameter is 1 mm. Each bronchiole divides and subdivides until it produces groups of many tiny air sacs, called alveoli.
Lungs
The pair of lungs are two organs in the thoracic cavity. They provide an interface for the exchange of respiratory gases in and out of the blood stream.
The lungs contain thousands of air sacs (the alveoli) which provide a huge surface area for the diffusion of respiratory gases into the blood stream and out of the blood stream.
The lungs are continuously expanded and contracted by the combined work of the diaphragm and the muscles in between ribs (intercostal muscles).
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