HUMAN RESPIRATORY SYSTEM 

       

The human breathing system consists of one pair of lungs and associated system of tubes for intake and removal of air through the nose  The lungs are situated in the thorax in a rib cage, the walls of which are formed by the ribs attached ventrally to the sternum and dorsally to the vertebral column, with intercostal muscles in between.

 Air is drawn into the lungs from nose, through a pair of nostrils into the nasal sacs.The nose and mouth cavities are separated by the palate so thate one can breathe through nose even when eating.The inside of the nasal cavity is moist and warm.It has mucus producing ciliated lining and numerous blood vessels close to the surface. The air is moistened and its temperature is adjusted as it passes over these surfaces. At the same time it is also cleaned, as the dust particles and germs get stuck in the mucus and are propelled  towards the pharynx by the cilia where they are swallowed or coughed out. 

      The air then passes into a long wind pipe, the trachea about 12cm long.Its upper part is called larynx or voice box which contains vocal cords.The opening of the larynx is called glottis having a cartilaginous flap like structure epiglottis which closes the glottis during swallowing of food and water.The walls of trachea are supported by incomplete rings of cartilages which keep it open all the time. 

The trachea branches into two bronchi which have almost the same structure as that of the trachea but are smaller in diameter.Each bronchus enters a lung, where it splits and resplits into numerous branches.These branches are called bronchioles. At the end of each bronchiole is a bunch of tiny air sacs or aliveoli, Where air and blood exchange gaeses takes place .The blood capillaries.Oxygen diffuses out into the blood and carbon dioxide from the blood diffuses into the alveoli.

Ventilation:

Ventilation means movement of air in and out. Breathing, one step of respiration,is often called ventilation. Breathing which takes place by the movement of the chest, can be divided into two step inspiration and expiration.Inspiration is air into your lung when you breathe and expiration when you out breathe of lungs . Ventilation happens about thoracic expansion of cavity and contracts.This is done by  two sets of muscles external and internal intercostals muscles .

  When breathing in (inspiration) the muscles of the diaphragm contract, this pulls the diaphragm downward making it flat.At the same time the external intercostal muscles contract.This pulls the rib cage upward outwards. These movement make the thorax  increses .As a result, the pressure inside it falls and the air rushes in to fill and expand the lungs.