Respiration is the process of gaseous exchange between an organism and its environment. In the higher animals and man the gaseous exchange between the tissues and environment is termed as internal or tissue respiration. The exchange of gases between the body and the environment taking place in the lungs is termed as external respiration. The external respiration constitutes processes of inspiration and expiration. Inspiration is an active muscular contraction, while expiration is merely a passive act of the relaxation of respiratory muscles.
The respiratory system is responsible for taking in Oxygen and giving off CO2 and water. It is divided into the upper respiratory tract and the lower respiratory tract.
Nose, mouth, the throat, the larynx, and numerous sinus cavities in the head.
The trachea, the bronchi and the lungs which contain bronchial tubes, bronchioles and alveoli or air sacs.
The two lungs which are the principal organs of the respiratory system, are situated in the upper part of the thoracic cage. They are inert organs, ie., they do not work by themselves, but function with the help of a muscular wall known as the diaphragm.
The pharynx is a tube approximately 12cm in length, which is a common opening for both digestive and respiratory system. It connects the oral cavity to the oesophagus (food tube) and the nasal cavity to the larynx and wind pipe. The opening into the larynx is oval in shape and guarded by the leaf like epiglottis.
The epiglottis folds down over the opening like a trap door while food or liquid is being swallowed. It prevents the entry of foreign substances into the respiratory passage. The closure of the epiglottis, when we swallow, is a reflex action and can be interfered with, if one attempts to talk and swallow at the same time. If this happens one may choke to death in the absence of immediate assistance.
From the pharynx, air passes through the trachea, which is 12cm long and 15cm in diameter. The tract, consists of a large number of C-shaped cartilage rings. The larynx or the voice box is at the top of the trachea and it is the vocal cords inside the box which by its coming together and going away from one another produces different sounds.
The trachea branches at its lower end into the right and left bronchi which enters the lungs. Within the lungs these bronchi repeatedly divide, forming microscopic tubes called bronchioles. Each bronchiole ends with several clusters of microscopic elastic air sacs called alveoli which is the functional unit of lungs. This resembles a bunch of grapes.
The paired lungs lie within the large cavity of the chest, the thoracic cavity. The lungs are greyish in colour and are spongy in appearance. The right lung has three lobes - upper, middle and lower, and the left lung has two lobes - upper and lower. The floor of the thoracic cavity is formed by the domelike muscular diaphragm. Each lung is enclosed by two layers of membrane called thepleural membrane. The chest cavity is also lined with this membrane. This layer being known as the parietal pleura, while the lung covering is called the visceral pleura. The thoracic cavity is flexible, capable of expanding and contracting along with the lungs.