The process of inhaling and exhaling is called breathing. This process accompanied by the breakdown of food to produce energy is called respiration. The mechanism of breathing varies, depending on the habitat and level of organisation of an animal.
In humans, the respiratory system includes respiratory tract and lungs. The respiratory tract includes the nose, pharynx, larynx, trachea and bronchi. Air enters through the nostrils into the nasal chamber and pharynx, which opens into the trachea through the glottis of the larynx. The larynx, also called the sound box, helps in sound production. The trachea is divided into two branches called the primary bronchi. Each primary bronchus undergoes repeated division to form the secondary and tertiary bronchi ending up in very thin terminal bronchioles. The terminal bronchioles connect to ducts called the alveolar ducts that open into the alveoli.
An average adult's lungs contain about 600 million alveoli. Each alveolus is covered by thin and fragile blood capillaries, which combine to form the pulmonary vein. The pulmonary vein carries purified blood to the heart and from the heart, the pulmonary artery carries impure blood to the lungs.
Bronchi, bronchioles and alveoli form a part of a pair of lungs – the left lung and the right lung. The left lung has two lobes and is slightly smaller because of the location of the heart near it. The right lung is bigger with three lobes.
Each lung is enclosed by the pleural membrane, which contains pleural fluid. The lungs are situated in the air-tight thoracic chamber. The thoracic chamber is supported by the vertebral column dorsally, by the sternum ventrally, by the ribs laterally and by the dome-shaped diaphragm on the lower side.
Based on the function, the respiratory system is divided into two parts – the conducting part and the respiratory part.