The respiratory system is responsible for taking oxygen and given off carbon dioxide and some water. It is divided into the upper and lower respiratory tract.
There are several organs involved in the Respiratory system, the first being the nose. This is part of the upper tract together with the mouth, throat, larynx and numerous sinus cavities in the head. Air breathed in through the nose is warmed and filtered before passing down the tract into the lungs. The lower tract includes the trachea, bronchi and the lungs, which contain bronchial tubes, bronchioles and alveoli (air sacs.)
 
The lungs are the principle organs of the respiratory system. These are situated in the upper part of the thoracic cage. They do not work on their own but function by a variation of atmospheric pressure, which is achieved by a muscular wall called the diaphragm. The diaphragm contracts and relaxes, which results in the alteration in the volume of the throat and therefore and alterations of atmospheric pressure within the lungs. When the diaphragm contracts it causes air to rush in, when it relaxes it pushes air out of the lungs.
The inspired air contains oxygen and passes into billions of minute air chambers called alveoli, which have very thin walls. Around these walls are capillaries of the pulmonary system. It is at this point that the fresh air gives off its oxygen to the blood and takes carbon dioxide from the blood, which is then expelled with the expired air. The trachea has rings of cartilage to prevent it collapsing. It passes through the oesophagus branching into two bronchi. The bronchi branch into smaller and smaller tubes ending up in the bronchioles, which have no cartilage in their walls and have clusters of thin walled air sacs called alveoli. The right lung has three lobes, upper, middle and lower. The left has two lobes, upper and lower.
 
The pleura are the serous membrane, which covers the lungs. The visceral layer is in close contact with the lung tissue and the parietal layer lines the chest wall. Between these layers is the pleural cavity. Air going into the lungs follows the same route as food to start with, but then reaches the epiglottis which permits the passage of air to lungs but closes when liquid or food are swallowed.