The<unk> s are the primary organs of the respiratory system in humans and most animals including a few fish, and some snails. In mammals and most other vertebrates, two<unk> s are located near the backbone on either side of the heart. Their function in the respiratory system is to extract oxygen from the air and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their different muscles to support and foster breathing. In earlier tetrapods, air was driven into the<unk> s by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians. In humans, the main muscle of respiration that drives breathing is the diaphragm. The<unk> s also provide airflow that makes vocal sounds including human speech possible. Humans have two<unk> s, a right<unk> , and a left<unk> . They are situated within the thoracic cavity of the chest. The right<unk>  is bigger than the left, which shares space in the chest with the heart. The<unk> s together weigh approximately , and the right is heavier. The<unk> s are part of the lower respiratory tract that begins at the trachea and branches into the bronchi and bronchioles, and which receive air breathed in via the conducting zone. The conducting zone ends at the terminal bronchioles. These divide into the respiratory bronchioles of the respiratory zone which divide into alveolar ducts that give rise to the alveolar sacs that contain the alveoli, where gas exchange takes place. Alveoli are also sparsely present on the walls of the respiratory bronchioles and alveolar ducts. Together, the<unk> s contain approximately of airways and 300 to 500 million alveoli. Each<unk>  is enclosed within a pleural sac of two membranes called pleurae; the membranes are separated by a film of pleural fluid, which allows the inner and outer membranes to slide over each other whilst breathing takes place, without much friction. The inner pleura also divides each<unk>  into sections called lobes. The right<unk>  has three lobes and the left has two. The lobes are further divided into bronchopulmonary segments and pulmonary lobules. The<unk> s have a unique blood supply, receiving deoxygenated blood from the heart in the pulmonary circulation for the purposes of receiving oxygen and releasing carbon dioxide, and a separate supply of oxygenated blood to the tissue of the<unk> s, in the bronchial circulation. The tissue of the<unk> s can be affected by a number of respiratory diseases, including pneumonia and<unk>  cancer. Chronic obstructive pulmonary disease includes chronic bronchitis and emphysema, and can be related to smoking or exposure to harmful substances. A number of occupational<unk>  diseases can be caused by substances such as coal dust, asbestos fibres, and crystalline silica dust. Diseases such as bronchitis can also affect the respiratory tract. Medical terms related to the<unk>  often begin with pulmo-, from the Latin pulmonarius (of the<unk> s) as in pulmonology, or with pneumo- (from Greek πνεύμων "lung") as in pneumonia. In embryonic development, the<unk> s begin to develop as an outpouching of the foregut, a tube which goes on to form the upper part of the digestive system. When the<unk> s are formed the fetus is held in the fluid-filled amniotic sac and so they do not function to breathe. Blood is also diverted from the<unk> s through the ductus arteriosus. At birth however, air begins to pass through the<unk> s, and the diversionary duct closes, so that the<unk> s can begin to respire. The<unk> s only fully develop in early childhood.
Lung
Kidney
Liver
Heart
Brain
Breast
Central nervous system
Peripheral nervous system