The human lungs are the organs of respiration in humans. Humans have two lungs, with the left being divided into two lobes and the right into three lobes. Together, the lungs contain approximately 2400 km (1500 mi) of airways and 300 to 500 million alveoli, having a total surface area of about 70 m2 (8,4 x 8,4 m) in adults — roughly the same area as one side of a tennis court.[1] Furthermore, if all of the capillaries that surround the alveoli were unwound and laid end to end, they would extend for about 992 km (620 mi). Each lung weighs 2.5 pounds, therefore making the entire organ about 5 pounds.

The conducting zone contains the trachea, the bronchi, the bronchioles, and the terminal bronchioles The respiratory zone contains the respiratory bronchioles, the alveolar ducts, and the alveoli. The conducting zone and the respiratory stuffers (but not the alveoli) are made up of airways. The conducting zone has no gas exchange with the blood, and is reinforced with cartilage in order to hold open the airways. The conducting zone warms the air to 37 degrees Celsius and humidifies the air. It also cleanses the air by removing particles via cilia located on the walls of all the passageways. The lungs are surrounded by the rib cage. The respiratory zone is the site of gas exchange with blood.

The sympathetic nervous system via noradrenaline acting on the beta receptors causes bronchodilation.
The parasympathetic nervous system via acetylcholine, which acts on the M-1 muscarinic receptors, maintains the resting tone of the bronchiolar smooth muscle. This action is related, although considered distinct from bronchoconstriction.
Many other non-autonomic nervous and biochemical stimuli, including carbon dioxide and oxygen, are also involved in the regulation process.
The pleural cavity is the potential space between the parietal pleura, lining the inner wall of the thoracic cage, and the visceral pleura lining the lungs.

The lung parenchyma is strictly used to refer solely to alveolar tissue with respiratory bronchioles, alveolar ducts and terminal bronchioles.[2] However, it often includes any form of lung tissue, also including bronchioles, bronchi, blood vessels and lung interstitium.[2]

Diseases
Diseases of the human lung belong to respiratory diseases. The following is a list of important medical conditions involving the lung. Many of these are caused or worsened by smoking. Lung disorders are generally handled by general practitioners, although patients may be referred to a pulmonologist.

Lung cancer
Emphysema is an enlargement of the air spaces in the lung, making it hard to breathe.
Asthma is an immunological disease which causes the bronchioles to narrow by inflammation and spasm of the lining of the airway wall.
Cystic fibrosis is a hereditary disease which causes the lung to produce abnormally viscous mucus.
A pulmonary embolism occurs when a blood clot obstructs an artery leading to the lung.
Tuberculosis is a transmittable bacterial infection of the lung, the most common infectious disease today.
Pneumonia is an infection of the lung, caused by bacteria, viruses or fungi.
Pneumoconiosis, an occupational lung disease caused by the inhalation of dust.
Bronchitis is an inflammation of the bronchi.
A collapsed lung (pneumothorax) can occur when one or both walls of the pleural cavity are penetrated by injury, allowing air to enter.
In pulmonary edema fluid from the capillaries enters the alveoli. This can be caused by weakness of the left side of the heart (resulting in a blood holdup in the lung), altitude sickness, or rarely inhaling toxic gases.
Lung pinprick condition is a hereditary disease which results in decreased lung capacity and occasional shortness of breath.
Transplantation now allows for a person to have a single lung transplant, a double-lung transplant, or a transplant of both the heart and lungs.