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 () of airways and 300 to 500 million alveoli, having a total surface area of about () in adults — roughly the same area as one side of a tennis court. Furthermore, if all of the capillaries that surround the alveoli were unwound and laid end to end, they would extend for about (). 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 ofgas 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 lungparenchymais strictly used to refer solely toalveolar tissue with respiratory bronchioles, alveolar ducts and terminal bronchioles. However, it often includes any form of lung tissue, also including bronchioles, bronchi, blood vessels and lung interstitium.

Physiology

Regarding lung volumes, total lung capacity (TLC) includes inspiratory reserve volume, tidal volume, expiratory reserve volume, and residual volume. The total lung capacity depends on the person's age, height, weight, sex, and normally ranges between 4,000 and 6,000 cm³ (4 to 6 L). For example, females tend to have a 20–25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. Smokers have a lower capacity than nonsmokers. Lung capacity is also affected by altitude. People who are born and live at sea level will have a smaller lung capacity than people who spend their lives at a high altitude. In addition to the total lung capacity, one also measures the tidal volume, the volume breathed in with an average breath, which is about 500 cm³. For a detailed discussion of the various lung volumes, see the article on lung volumes.

Typical resting adult respiratory rates are 10–20 breaths per minute with 1/3 of the breath time in inhalation.

Human lungs are to a certain extent 'overbuilt' and have a tremendous reserve volume as compared to the oxygen exchange requirements when at rest. This is the reason that individuals can smoke for years without having a noticeable decrease in lung function while still or moving slowly; in situations like these only a small portion of the lungs are actually perfused with blood for gas exchange. As oxygen requirements increase due to exercise, a greater volume of the lungs is perfused, allowing the body to reach its CO₂/O₂ exchange requirements.

An average human breathes around 11,000 litres of air (21% of which consists of oxygen) in one day.

Mucus-secreting cells, cells with tiny hairs called cilia or Cilium, and cells from the immune syste