Human lung

The human lungs are the human organs of respiration.

Humans have two lungs, with the left being divided into two lobes and the right into three lobes. Together, the lungs contain approximately 1500 miles (2,400 km) of airways and 300 to 500 million alveoli, having a total surface area of about 75 m² in adults — roughly the same area as 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 620 miles.

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Organization

• 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 zone (but not the alveoli) are made up of airways.

The conducting zone has no gas exchange with the blood, and is reinforced with cartilage and smooth muscle, which are very strong. Smooth muscle has variable resistance to air flow. The conducting zone warms the air to 37 degrees Celsius and humidifies the air. It also cleanses the air by removing particles.

The respiratory zone is the site of gas exchange with blood.

The smooth muscle tone in bronchioles, and therefore bronchiolar diameter, is controlled by:

• the sympathetic nervous system via noradrenaline acting on the beta receptors and causes bronchodilation
• the parasympathetic nervous system via acetylcholine which acts on the muscarinic receptors and causes bronchoconstriction
• many other non-autonomic nervous and biochemical stimuli including, for example, carbon dioxide.

The intrapleural space is the potential space between the pleura lining the inner wall of the thoracic cage and the pleura lining the lungs.

Physiology

Total lung capacity (TLC) includes inspiratory reserve volume, tidal volume, expiratory reserve volume, and residual volume.^[2] 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, 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 inspiration.

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.

Diseases

The following is a list of important medical conditions involving the lung. Many of these are caused or worsened by smoking.

• 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.
• 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.

Modification of substances

The lungs convert angiotensin I to angiotensin II. In addition, they remove several blood-bourne substances, e.g. PGE₁, PGE₂, PGF_2α, leukotrienes, serotonin, bradykinin. ^[3]

See also

• Development of human lung

References