Human lung
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The human lungs are the human organs of respiration.
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Trachea branches into bronchi |
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 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 acetyl choline which acts on the muscarinic receptors and causes bronchoconstriction
- many other non-autonomic nervous and biochemical stimuli including, for example, carbon dioxide.
Details
Humans have two lungs, with the left being divided into 2 lobes and the right into 3 lobes. These contain approximately 1500 miles (2,400 km) of airways and 300 million alveoli, having a total surface area of about 140 m2 in adults — roughly the same area as a tennis court. Furthermore, if all of the capillaires that surround the alveoli were unwound and laid end to end, they would extend for about 620 miles.
Total lung capacity 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. It normally ranges between 4,000 and 6,000 cm3. 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 spends 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 adult resting breathing pattern has a breath rate of 10-20 breaths per minute with 1/3 of the breath time in inspiration.
Development
The larynx, trachea, bronchi and lungs begin to form during the fourth week of embryonic development. At this time, the respiratory diverticulum (lung bud) appears ventrally to the caudal portion of the foregut. The location of the diverticulum along the gut tube is directed by various signals from the surrounding mesenchyme, including fibroblast growth factors. As the lung bud grows, its distal end enlarges to form the tracheal bud. At the same time the future trachea separates from the foregut through the formation of tracheoesophageal ridges, which fuse to form the tracheoesophageal septum.The tracheal bud divides into two primary bronchial buds. During the fifth week of development, the bronchial buds enlarge to form right and left main bronchi. These continue to develop into secondary and tertiary bronchi.
The maturation of the lungs occurs in several phases:
- Pseudoglandular period
- *This stage occurs during weeks 6 to 16. The developing lung resembles an exocrine gland at this time. By the end of this period, all of the major lung elements, except those required for gas exchange (e.g. alveoli), have appeared. Respiration is not possible during this phase, and fetuses born during this period are unable to survive.
- Canalicular Period
- *Lasting from weeks 16 to 26, the lumens of the bronchi enlarge and lung tissue becomes highly vascularized during the canalicular period. By week 24, respiratory bronchioles and alveolar ducts have developed from the terminal bronchioles. Respiration is possible towards the end of this period, but few fetuses born during this time will survive.
- Terminal Saccular Period
- *The important blood-air barrier is established during the terminal saccular period. This period lasts from week 26 to birth. Specialized cells of the respiratory epithelium appear at this time, including type I alveolar cells across which gas exchange occurs, and type II alveolar cells which secrete pulmonary surfactant. This surfactant is important in reducing the surface tension at the air-alveolar surface, allowing expansion of the terminal saccules.
- Alveolar period
- *This stage lasts from birth to 8 years of age. During this stage the terminal saccules, alveolar ducts, and alveoli increase in number.
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.
References
- Moore, Keith L. and T.V.N. Persaud. The Developing Human: Clinically Oriented Embryology, 7th ed. (2003). ISBN 0-7216-9412-8
- Sadler, T.W. Langman's Medical Embryology, 9th ed. (2004). ISBN 0-7817-4310-9
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