The human lungs are paired organs, located on either side of the heart and occupying a large portion of the chest cavity from the collarbone to the diaphragm. Air enters the body through a series of passages, beginning with the nose or mouth. It travels to the chest cavity through the trachea, which divides into two bronchi, each of which enters a lung. The bronchi divide and subdivide into a network of countless tubules. The smallest tubules, or bronchioles, enter cup-shaped air sacs known as alveoli, which number about 700 million in both lungs. Each alveolus is surrounded by a net of capillaries. As blood flows through these vessels, carbon dioxide passes into the alveoli, and oxygen diffuses into the bloodstream. The capillaries are part of a vast network of pulmonary blood vessels that connect the lungs directly to the heart via the large pulmonary arteries and veins. The alveoli are clustered in groups, or lobules, and the lobules are clustered into lobes.
In humans, the left lung has two lobes; the right lung three. The lungs are covered by a thin membrane called the pleura. They are expanded and contracted (thereby inhaling and exhaling air) by the combined movement of the diaphragm and the rib cage, which is alternately raised (expansion) and lowered (contraction) by the chest muscles. In recent years, smoking has been found to cause severe and sometimes fatal diseases of the lung, such as cancer and emphysema. Pneumonia is an inflammation of the lung tissue caused by various agents or organisms such as viruses. Asthma, a hypersensitivity or allergic response to some stimuli, covers a range of severity and is characterized by bronchial spasms and difficult breathing. See respiration.
Distention of blood vessels in the lungs and filling of the pulmonary alveoli with blood. It results from infection, hypertension, or inadequate heart function (e.g., left-sided heart failure). Congestion seriously impairs gas exchange, leading to breathing difficulty, bloody discharge in sputum, and bluish skin tint.
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Malignant tumour of the lung. Four major types (squamous-cell carcinoma, adenocarcinoma, large-cell carcinoma, and small-cell carcinoma) have roughly equal prevalence. Most cases are due to long-term cigarette smoking. Heavy smoking and starting smoking earlier in life increase the risk. Passive inhalation (“secondhand smoke”) is linked to lung cancer in nonsmokers. Other risk factors include exposure to radon or asbestos. Symptoms, including coughing (sometimes with blood), chest pain, and shortness of breath, seldom appear until lung cancer is advanced, when treatment with surgery, chemotherapy, and radiation or some combination of the three is less effective. Most patients die within a year of diagnosis.
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Either of two light, spongy, elastic organs in the chest, used for breathing. Each is enclosed in a membrane (pleura). Contraction of the diaphragm and the muscles between the ribs draw air into the lungs through the trachea, which splits into two primary bronchi, one per lung. Each bronchus branches into secondary bronchi (one per lobe of lung), tertiary bronchi (one per segment of lung), and many bronchioles leading to the pulmonary alveoli. There oxygen in the inspired gas is exchanged for carbon dioxide from the blood in the surrounding capillaries. Adequate tissue oxygen supply depends on sufficient distribution of air (ventilation) and blood (perfusion) in the lungs. Lung injuries or diseases (e.g., emphysema, embolism, pneumonia) can affect either or both.
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