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Either of two cone-shaped spongy organs of respiration contained within the pleural cavity of the thorax.
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LUNGS ; SEE: alveolus for illus.

The lungs are connected with the pharynx through the trachea and larynx. The base of each lung rests on the diaphragm, and each lung apex rises from 2.5 to 5 cm above the sternal end of the first rib, the collarbone (clavicle), supported by its attachment to the hilum or root structures. The lungs include the lobes, lobules, bronchi, bronchioles, alveoli (air sacs), and pleural covering.

The right lung has three lobes and the left two. The lungs contain 300,000,000 alveoli, and their respiratory surface is about 70 sq m. Respirations per minute are 12 to 20 in an adult. The total capacity of the lung varies from 3.6 to 9.4 L in men and 2.5 to 6.9 L in women.

The left lung has an indentation (the cardiac depression), for the normal placement of the heart. Behind this is the hilum, through which the blood vessels, lymphatics, and bronchi enter and leave the lung.

Air travels from the nasal passages to the pharynx, larynx, and trachea. Two primary bronchi, one on each side, extend from the trachea. The primary bronchi divide into secondary bronchi, one for each of five lobes. These further divide into a great number of smaller bronchioles. The pattern of distribution of these into the segments of each lobe is important in pulmonary and thoracic surgery. There are about 10 bronchopulmonary segments in the right lung and 8 in the left, the actual number varying. There are 50 to 80 terminal bronchioles in each lobe. Each of these divides into two respiratory bronchioles, which in turn divide to form 2 to 11 alveolar ducts, from which the alveolar sacs and alveoli arise.

In the alveolus, blood and inspired air are separated only by the cells of the alveolus and of the pulmonary capillary. This respiratory membrane is thin (0.07 to 2.0 μm) and permits oxygen (O) to diffuse into the blood and carbon dioxide (CO2) to diffuse from the blood to the air.

The lungs are innervated by parasympathetic fibers via the vagus nerve and sympathetic fibers from the anterior and posterior pulmonary plexuses to the smooth muscle in the walls of the bronchial tree.

The bronchial arteries and veins circulate blood to the bronchial tree. The pulmonary arteries and veins circulate the blood involved in gas exchange.

The purpose of the lung is to bring air and blood into contact so that O can be added to the blood and CO2 removed. This is achieved by two pumping systems, one moving a gas, and the other a liquid. The blood and air are brought together so closely that only approx. 1 μm of tissue separates them. The volume of the pulmonary capillary circulation is 150 mL, but this is spread out over a surface area of approx. 750 sq ft (69.68 sq m). This capillary surface area surrounds 300 million air sacs (alveoli). The blood that is low in O but high in CO2 is in contact with the air that is high in O and low in CO2 for less than 1 second.
SEE: respiratory defense mechanism

Inspection: The examiner determines the respiratory rate by unobtrusively watching the patient's chest rise and fall and counting the number of breaths per minute. In adults, a normal respiratory rate at rest is about 12 breaths per minute. While counting the respiratory rate, the examiner can observe other breathing characteristics. Dyspneic patients breathe rapidly, often laboring to draw breath even when at rest. Retractions of the intercostal and supraclavicular spaces are visible during inspiration. Sleep apnea is characterized by episodes of stalled breathing followed by periods of respiratory compensation. Regular slow breathing is normal.

Palpation: In healthy people, the chest and lung transmit a vibration (fremitus) during speech. An abnormal fremitus may be felt in chronic obstructive lung diseases or obesity, in which the vibration is diminished, and in pneumonia, in which it is increased over the infected lobe.

Percussion: Tapping on the chest wall over healthy lung results in a hollow, resonant sound. The hollow character of the resonance is sometimes exaggerated in emphysematous lungs or in pneumothorax but muffled by pleural effusions or pulmonary consolidation.

Normal breath sounds: In healthy people, breath sounds (vesicular breath sounds) are produced by air passing in and out of the airways, are low-pitched, and have a frequency of 200 to 400 cycles per second (cps); frequency rarely exceeds 500 cps.

Bronchial and tracheal breath sounds: These sounds, higher-pitched and louder than vesicular breath sounds, are produced by air passing over the walls of the bronchi and trachea, and are normally heard only over the bronchi and trachea.

Amphoric and cavernous breathing: These two nearly identical sounds are loud, with a prolonged, hollow expiration. The pitch of amphoric breathing is slightly higher than that of the cavernous type and may be imitated by blowing over the mouth of an empty jar. It is heard in bronchiectatic cavities or pneumothorax when the opening to the lung is patulous, in the consolidation area near a large bronchus, and sometimes over a lung compressed by a moderate effusion.

Harsh inspiratory sounds are typical of stridor, a medical emergency. Expirations that are prolonged and musical are characteristic of wheezing.

Friction: This sound is produced by the rubbing together of roughened pleural surfaces. It may be heard in both inspiration and expiration. Friction often resembles crackle, but it is more superficial and localized than the latter and is not modified by cough or deep inspiration.

Metallic tinkling: A silvery bell-like sound heard at intervals over a hydropneumothorax or large cavity. Speaking, coughing, and deep breathing usually induce this sound. It must not be confused with a similar sound produced by liquids in the stomach.

Crackles: Abnormal bubbling sounds heard in air cells or bronchi.

Succussion-splash or hippocratic succussion: A splashing sound produced by the presence of air and liquid in the chest. It may be elicited by gently shaking the patient during auscultation. This sound nearly always indicates either a hydropneumothorax or a pyopneumothorax, but it has also been detected over very large cavities. The presence of air and liquid in the stomach produces similar sounds.

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