The lungs are the two organs involved in the supply of oxygen to the body and the elimination of carbon dioxide from the blood, or gas exchange between air and blood (process known by the name of gas exchange). Located in the chest cavity, the lungs are surrounded by a serous membrane, the pleura, which is essential for the performance of their function. The lungs are separated by a space between the spine and the sternum, the mediastinum, which inside includes the heart, the oesophagus, trachea, bronchi, thymus gland, and great vessels. Each of the two lungs has, at the upper end, an apex that extends upward to the base of the neck and, it rests on the diaphragmatic muscle at the lower end.
The main task of the lungs is to get the blood load of carbon dioxide and waste products from the peripheral circulation and to clean it up: once cleaned, blood is then sent to the heart from where it is delivered to organs and tissues. The lungs have a high degree of elasticity, which promotes the expulsion of air during exhalation. As in the case of kidneys, only one lung is sufficient to ensure the functioning of the whole process.
What are the lungs?
The right lung comprises three lobes (upper, middle and lower) separated by a slit and a horizontal oblique, while the left lung contains two lobes (upper and lower) separated by a slit oblique. The lobes are further divided into bronchopulmonary segments, each of which is served by a segmental bronchus; the segmental bronchi in turn are divided into gradually smaller and smaller structures until they reach the alveoli, which are responsible for gas exchange between the air and the blood.
The spaces within which the lungs are located are called pulmonary lodges; these are bounded at the lower end by the diaphragm, by upper structures that are located in the upper chest (as the brachial plexus), on the side by the rib cage and the intercostal muscles and internally by the mediastinum.
The lungs are enveloped in a serous membrane, the pleura, formed by two sheets called the pleural layers: the parietal pleural leaflet, which covers the lungs externally and separates them from the chest wall; and the package visceral pleural layer, which adheres to the inner surface of the lung. The two pleural layers are in continuous contact with each other: to ensure and facilitate the sliding, there is a thin film of pleural fluid, which is located in the space between the two layers. Its presence is essential to allow the lungs to follow the movements of the muscles to which they subscribe during breathing. The two sheets delimit a virtual cavity, the pleural cavity, which does not communicate with the exterior nor with other organs and within which a negative pressure is formed, that allows the lungs to expand as we inhale.
The shape of the lungs resembles that of two cones. Their size varies according to the development of the organism: in adult males they generally measure at most about 25 centimetres in length, while the average diameter is about 16 cm. The left lung is slightly smaller than the right. In women these values are usually lower. The weight of the lungs varies greatly depending on the stage of physical development: in a full-term pregnancy fetus the lung weighs 60-65 grams, while in an infant it reaches 80-100 grams. In adulthood the right lung in humans can weigh around 680 grams, while the left lung around 620 grams (in women, the weight varies with size).
Lung capacity – i.e. the amount of air that may be contained in the lungs – varies from person to person and according to the phase of respiration: in an ordinary inhalation phase the lungs may contain 3400-3700 cm3 of air while forced inhalation can reach 5000-6000 cm3.