Two sponges that begin as a gut outpouching
Lungs swap gases for many animals, pulling oxygen from the air into the blood and returning carbon dioxide. A human pair weighs about 1.3 kilograms, and they are not twins: the right lung is bigger, with three lobes, while the left makes room for the heart and has only two.
In people the lungs fill the rib cage on either side of the heart. Each is roughly a cone, with a rounded tip reaching just above the first rib into the base of the neck and a broad hollow base resting on the dome of the diaphragm. Two thin membranes, the pleurae, wrap them: one lines the ribs, the other coats the lung, and a film of lubricating fluid between them lets the surfaces slide. Airways, blood vessels and lymph nodes enter at a central recess called the hilum.
Folds of the inner pleura cut each lung into lobes, which divide into segments, each fed by its own bronchus and artery, and then into lobules. The right lung has an upper, middle and lower lobe separated by a horizontal and an oblique fissure; the left has two lobes plus the lingula, a small tongue-shaped flap that mirrors the right’s middle lobe. The left also carries a cardiac notch and a deeper hollow for the heart. Weights vary widely: the right lung ranges from 155 to 720 grams in men, the left from 110 to 675 grams.
The branching airways end in alveoli, the tissue where exchange happens, and elastin fibres give the lungs the stretch and recoil that breathing demands. The airways are lined with ciliated cells and mucus-making goblet cells. Lungs begin in the embryo as a pouch off the foregut, and because the fetus floats in amniotic fluid they do not breathe; blood bypasses them through the ductus arteriosus, which closes after birth once air flows in.
Pneumonia and lung cancer attack the tissue, and chronic obstructive pulmonary disease and emphysema can follow smoking or exposure to harmful substances. Lung-related medical words start with pulmo- from Latin or pneumo- from Greek.
Source: Lung
