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This is the latest accepted revision, reviewed on 8 March 2019. The lungs are the primary organs of the respiratory system in humans and many other animals including a few fish and some snails. Humans have two lungs, a right lung and a left lung. They are situated within the thoracic cavity of the chest.

The right lung is bigger than the left, which shares space in the chest with the heart. The lungs together weigh approximately 1. The tissue of the lungs can be affected by a number of diseases, including pneumonia and lung cancer. In embryonic development, the lungs begin to develop as an outpouching of the foregut, a tube which goes on to form the upper part of the digestive system.

The lungs are located in the chest on either side of the heart in the rib cage. They are conical in shape with a narrow rounded apex at the top, and a broad concave base that rests on the convex surface of the diaphragm. Both lungs have a central recession called the hilum at the root of the lung, where the blood vessels and airways pass into the lungs. The lungs are surrounded by the pulmonary pleurae. The main or primary bronchi enter the lungs at the hilum and initially branch into secondary bronchi also known as lobar bronchi that supply air to each lobe of the lung. The right lung has both more lobes and segments than the left. It is divided into three lobes, an upper, middle, and a lower, by two fissures, one oblique and one horizontal.

The upper, horizontal fissure, separates the upper from the middle lobe. The lower, oblique fissure, separates the lower from the middle and upper lobes, and is closely aligned with the oblique fissure in the left lung. The mediastinal surface of the right lung is indented by a number of nearby structures. The heart sits in an impression called the cardiac impression. The left lung is divided into two lobes, an upper and a lower, by the oblique fissure, which extends from the costal to the mediastinal surface of the lung both above and below the hilum.