In human beings these are the main respiratory organs. These are a pair of conical organs enveloped by a serious membrane – the pleura. The lungs occupy a greater part of the thoracic cavity. The two lungs are separated from each other by the mediastinum. Each lung is divided by deep fissures into lobes. The right lung has three lobes – the upper, middle and lower while the left lung has two lobes the upper and the lower. The lung is also described as having a mediastinal and a costal surface, an apex and a base.
The outer (costal) surface is in contact with the wall of the pleural cavity while the medial surface of the lung is applied to the mediastinum. The main feature here is the presence of hilum where the main bronchus and pulmonary artery enter and the pulmonary veins leave the lung. Lymph nodes are present at the hilum which may get enlarged by diseases.
The apex of the lungs rises into the root of the neck for about an inch above the clavicle. The base is concave and is related to the upper part of the diaphragm.
Structurally the lungs are spongy and each lobe of the lung is made up of a number of small lobules consisting of alveoli and “bronchioles which form larger bronchi. In addition to this there is a framework of interstitial or fibrous tissue in which are found the blood vessels and lymphatics.
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The pleura:
This is a serious membrane and is made up of two layers – the parietal and the visceral. The visceral layer forms the outer cover of the lung which it encloses completely except at the region of the hilum. The parietal layer lines the interior of the chest wall and upper surface of the diaphragm. Both the layers are smooth and shiny and are always kept moist by a small amount of serous fluid acting as a lubricant. This helps the two surfaces to glide smoothly over each other during the process of breathing.