(3.36)

Before looking at the lungs, it's important to understand the structures that contain them. The thorax, the two pleural cavities that lie within it, and the structures that produce respiratory movement are shown in Tape 3 of this Atlas.

Seen from in front, the lungs are here. Seen from behind, they're here. Seen from either side, the lung is here. A large part of each lung lies behind the heart.

The lung extends from the ribs in front, to the ribs behind, and from the dome of the pleural cavity, down to the diaphragm. With each breath in, and each breath out there's an increase and a decrease in the volume of the lungs.

Here are the lungs by themselves, seen from in front. They're being inflated with air so as to maintain their shape. Here's the trachea, here are the two principal bronchi. This is the apex of each lung, this is the base. The space between the two lungs is occupied by the heart and great blood vessels.

Each lung has three surfaces. The medial or mediastinal surface is marked by the hilum where the bronchi and blood vessels enter; the concave inferior suface rests on the diaphragm; and the large convex costal surface faces the rib cage.

Each lung is divided by deep fissures into lobes. The right lung has a superior lobe, a middle lobe, and an inferior lobe. The left lung has only a superior and an inferior lobe.

Here are the lungs seen from behind. The fissures that separate the lobes extend all the way round onto the mediastinal surfaces. The space between the posterior parts of the lungs is occupied by the vertebral bodies.

To see the lungs in a more intact setting we'll look at a dissection in which the anterior chest wall has been removed. Again the lungs are being inflated with air to keep them expanded. The surface of these lungs is discolored by trapped carbon particles that come from a lifetime's exposure to smoke.

The two lungs are separated by the mediastinum, the midline partition that's occupied largely by the heart and great vessels, and also by fat and lymph nodes, and in childhood by the thymus gland.

The surface of the lung is formed by the visceral pleura. The visceral pleura covers the lung completely, except here at the hilum, where it becomes continuous with the parietal pleura.

Taking a close look at the edge of the lung we can see the alveoli, the tiny air spaces where gas exchange occurs. The'yre opening progressively as we slowly inflate the lung.

Here we're letting the lung collapse, so that we can see the two separate pleural surfaces, parietal and visceral. The two surfaces are normally in close sliding contact with nothing between them except a film of serous fluid.