The lungs are the organs in which gas diffusion takes place through the extremely thin and delicate lung tissues (see Fig. 14-2). The two lungs are set side by side in the thoracic (chest) cavity. Between them are the heart, the great blood vessels, and other organs of the mediastinum, the space between the lungs, including the esophagus, trachea, and lymph nodes. On its medial side, the left lung has an indentation that accommodates the heart. The right lung is subdivided by fissures into three lobes; the left lung is divided into two lobes. Each lobe is then further subdivided into segments and then lobules. These subdivisions correspond to subdivisions of the bronchi as they branch throughout the lungs. Each primary bronchus enters the lung at the hilum and immediately subdivides. The right bronchus divides into three secondary bronchi, each of which enters one of the three lobes of the right lung. The left bronchus gives rise to two secondary bronchi, which enter the two lobes of the left lung. Because the bronchial subdivisions resemble the branches of a tree, they have been given the common name bronchial tree. The bronchi subdivide again and again, becoming progressively smaller as they branch through lung tissue. The smallest of these conducting tubes are called bronchioles. The bronchi contain small bits of cartilage, which give firmness to their walls and hold the passageways open so that air can pass in and out easily. As the bronchi become smaller, however, the cartilage decreases in amount. In the bronchioles, there is no cartilage at all; what remains is mostly smooth muscle, which is under the control of the autonomic (involuntary) nervous system.
Figure 14-2 The respiratory system. (A) Overview. (B) Enlarged section of lung tissue showing the relationship between the alveoli (air sacs) of the lungs and the blood capillaries. (C) A transverse section through the lungs.
The Alveoli At the end of the terminal bronchioles, the smallest subdivisions of the bronchial tree, there are clusters of tiny air sacs in which most gas exchange takes place. These sacs are known as alveoli (sing. alveolus) (see Fig. 14-2). The wall of each alveolus is made of a single-cell layer of squamous (flat) epithelium. This thin wall provides easy passage for the gases entering and leaving the blood as the blood circulates through the millions of tiny capillaries covering the alveoli. Certain cells in the alveolar wall produce surfactant, a substance that reduces the surface tension (“pull”) of the fluids that line the alveoli. This surface action prevents collapse of the alveoli and eases expansion of the lungs. There are about 300 million alveoli in the human lungs. The resulting surface area in contact with gases approximates 60 square meters (some books say even more). This area is equivalent, as an example, to the floor surface of a classroom that measures about 24 by 24 feet. As with many other systems in the body, there is great functional reserve; we have about three times as much lung tissue as is minimally necessary to sustain life. Because of the many air spaces, the lung is light in weight; normally, a piece of lung tissue dropped into a glass of water will float. Figure 14-6 shows a microscopic view of lung tissue.
Figure 14-6 Lung tissue viewed through a microscope.
The pulmonary circuit brings blood to and from the lungs. In the lungs, blood passes through the capillaries around the alveoli, where gas exchange takes place.
The Lung Cavities and Pleura
The lungs occupy a considerable portion of the thoracic cavity, which is separated from the abdominal cavity by the muscular partition known as the diaphragm. A continuous doubled sac, the pleura, covers each lung. The two layers of the pleura are named according to location. The portion of the pleura that is attached to the chest wall is the parietal pleura, and the portion that is attached to the surface of the lung is called the visceral pleura. Each closed sac completely surrounds the lung, except in the place where the bronchus and blood vessels enter the lung, a region known as the root of the lung. Between the two layers of the pleura is the pleural space containing a thin film of fluid that lubricates the membranes. The effect of this fluid is the same as between two flat pieces of glass joined by a film of water; that is, the surfaces slide easily on each other but strongly resist separation. Thus, the lungs are able to move and enlarge effortlessly in response to changes in the thoracic volume that occur during breathing.