The Respiratory System 153 



visceral pleura is adherent to the lungs. The nearly obliterated space 

 between these pleura is the pleural cavity. Since the two membranes 

 are kept moist by secretions of mucus, they normally move smoothly 

 against one another. Pleurisy results when they become inflamed and 

 friction occurs between them. Medially the walls of the pleura press 

 against each other to form the mediastinal septa. In the region of the 

 heart, the pleura separate from one another, forming a cavity, the 

 mediastinum in which are located the heart, many blood vessels, and 

 other visceral structures. 



The lung tissue is very elastic and spongy, owing to the numerous 

 small air spaces within it. The lung itself consists of numerous blood 

 vessels, nerves, bronchioles, with their terminal air sacs, and much elastic 

 connective tissue. The two lungs contain approximately 700,000,000 

 or more alveoli, each having a diameter of about 0.1 mm. This presents 

 an enormous area for respiratory exchange. 



The wall of each alveolus consists of an extensive network of 

 anastomosing capillaries interspersed with elastic and reticular connective 

 tissue fibers. This very intimate contact between the thin-walled 

 capillaries and the inspired air permits free exchange of gases. 



THE PHYSIOLOGY OF RESPIRATION 



All terrestrial animals live in a sea of air; it permeates nearly 

 all spaces, and is essential to most animals for the utilization of food for 

 the production of energy. 



The greater part of the atmosphere is actually made up of nitrogen 

 which plays no essential role in respiration. It forms 78 per cent of 

 the air while oxygen forms but 21 per cent. The remaining 1 per cent 

 of air is made up of carbon dioxide, water vapor, and rare gases. The 

 amount of dissolved oxygen in water varies, depending upon rate of 

 flow, depth, and other factors, but is much lower than that of air. 

 Aquatic animals, therefore, tend to be more sluggish than do terrestrial 

 animals. 



The need for oxygen varies within the dififerent animal groups and 

 even in the tissues within the individual. Even in man, there are 

 anaerobic processes, that is, energy production without the use of oxygen. 

 This is especially well illustrated by muscle activity in which an oxygen 

 debt can be acquired. Some animals, especially intestinal parasites, 

 are able to live without free oxygen. They utilize glycogen for the pro- 

 duction of energy in a manner similar to that noted previously for 



