Biology of Man 491 



place through the integument (skin) (Fig. 228) which has been pre- 

 viously described. However, it is estimated that the total inner surface 

 of the lungs is about 90 square meters or more than one hundred times 

 the total skin area of the body. The vital capacity of the average adult 

 lungs represents the maximum volume which can be exchanged in a 

 single respiration. This is about 4,000 c.c. (eight pints), although it 

 varies with different individuals and conditions. The following phe- 

 nomena occur in the lungs : ( 1 ) loss of about 5 per cent of the oxygen 

 from the inhaled air, (2) gain of about 4 per cent of carbon dioxide, 

 (3) gain of about 1 per cent of nitrogen, (4) saturation of the expired 

 air with moisture (about 1 pint daily), (5) warming the expired air to 

 nearly that of the blood (98.6° F.), thereby losing body heat, (6) trans- 

 fer of oxygen and carbon dioxide through the thin-walled air sacs of 

 the lungs (Figs. 234, 235, 243, and 244). 



Respiration involves ( 1 ) the exchange of gases between the respiratory 

 membrane and the capillaries of the pulmonary circulation within the 

 lungs, known as external respiration and (2) the exchange of gases 

 between the capillaries of the systemic circulation and the body tissue 

 known as internal respiration. 



The respiratory system is composed of the nose, pharynx, larynx (voice 

 box or "Adam's apple"), trachea (windpipe), bronchi, and lungs (Figs. 

 234, 235, 243 and, 244). The nose is divided by a partition (septum) 

 to form two wedge-shaped cavities which are lined by a highly vascular, 

 mucous membrane, the upper layer of which is ciliated. Sinuses in the 

 bones are associated with the nasal cavities so that inflammations may 

 spread to the sinuses easily. The lateral surface of each nasal cavity 

 has three light, spongy, bony projections called conchae to make the 

 upper part of the nasal passages very narrow. The nose has the follow- 

 ing functions: (1) to act as a sounding board for the voice (organ of 

 phonation), (2) to give warmth and rnoisture to the inhaled air, (3) to 

 remove dust and other foreign materials by hair, cilia, and mucus (se- 

 creted by goblet cells), and (4) to detect odors by means of the olfactory 

 nerve endings in the upper passages. 



The pharynx is a common cavity (Fig. 234) which connects the nasal 

 cavities with the larynx as well as the mouth with the esophagus. Be- 

 cause of this dual function it is impossible to inhale air and swallow 

 food at the same time. 



The larynx is a cartilaginous box which forms the prominence in the 

 midline of the front part of the neck. Within the laryngeal cavity are 

 two folds of mucous membrane extending from front to back but not 



