82 HUMAN PHYSIOLOGY 



After expansion of the thorax the atmospheric pressure 

 stretches the lung-sac and enlarges it. 



Even in the expiratory position of the thorax, the walls 

 of the lungs are stretched. If, in a dead body, the thoracic 

 wall is opened so that the air can enter the pleural cavity, 

 the lungs withdraw from the thoracic walls. If, previous to- 

 opening the thorax, a manometer is connected with the 

 trachea, this will, on opening the thorax, indicate the 

 pressure which the tension of the pulmonary wall exerts. 

 In .the pleural cavity, in the expiratory position, there is a 

 corresponding negative pressure of about 3-5 mm Hg. 

 During ordinary inspiration this is increased by about 9 mm ; 

 during forced inspiration, by 3040 mm. 



2. THE VARIATION IN PRESSURE OF PULMONARY 

 AIR DURING RESPIRATION ; RESPIRATORY CA- 

 PACITY 



During the inspiratory dilation, the pressure of the air in 

 the lungs sinks, while during expiration it rises. This 

 decrease in pressure causes the external air to rush in ; the 

 increase in pressure during expiration forces the air out of 

 the lungs. These respiratory changes in pressure amount, 

 in quiet breathing, to 1-3 mm Hg; in forced respiration 

 they are greater. 



The respiratory volume is determined by exhaling into an instru- 

 ment used for measuring the volume of gases (gasometer, spirom- 

 eter) or by letting the inhaled or exhaled air pass through a, 

 gas-meter. 



A part of the inspired air does no service in the gas-exchange, 

 as it does not reach the alveoli but remains in the air-passage 

 (trachea, bronchi, nose). The size of this "dead space" is 

 100-150 cc. 



Tidal air is the air inhaled and exhaled during quiet- 

 respiration ; in the adult male it is about 500 cc. 



Complcmcntal air is the air which can be inhaled, in 

 excess of the tidal air, by forced inspiration ; it is about 

 2500 cc. 



Supplemental air is the air which can be expelled, besides, 



