RESPIRA TION 



209 



surface of the lungs of a man is about 100 square metres (sixty 

 times greater than the area of the skin), of which, perhaps, 75 square 

 metres are occupied by capillaries. The average thickness of this 

 immense sheet of blood has been reckoned to be equal to the diameter 

 of a red blood-corpuscle, or, say, 8 t*.. This would give 600 c.c. 

 (630 grm.) as the quantity of blood in the lungs, which is probably 

 somewhat too low an estimate. 



If we take the pulmonary circulation-time as 13 seconds (p. 125), 



and the quantity of blood in the lungs as 700 grm., then 



= 194 kilos of blood will pass through the lungs in an hour, or 

 4,656 kilos (say, 4,400 litres) in twenty-four hours. This would fill 

 a cubical tank in which the man could almost stand upright with the 

 lid closed. 



Mechanical Phenomena of Respiration. 



The lungs are enclosed in an air-tight box, the thorax ; or it 

 may be said with equal truth that they form part of the wall 

 of the thoracic cavity, and the part which has by far the greatest 

 capacity of adjustment. The alveolar surface of the lungs is in 

 contact with the air. The pleura, which covers their internal 

 surface, is reflected over the chest-walls and diaphragm, so as to 

 form two lateral sacs, the pleural cavities. In health these are 

 almost obliterated, and the visceral and parietal pleurae, 

 separated and lubricated by a few drops of lymph, glide on each 

 other with every movement of respiration. But in disease the 

 pleural cavities may be filled and their walls widely separated by 

 exudation, as in pleurisy, or by blood, as in rupture of an aneurism, 

 or by air in the condition known as pneumo- thorax. Between 

 the two pleural sacs lies a mesial space, the mediastinum, 

 commonly divided into an anterior mediastinum in front of the 

 heart, and a posterior mediastinum behind it. The pleural and 

 pericardial sacs and the mediastinum constitute together the 

 thoracic cavity. The external surface of the chest-wall and the 

 alveolar surface of the lungs are subjected to the pressure of the 

 atmosphere, to which the pressure in the thoracic cavity (intra- 

 thoracic pressure) would be exactly equal if its boundaries were 

 perfectly yielding. But in reality the in tra- thoracic pressure is 

 always normally something less than this. For even the lungs, 

 the least rigid part of the boundary, oppose a certain resistance 

 to distension, and so hold off, as it were, from the thoracic cavity 

 a portion of the alveolar pressure ; and in any given position of 

 the chest the intra-thoracic pressure is equal to the atmospheric 

 pressure minus this elastic tension of the lungs. 



The object of the respiratory movements is the renewal of the 

 air in contact with the alveolar membrane in other words, the 

 ventilation of the lungs. Two main methods are followed by 

 sanitary engineers in the ventilation of buildings : they force air 



