LUNGS IN MAN 111 



100,000 kilometres or two and a half times round the globe, 

 and their total surface 6300 square metres." 



In an ordinary healthy adult the heart contracts from 70 

 to 80 times a minute, but in infants and youths it is quicker, 

 and in old men much slower. In fact, the number of pulsations 

 per minute greatly diminishes throughout life. It has been 

 said that the work of the heart in a day is equivalent to that 

 done by an able-bodied labourer working hard^ for two hours. 

 Its muscles are the most hard- worked of the body : night and 

 day they have no rest. 



In man the lungs are not very big. Perhaps on an average 

 they equal in bulk, when collapsed, both closed fists. But tliey 

 expose an enormous area to the air which they breathe in, and 

 to the blood which circulates in their tissues. The object of all 

 breathing organs is to afford as much surface as possible so 

 as to facilitate the interchanges of gas. A lung consists of an 

 enormous number of little pockets or alveoli, of which there 

 are said to be in the human lungs some 725,000,000. Their 

 total surface has been calculated at from 90 to 95 square 

 metres (107-6 to 113-5 square yards), an area which would 

 carpet a large room of 30 feet by 36 feet. Since a suit of 

 clothes for an average man takes 3 to 3} square yards, the 

 internal surface of the two lungs is equal to that of some 

 thirty suits. Normally the total quantity of air which passes 

 in and out of the lungs of an adult at rest in 24 hours varies 

 from 400,000 to 680,000 cubic inches. 



A man can live some eight weeks without food and more 

 than a week without water, but only for a few minutes without 

 oxygen; and this is true of all warm-blooded vertebrates. 

 Lack of air induces death much quicker than lack of drink 

 or food. Diving birds as a rule only remain under water for 

 something less than a minute, and the most expert pearl- 

 fishery divers in the East must return to the surface after two 

 or three minutes. On the other hand, so highly adapted are 

 whales and other Cetacea to their watery surroundings tiiat 

 sperm whales have been known to remain beneath the surface 

 from an hour to an hour and twenty minutes; and many 

 species remain under water from twenty minutes to an hour. 



1 This estimate is a pre-war one. 



