74 APPLIED PHYSIOLOGY 



sacrificed to facilitate their function that of taking up 

 and giving off oxygen. Their shape, for instance that 

 of a biconcave disc is such as to present the largest 

 possible surface compatible with their free movement in 

 the blood-stream. Collectively, therefore, they offer in 

 the lungs and in the tissues a large area over which 

 gaseous exchange can take place an area which has 

 been not inaptly termed the * internal respiratory 

 surface.'* Further, they are enclosed in a smooth 

 membrane of great elasticity, which enables them easily 

 to wriggle their way through the most tortuous capillary 

 channels. This remarkable elasticity is a sign of health 

 in the corpuscle, and is lost in many diseases of the 

 blood in which the disc form gives place to various 

 irregularities of shape (poikilocytosis). Each corpuscle 

 is stuffed full of haemoglobin, but the exact mode in 

 which this is disposed whether it is partly in solution 

 in the corpuscle or loosely united in an amorphous form 

 to a stroma of nucleo-protein is still disputed. In 

 addition, the red corpuscles contain a considerable 

 amount of lecithin and cholesterin, which probably form 

 a sort of waterproof coating to their walls. So long as 

 this impermeable membrane is in a living condition it 

 prevents the diffusion out of the contents of the 

 corpuscles, but if it be killed, diffusion begins, because 

 the plasma and the contents of the corpuscles are not 

 iso-tonic. Chilling seems to kill the membrane and 



* Assuming that the body contains 3^ litres of blood, with 

 5,000,000 red cells in each cubic millimetre, then the 'internal 

 respiratory surface ' will amount to 2,500 square metres, or more 

 than 1,000 times the external body surface (Buckmaster) 



