638 HANDBOOK OF PHYSIOLOGY '^ CIRCULATION I 



FIG. 5. Basic types of compartmented systems analyzable with RC analogs and typical curves 

 obtained with analog computer. 



compartmental system and circulatory problems are 

 the following; 



Gregg (30) discusses the use of the indirect type of 

 analog computer to problems such as the removal by 

 the kidney of Diodrast injected into the bloodstream. 

 Solomon & Gold (66) have used analog computers in 

 studies of potassium transport. Warner (72) describes 

 the use of an analog computer in studies of regulation 

 of the circulation and the response to transient 

 disturbances such as the Valsalva maneuver. Fish (24) 

 discusses the general problem of compartmental 

 system analysis by analogs, with applications to the 

 interpretation of excretion data. 



Bauer & Ray (3) used an analog computer to solve 

 problems of strontium metabolism involving distin- 

 guishing between the uptake in bone by accretion 

 and the uptake by exchange. Rollinson & Rotblat 

 (53) describe a fairly elaborate electrical analog 

 system designed to simulate the metabolism of 

 iodine. Parrish el al. (47) have used an analog com- 

 puter in studies of pulmonary circulation. 



DiGiT.\L COMPUTERS. As their name implies, digital 

 computers arc essentially number-handling machines. 

 The familiar desk calculator and the "giant brains" 

 or electronic digital computers have in common the 

 basic capabilities of being able to store (or "remem- 

 ber") numbers and to add or subtract pairs of these 

 numbers. 



Almost all other computations, multiplication, 

 division, extraction of square roots, taking logarithms, 

 raising to powers, etc. are achic\ cd through sequences 



of basically addition operations, the larger machines 

 differing from the smaller ones in such features as 

 the capacity and accessibility of the memory, the 

 degree of automation, and the speed of operations. 

 The large machines perform tens of thousands of 

 operations per second, so computations which would 

 be absurd to attempt with a desk computer become 

 entirely reasonaljle to achieve with an electronic 

 computer. 



In addition to performing the basic arithmetical 

 operations, digital computers can make certain 

 simple logical decisions such as choosing between 

 alternative secjuences of operations, stopping repetitive 

 operations when some arbitrary condition is attained, 

 etc. 



The big job for the user of a digital computer 

 consists of figuring out how to tell the machine what 

 he wants it to do, or programming. The machines 

 have built-in programs for the basic operations, so 

 that it is not necessary to prescribe every detail, but 

 it is necessary to establish the sequence of operations 

 desired. 



Although it is more ditticult and more expensive to 

 set up a problem on a digital computer than on an 

 analog computer, the digital computer can handle a 

 much wider variety of problems (including nonsteady- 

 state conditions, for example) and gives a higher 

 degree of precision in the answer if warranted by the 

 data. Because of the complexity of the subject, 

 including the differences among the variety of 

 machines available, it is not feasil^le to present a 

 complete or even an adequate discussion of the 



