CIRCULATION STUDIES 421 



Circulation Studies 



In estimation of the indirect effects of irradiation between the various 

 tissues the circulation must be considered a factor of basic importance. 

 If substances are produced which are removed by either the Hver or the 

 kidney with high efficiency, their effective time of detectable concen- 

 tration in the blood will be very low. For example, if a material is dis- 

 tributed in the blood alone and removed by the liver with 90 per cent 

 efficiency, it will fall to 10 per cent of the initial concentration in 10 

 min (since the liver circulation is greater than one volume of blood per 

 minute per volume of tissue), and if the substance is produced at a 

 steady state, the level of circulating material in the blood at any time 

 will be only 3 per cent of the quantity produced per hour. The organs 

 and tissues with the greatest circulation will necessarily be exposed to 

 the greatest amount of all labile material related to radiation effects 

 that are carried by the blood. 



Animals joined together in parabiosis have been used to study indirect 

 effects of radiation. The common bridge of tissue between such animals 

 is a site of exchange of materials and, of more importance, of the inter- 

 mingling of blood. Because of the fact that the average blood perfusion 

 rate of skin and connective tissue is low (13), approximately 0.02 volume 

 of blood per volume of tissue per minute, the relative exchange across 

 the tissue bridge of parabionts must be insignificant compared to the to- 

 tal cardiac output. From the fact that 15 per cent of the cardiac output 

 is used by the non-visceral tissues and from the volume of the tissue 

 concerned in the tissue bridge, the maximum exchange between para- 

 bionts can be placed at 1.5 per cent of the cardiac output per minute, 

 assuming that all the blood from one animal that reaches the tissue 

 bridge is ultimately collected in the venous drainage of the other. This 

 small exchange means that the parabiont cannot be used to detect ma- 

 terials which are of short biological duration in the body, since obvi- 

 ously most of such material will be consumed in the animal in which it 

 is produced before it can pass the tissue bridge, and ineffective amounts 

 will be carried to the partner. Since these circulatory exchange factors 

 are basic to interpretation of induced effects in the parabiotic states, a 

 study has been made to quantify the above principle. The measure- 

 ment of physical exchange of blood between rats in parabiosis indicates 

 0.66 per cent of the blood volume per minute, which would be approxi- 

 mately the same value if it were present as percentage of the cardiac 

 output (14). Exchange of an electrolyte is much slower because of the 

 greater space occupied in either animal relative to the quantity of blood 



