252 BIOLOGICAL EFFECTS OF IONIZING RADIATIONS 



but it could just as well be modification of one of the catalysts of the syn- 

 thesis of DNA, or the membrane which contains them. 



There is some direct information on DNA in solution, however. By vis- 

 cosity and titration methods it has been found that the molecule is shattered 

 by X and a rays, to an amount of about 1.5 x 10 11 chain-breaks per gram 

 of DNA per rad absorbed. The analogy with the effect of ultrasound on 

 viruses is usefully drawn at this point, for ultrasound quite literally shakes 

 the molecule to pieces. 



There is also some semidirect information on DNA in vivo. Thus, T. T. 

 Puck 4 and others have allowed irradiated human cells to culture, and have 

 measured, not the LD 50 , but the "reproductive death" — the irradiation dose 

 which is just sufficient to cause the cells not to reproduce. These cells are 

 not killed by the radiation, but often show abnormalities, such as growing 

 to a huge size or showing a change in metabolic rates. Reproductive 

 "death" is relatively very sensitive, its L"D" 50 being 25 to 40 rads in human 

 cells. The corresponding sensitivity, a, is about 2000 (compare with the 

 values in Table 9-4). 



Irradiation of Organs and Tissues 



The histologic and pathologic changes in tissues resulting from irradiation 

 are properly part of the subject matter of radiology, and will not be dis- 

 cussed here. However, as illustration, some of the results of whole- and 

 partial-body irradiation are listed below, with no explanation, as simple 

 statements. 



Just as some parts of the cell are more radiation-sensitive than others, also 

 some tissues and organs are more sensitive than others. The analogy goes 

 further. Some parts of the human body can be irradiated relatively heavily 

 without severe general damage; others are very radiation-sensitive. The fol- 

 lowing list includes the most sensitive. 



(1) Red blood cell manufacture slows down in the bone marrow. 



(2) Manufacture of lymphocytes in the spleen is drastically reduced and 

 cannot replace fast enough those killed by irradiation of the general lym- 

 phatic circulation system. 



(3) The skin shows reddening or blistering, after only 140 rem; larger 

 doses can precipitate skin cancer. 



(4) Impairment of secretion or of assimilation occurs in the alimentary 

 canal, mostly as a result of membrane destruction. Sloughing off of the 

 mucous lining of the canal is an early symptom of damage and often results 

 in death due to infection. 



(5) The critically important steady-states in the adrenal glands are upset. 

 Because these are the source of certain rate-controlling molecules, the hor- 



