The early response to lethal doses of ionizing radiation in the chicken and chick embryo is characterized by rapid structural and functional changes in the vascular system. Increase in exposure time from a few minutes to a few hours has been shown to reduce the dose effectiveness for this injury mechanism. An x-ray exposure to 1100 R delivered in 12 minutes resulted in more than 95% mortality within 24 hours; when the exposure time was increased to 240 minutes, 1100 R represented a sublethal dose, 1800 R a 50% lethal dose, and 2200 R a 95% lethal dose. Effects of these exposures on the microcirculation were studied in the explanted chick embryo. The extent of injury to the vascular system could be correlated with the amount of lethality produced by the irradiation. A 240-minute exposure to 1100 R produced little effect on the microcirculation. Only after exposure to 2200 R was there a rapid circulatory collapse comparable to that produced by a 12-minute exposure to only 1100 R. Degenerative changes in individual endothelial cells appeared to be a direct effect, which was reduced when the exposure time was increased.

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