Which ARS subsystem has a threshold of about 2 Gy (200 rad)?

• A. Cardiovascular
• B. Hematopoietic
• C. Gastrointestinal
• D. Neurological

Answer: (B)

The idea tested is how different body systems respond to radiation in acute radiation syndrome. Each subsystem has a dose threshold where damage becomes clinically evident, and the hematopoietic system sits at a relatively low threshold.

Bone marrow is highly radiosensitive because it contains rapidly dividing cells that continually replenish blood cells. When exposure reaches about 2 Gy (200 rad), you start to see suppression of blood cell production—neutrophils drop first, followed by platelets and other blood lines. This hematopoietic suppression weakens the immune system and impairs clotting, increasing the risk of infection and bleeding. Those effects define the hematopoietic subsyndrome, which becomes the dominant concern around this dose.

In contrast, the gastrointestinal subsyndrome requires a higher dose (roughly 6 Gy or more) to severely disrupt the mucosal lining and absorptive functions of the gut, leading to different clinical consequences. The neurological/cardiovascular syndromes need even larger doses—on the order of tens to hundreds of Gy—before rapid, catastrophic failure occurs.

So, a threshold around 2 Gy points to the hematopoietic subsystem because of the bone marrow’s extreme sensitivity and the consequential drop in blood cell counts that follows radiation exposure at that level.