What high-G exposure effect most directly leads to G-LOC?

• A. Cerebral hypoperfusion from reduced venous return
• B. Increased cerebral blood flow
• C. Hyperventilation
• D. Increased heart rate

Answer: (A)

During high-G exposure the main issue is cerebral hypoperfusion caused by reduced venous return. The strong gravitational force causes blood to pool in the legs and abdomen, which lowers the amount of blood returning to the heart. This drop in venous return reduces cardiac output and cerebral blood flow, so the brain receives less oxygen and nutrients. When cerebral perfusion falls below what the brain needs, consciousness is lost, resulting in G-LOC.

Other effects aren’t the direct trigger. Increasing cerebral blood flow would help prevent LOC, not cause it. Hyperventilation lowers carbon dioxide, leading to cerebral vasoconstriction and reduced blood flow to the brain, which can worsen perfusion but is not the primary mechanism for G-LOC. An increased heart rate is a compensatory response to maintain blood pressure and perfusion, not the cause of LOC.