Final answer:
When Brian's testosterone levels drop, the hypothalamus releases GnRH, stimulating LH production, which in turn prompts the testes to secrete testosterone, regulated by a negative feedback loop.
Explanation:
The sequence of events when the testosterone level in Brian's blood drops below the normal range is:
- The hypothalamus secretes gonadotropin-releasing hormone (GnRH)
- GnRH stimulates the pituitary gland to secrete luteinizing hormone (LH)
- LH stimulates the testes to secrete testosterone
This process is regulated by a negative feedback loop involving testosterone, GnRH, LH, and FSH to maintain testosterone levels.
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