During a rapid response to hypocalcemia, from where is calcium released first?

During a rapid response to hypocalcemia, from where is calcium released first?

• A. Lacunar spaces
• B. Osteons
• C. Trabeculae
• D. Endosteum

Answer: (A)

When calcium drops rapidly, the body first draws from the most quickly exchangeable bone calcium pool: the interstitial fluid in the lacunar-canalicular system surrounding osteocytes. These lacunar spaces contain bone fluid that can quickly equilibrate with plasma Ca, so calcium is released into the circulation from this reservoir first. In contrast, release from mineralized matrix in osteons or trabeculae requires active remodeling at the endosteal surfaces and osteoclast-mediated resorption, which takes more time. Endosteum-related remodeling contributes afterward as the slower, larger calcium pool is mobilized.

When calcium drops rapidly, the body first draws from the most quickly exchangeable bone calcium pool: the interstitial fluid in the lacunar-canalicular system surrounding osteocytes. These lacunar spaces contain bone fluid that can quickly equilibrate with plasma Ca, so calcium is released into the circulation from this reservoir first. In contrast, release from mineralized matrix in osteons or trabeculae requires active remodeling at the endosteal surfaces and osteoclast-mediated resorption, which takes more time. Endosteum-related remodeling contributes afterward as the slower, larger calcium pool is mobilized.