Which structure is primarily responsible for calcium metabolism in the body?

The correct choice is bone, as it serves as the primary reservoir for calcium in the body and plays a crucial role in calcium metabolism. Bones store approximately 99% of the body's calcium content, and they release calcium into the bloodstream as needed to maintain physiological balance. This release is regulated by hormones such as parathyroid hormone and calcitonin, which respond to the body’s calcium levels.

Bone tissue is dynamic and continuously remodeled throughout life, undergoing a process called bone remodeling that is essential for maintaining calcium homeostasis. When calcium levels in the blood drop, osteoclasts—cells that break down bone—are stimulated to release the stored calcium. Conversely, when calcium levels are high, osteoblasts—cells involved in bone formation—can be activated to store excess calcium in the bone matrix.

Other structures such as muscles, cartilage, and ligaments do not play a direct role in calcium metabolism. Muscles utilize calcium primarily for contraction, but they do not regulate calcium levels in the body. Cartilage and ligaments, while important for structural support and joint function, do not store or manage calcium as bones do. Therefore, bone is distinctly responsible for calcium metabolism and is essential for maintaining overall calcium balance in the body.