Bone as a tissue in constant turnover
Bone is not static. Cells called osteoclasts resorb old bone while osteoblasts lay down new matrix, and the two processes run continuously throughout life. Net bone balance is the difference between them: in youth deposition outpaces resorption, peak bone mass is reached in the late twenties or so, and later in life resorption tends to win, gradually lowering density.
The diagram frames balance as a sum of inputs rather than calcium alone, because calcium intake is only useful if the surrounding system can place it into bone.
The supporting cast: vitamin D, K2, protein, and load
Vitamin D is needed to absorb dietary calcium efficiently. Vitamin K2 activates proteins (such as osteocalcin) that help bind calcium into bone matrix. Adequate protein supplies the collagen scaffold, and mechanical load from weight-bearing activity is one of the strongest signals telling bone to build. None of these substitutes for the others.
This is general educational information, not medical advice. Calcium and vitamin D needs vary by age and condition, and very high calcium supplement doses are not automatically better, so dosing should be discussed with a qualified clinician.