Bone is the body's calcium bank, holding about 99 percent of its calcium. Hormones move calcium in and out of bone to keep the blood level in a narrow range.
Why blood calcium matters
Nerves, muscles and the heart need a stable calcium level to work. Low calcium causes tingling and muscle cramps; high calcium causes weakness, constipation and confusion.
Parathyroid hormone
When blood calcium falls, the parathyroid glands release PTH. It frees calcium from bone, keeps calcium in the kidneys and activates vitamin D.
Vitamin D
Vitamin D is made in the skin with sunlight and activated in the liver and kidneys. Its main job is to increase calcium absorption from the gut.
Calcitonin
Calcitonin from the thyroid gland lowers blood calcium by slowing bone breakdown. Its role in adults is small compared with PTH.
When it fails
Lack of vitamin D causes rickets in children and osteomalacia in adults, soft bones that bend or ache. Chronic kidney disease disturbs vitamin D activation and weakens bone.
| Hormone | Made in | Effect on blood calcium |
|---|---|---|
| PTH | Parathyroid glands | Raises |
| Active vitamin D | Skin, liver and kidneys | Raises |
| Calcitonin | Thyroid gland | Lowers |
At the bedside
Calcium is part of the basic metabolic panel; correct it for a low albumin before you interpret it. Low calcium can prolong the QT interval on the ECG.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).