Your body stores calcium in the bones, but also maintains a constant level of calcium in the blood. If the blood calcium level falls, then the parathyroid glands in your neck release a hormone called parathyroid hormone. Parathyroid hormone increases calcium reabsorption from the distal tubule of the nephron to restore the blood calcium level. Parathyroid hormone also stimulates calcium release from bone and calcium absorption from the intestine.
In addition to parathyroid hormone, your body also requires vitamin D to stimulate calcium absorption from the kidney and intestine. Vitamin D is found in milk products. A precursor to vitamin D (cholecalciferol) is made in the skin and processed in the liver. However, the final step that converts an inactive form of cholecalciferol into active vitamin D occurs in the proximal tubule of the nephron. Once activated, vitamin D stimulates calcium absorption from the proximal tubule and from the intestine, thereby increasing blood calcium levels.
Kidney stones are often caused by problems in the kidney’s ability to handle calcium. In addition, the kidney’s role in maintaining blood calcium is important in the bone disease osteoporosis that afflicts many elderly people, especially women.
As you can see, the kidneys perform many functions that are important to your body:
- Controlling the composition of your blood and eliminate wastes—filtration/reabsorption/secretion method
- Influencing blood pressure—renin secretion
- Helping to regulate your body’s calcium—vitamin D activation