Your bones are made from calcium phosphate. So are your teeth. But your body uses calcium in other tissues, such as muscles, the heart, and the nerves.
When you are growing, your bones need to grow, and so you need calcium. If you get regular exercise, your bones feel the forces and respond by getting stronger. If you do not get enough exercise, the bones weaken, and can break more easily.
When your body needs calcium for the muscles and nerves, it can get it by dissolving bone tissue to get more calcium into the blood. You will gradually lose calcium from your bones if you do not get more calcium in your diet. You need Vitamin D in order to process and absorb calcium from your food.
Most bone building occurs when you are young. Later in life, your body will use the calcium you stored in your bones when you were growing. In older people, and especially in women, the loss of calcium can make the bones weak and brittle. So it is important to get plenty of calcium, Vitamin D, and exercise when you are young. Your body is banking calcium.
As you get older, less calcium is needed, since your bones are not growing. But getting adequate calcium to replace what is lost is still important, as is exercise and Vitamin D.
Calcium is an element, so your body cannot make it, but must get it from the environment. Besides Vitamin D, other things that help absorb calcium from the diet are protein, magnesium, and phosphorus. Some foods, such as spinach, sugar, alcohol, and coffee, can prevent calcium absorption.
An adult woman has about two pounds of calcium in her body. An adult man has about three pounds. All but 1% of it is in the bones.
Calcium helps muscles contract, it helps blood clot, it controls blood pressure, and helps prevent gum disease. Lack of calcium can cause leg cramps, muscle spasms, and nerve sensitivity.