Although calcium is found in only a limited number of foods, it is one of the most plantiful - and vital - nutrients in our bodies. The average adult carries anywhere from two to four pounds of calcium, depending on size. About 90 per cent of it is in the bones and tooth, and that tiny - but so critical - other 1 per cent is in the soft tissues, blood plasma and the fluids inside and outside every cell.

This minute amount of circulating calcium is necessary to maintain a number of vital body function. It helps the blood clot properly. The muscles, including the heart itself, need calcium to properlyu contract and relax. Nerves use it to transmit impulsea. Calcium also helps the cell membranes maintain their structure and cling together. Finally, this "free" calcium activates or inhibits several enzyme systems.

We would be in great danger if our blood supply of calcium were to be exhausted. But fortunately, the bones act as a reservoir of calcium from which the body can draw when blood levels are low.

The body has a marvelous set of controls to regulate the calcium balance. Two hormones - the parathyroid hormones and calcitonin, in conjunction with a metabolite of vitamin D (commonly known as 1,25-CHCC - keep the blood level at a constant of 10 miligrims for every 100 milliliters of blood.

When the calcium level drops, bringing on a condition known as hypocalemia, parathoid hormone in secreted, which, in turn, signals for the production of more 1,25-DHCC. This combined action increases the absorption of calcium from the digestive system, and if necessary, from the bones.

Too much calcium, on the other hand, also is undesirable. When the level in the blood climbs above 10 milligrim, producing a condition known as hypercalcernia, calcitinin is secreted, a production of 1,25-DHCC goes down. Intestinal absorption of the mineral is slowed, and calcium is again deposited in the bones.

Although the bones act as a reservoir of calcium, it is important that we take in enough of the mineral in our daily diet. Most people know that children need calcium for strong teeth and growing bones, and that pregnant and nursing mothers also require dietary calcium. But many people have the mistaken notion that adults don't require calcium.

As you age, bones do cease to grow. But bones like any other body tissue, is constantly being renewed, with old tissues breaking down and new being formed. (Sadly this is not true for the teeth.) Also we lose 250 to 400 milligrams of calcium a day in urine, feces and sweat. Women also lose calcium during pregnancy, when 25 to 30 grams are diverted to the fetus, and when breast milk is being produced. So we never outgrow our needs for dietary calcium.

As we said earlier, there are limited sources of calcium in our diets. But assuming that intake is adequate - that is, at or above the Recommended Dietary Allowance of 800 milligrams a day and 1,200 for pregnant and lactating women - a number of other factors determine how well calcium is absorbed, retained and utilized. For example, the diet must also contain enough phosphorus, magnesium, flouride and other minerals, as well as proper balance of vitamin D. A shortage of vitamin D can produce rickets and contribute to osteoporosis or the loss of bone sturcture around the teeth - peridontal disease. Excess of vitamin D causes too much calcium to be drawn from the bones and deposited in soft tissues, with a danger of permanent kidney damage and serious heart disorders.

Substances in foods also may inhibit calcium absorption. Oxalic acid in such foods as rhubarb, cocoa and spinach and the phytic acid in the bran of cereals "tie up" some calcium. A diet very high in fat can have the same effect. Surprisingly, diets comparatively low in protein seem to enhance absorption and retention of calcium, as does getting the mineral in small amounts throughout the day.

Inactivity also affects calcium retention. Patients recovering from a serious operation may lose substantial amounts in a short time. Astronauts face the same problem.

How do you ensure getting enough calcium? It is difficutl without consuming milk and milk products, which provide about 75 per cent of our total calcium. However, there are others food sources, such as turnips, mustard and dandelion greens, collards or kale. A half cup of each of these provides about 140 milligrams. Two-thirds of a cup of broccoli has about 99 milligrams, a medium-size orange about 60, and a medium artichoke about 100. Small fish eaten with the bones also are good sources. An ounce of sardines contains 120 milligrams. If for some reason you cannot drink milk or eat cheese or other milk products, it would be good to ask you physician whether you are one of those few people who need to take a calcium supplement.