Calcium is the most abundant mineral in the body. For an average adult there are about 15 g of calcium for every 1 kg of body weight. This adds up to approximately 1 kg of total calcium for an average size adult. Ninety-nine percent of this calcium is stored in the bone. The human body requires a carefully balanced interplay of daily intake of calcium and excretion of calcium in the urine to maintain a normal total body calcium.
Unlike total body calcium, the measured serum calcium ranges from only 8.5-10.5 mg/dL. This is because the vast majority of the body calcium stores are locked up in the bones. An only modest deviation in this relatively narrow range of normal serum calcium levels will result in the signs and symptoms of hyper- or hypocalcemia, states "Ganong’s Review of Medical Physiology."
Pound for pound, the vast majority of the calcium in the body functions to provide strength and support to the skeletal system. It is the presence of calcium which causes bones to appear white on an x-ray. The relatively small amount of calcium that circulates in the serum and in the body's cells serves a number of electrochemical functions. Most notably, calcium is necessary for muscular contraction and various types of neurochemical signal transduction.
The balance of calcium in the serum is maintained through the actions of parathyroid hormone, or PTH. PTH is secreted by the parathyroid gland in response to low serum calcium levels. This homeostasis raises calcium levels by stimulating the breakdown of bone and the release of its mineral stores. It also increases resorption of calcium by the kidney and absorption of calcium by the intestine. Elevated calcium levels stimulate the hormone calcitonin to inhibit the breakdown of bone, resulting in greater storage of calcium in the skeletal system.
Elevated levels of calcium in the serum, or hypercalcemia, are typically asymptomatic until the level of calcium rises above 12 mg/dL. The most likely causes are an elevated level of parathyroid hormone or cancer. According to "Tintinalli's Emergency Medicine," the symptoms of hypercalcemia are vague and may include generalized weakness, confusion, bone pain, cardiac dysrhythmias, weight loss and kidney stones. Treatment revolves around rehydration and medications to increase elimination of calcium by the kidney and decrease the mobilization of calcium from the bones.
Low serum calcium levels, states "Harrison's Principles of Internal Medicine," are typically only found in severely ill patients who may have renal failure, shock or sepsis. They could also be caused by a variety of medications such as diuretics and anti-seizure drugs. A classic initial symptom of hypocalcemia is a tingling sensation around the mouth or in the fingertips. It can cause twitching contractions of the muscles when stimulated, such as during a blood pressure measurement. Treatment is typically as simple as taking oral calcium supplementation.