Calcium and phosphate are minerals that are important for many functions in the body. The levels of calcium and phosphate are tightly regulated physiologically in order to perform these functions. Diseases can disrupt normal calcium and phosphate levels, and these abnormal levels can lead to further problems.
Functions
Calcium and phosphate are both found in bone. The minerals impart strength to bone and also serve as a reservoir in case the blood levels of calcium or phosphate drop. Calcium is also important in cellular communication. For example, signals from the brain travel through the nerves to tell a muscle to contract. When the signal reaches the area where the nerve meets the muscle, called the neuromuscular junction, calcium enters the nerve and signals the release of the chemical acetylcholine. Acetylcholine binds to the muscle and causes calcium levels to rise in the muscle, which leads to contraction. Calcium has many such functions as a messenger. Phosphate can also serve as a chemical messenger, and also is important in reactions requiring the addition of energy. One of the main energy carrying molecules of the cell, ATP, contains three phosphate groups.
Levels
Normal calcium levels can be measured in the blood as total calcium or free calcium. Total calcium is all the calcium found in a given unit of blood with the normal range being 9 to 10.5 mg/dL. Free, or ionized calcium, is the measure of calcium that is not bound to protein, such as albumin in the blood. Normal ionized calcium levels are 4.5 to 5.6 mg/dL. Phosphate levels are normally 2.5 to 4.5 mg/dL.
Regulation
Calcium is absorbed from the diet through the intestines. This process is regulated by vitamin D, which is obtained in the diet or synthesized by sunlight on the skin, the kidneys and liver. The kidneys excrete and reabsorb calcium depending on the blood levels. In addition, the bone provides calcium to the blood if needed. Parathyroid hormone and vitamin D increase calcium levels, and calcitonin, also released by the parathyroid, decreases calcium levels. Phosphate levels are decreased by parathyroid hormone and increased by vitamin D.
Disorders
Abnormal calcium levels tend to cause nerve and muscle dysfunction. Cancer and overly active parathyroid glands, called hyperparathyroidism, are two common causes of high calcium. Kidney failure is a common cause of low calcium, largely through the decreased synthesis of vitamin D. In contrast, kidney failure tends to cause high phosphate levels, which can lead to complications such as hardening of the arteries. Low phosphate levels are caused by a number of conditions, including hyperparathyroidism and diarrhea.
References
- "Physiology"; Linda S.Costanzo; 4th Ed 2008
- "Molecular Biology of the Cell"; Bruce Alberts et al.; 4th Ed 2002
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci et al.; 17th Ed 2008
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