Calcium is the most abundant mineral found in bone. Only a small percentage of total body calcium is found in blood. Blood calcium plays a major role in the regulation of heart rate and rhythm, because it is involved in the generation and transmission of current from the pacemakers to the heart muscle. Low blood calcium increases the risk for ventricular tachycardia, or too rapid contractions of the lower chambers of the heart.
Blood calcium is regulated by the thyroid and parathyroid glands. Calcitonin -- a hormone produced by the thyroid gland -- is released when blood calcium is increased. When blood calcium is low, the parathyroid glands release parathyroid hormone, or PTH, which stimulates the breakdown of bone to release calcium to the blood. PTH also acts in the kidneys, thereby reducing calcium excretion. Blood calcium is also maintained by vitamin D, which facilitates the absorption of intestinal calcium.
Sources of Calcium and Reduced Calcium Intake
Major sources of dietary calcium include diary products, such as milk, cheese and yogurt, and eggs. Certain kinds of fish, such as sardines and pilchards, can contain significant amounts of calcium. Other sources of calcium include shellfish, almonds, peanuts, chickpeas and beans. Reduced dietary intake of calcium can result in calcium deficiency. According to Children's Hospital and Medical Center in Omaha, Nebraska, the lack of calcium results in the prolongation of the QT interval, which reflects the time it takes for the heart muscle to contract and relax. In addition to ventricular tachycardia, a prolonged QT interval is also associated with increased risk for ventricular arrhythmias, or irregular heart rhythm.
Calcium and Pacemakers
Most cells have stable resting membrane potentials, or the ability to drive electrical current when excited. The resting membrane potential is maintained by the distribution of positive and negative charged ions across both sides of the cell membrane. At rest, calcium concentration in pacemakers -- cells in the heart that determine the heart rate -- is low compared to the outside. When pacemakers drive current, calcium channels in the membranes open, thereby allowing calcium to rush into the cells. Pacemakers have unstable resting membrane potentials, which result from leaky channels in the cell membrane that allow positive ions to enter the cell; this "leakiness" of the cell membrane, means that pacemakers can self-excite at regular intervals.
Causes of Low Calcium
Deficiency of blood calcium results from the interruption of mechanisms involved in calcium absorption and regulation. Vitamin D deficiency results in decreased calcium absorption, and poor calcium absorption is associated with low blood calcium, irrespective of adequate dietary calcium intake. Diseases that affect the absorptive surface of the small intestines, thereby resulting in malabsorption of nutrients, including calcium, may result in low blood calcium. Intake of foods containing oxalate -- a compound that binds to intestinal calcium, thereby preventing its absorption -- can result in low blood calcium. Oxalate is found in spinach, rhubarb, green beans and beets. Certain fruits, such as oranges, cranberries, grapes, plums, apples and pears also contain oxalate.