The heart is a pump that delivers blood to the organs, including itself. It is largely composed of specialized muscle fibers called myocytes, and has its own blood vessels, connective tissues and nerves. In order to coordinate contraction, the nerves and myocytes communicate with an electrical charge. The charge moves through charged minerals called electrolytes, such as calcium.
Electrolytes and Nerve and Muscle Transmission
Cells have a covering called the cell membrane. The outside and inside of cells have different compositions of electrolytes. A change in the composition can signal a cell to perform an activity or pass on a message. For example, a nerve cell can receive a chemical signal that opens protein channels in its cell membrane. The channels allow sodium, normally in high concentration outside the cell, to rush in. Sodium is positively charged; this makes the cell positive. Proteins inside the nerve cell respond to this charge by opening more channels that allow calcium to enter the nerve. The calcium then signals the nerve to release the chemical acetylcholine. Acetylcholine binds to a nearby muscle cell. Through a similar chain of events, calcium enters the muscle cell and tells it to contract.
Normal Calcium and the Heart
Calcium serves three general purposes in the heart. First, it helps regulate the speed and pace of contraction in the heart's pacemaker cells. Second, it signals the muscle to contract. Third, it tells the heart how forcefully to contract.
Hypercalcemia and the Heart
High calcium levels -- hypercalcemia -- are predominantly caused by cancer and an overactive parathyroid gland. A high calcium level affects the nervous system, heart, bones and kidneys. The heart is prone to disturbances for several reasons. First, the increased calcium increases the strength of contraction up to a point, then weakens the strength of contraction. The length of time for contraction is shortened, as is the time for relaxation of the muscle. The pacemaker cells of the heart do not function well, and can fail to tell the heart when to beat. This can even lead to the heart stopping.
Hypocalcemia and the Heart
Hypocalcemia, or low calcium levels, causes nerve, blood and heart problems. The predominant effect on the heart muscle is a weakened force of contraction that can lead to heart failure. The heart can become electrically unstable and beat ineffectively; this results in inadequate delivery of blood and in the worst case scenario can also lead to death if not corrected.
References
- "Physiology"; Linda S.Costanzo; 2008
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci et al.; 2008
- "Circulation "; American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Part 10.1: Life-Threatening Electrolyte Abnormalities; American Heart Association; 2005
