When most people think of calcium inside the body, they think of growing and maintaining bone structure. While this is one role of calcium, there are many others. In fact, the heart couldn't beat and your muscles couldn't contract without calcium. Calcium is a major player in the electrical activity of the cardiac cycle.
Anatomy
The heart is made up of cardiac muscle fibers with the interesting ability to spontaneously contract. The SA node is a group of cells that starts off the heartbeat. A spontaneous excitation of these cells acts as a wave of electrical conductivity to its neighboring cells and onward to result in a rhythmic contraction of the atrium and then the ventricles.
Spontaneous Excitation
The SA node is preset to fire between 60 to 100 beats per minute. This is due to an unstable resting membrane potential. The resting membrane potential refers to the amount of positive or negative ions inside and outside of a cell. The cardiac muscles rest at about negative 60 mV. As positive ions drift into the cell, the cell's pole increases until it reaches zero at its peak.
Calcium and Depolarization
Calcium acts as one of the ions that depolarizes cardiac cells. As depolarization occurs, sodium, which is a positive ion, makes its way across the membrane of the cell. Once the cell has reached negative 40 mV, fast calcium channels open, which, in turn, opens the gates for calcium outside of the cell to rush in. Calcium is another positively charged ion and this flood of calcium makes it possible for the cell to contract.
Calcium and the Cardiac Cell
The cardiac cell would not be able to contract if not for calcium. Calcium release is prompted by the opening of storage sites just outside of the nucleus of the cell, in the membrane. The impulse to release calcium travels through the membrane by way of t-tubules, which connect the inside of the cell to the membrane. After the contraction has occurred, the cell goes back to its resting state and the calcium is returned.
Calcium Recommendations
Now that you know calcium does more than just build strong bones, you should know how much calcium is necessary in your diet. For an adult man or woman, the recommended intake of calcium is 1,000 mg per day. If you are over 51, your recommendation goes up to 1,200 mg per day. For most, as long as you eat a healthy diet rich in milk, cheese or other low-fat dairy products, you will meet the recommended standards.
References
- "Anatomy and Physiology"; Kenneth S. Saladin; 2004
- "Exercise Physiology"; George A. Brooks, Thomas D. Fahey, Kenneth M. Baldwin; 2005
- "Nutrition: Concepts and Controversies"; Francis Sizer and Eleanor Whitney; 2004.
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