Bones contain most of the calcium of the human body. The calcium in the extracellular and intracellular
compartments, although a small amount compared to the bones, is a major player in the function of nerves,
skeletal muscle, the heart, smooth muscle and blood coagulation. Therefore, the plasma concentration of calcium
is regulated within narrow limits. Even the absorption of calcium from the GI tract is regulated.
Extracellular calcium, like potassium, must be kept relatively constant. When extracellular calcium
concentration is abnormally low (hypocalcemia), nerve and muscle cell membranes become hyperexcitable.
Amongst other things, this can lead to muscle spasms. On the other hand, if the calcium concentration is too
high (hypercalcemia) there is the possibility of cardiac arrhythmias and decreased neuromuscular excitability.
Unlike sodium and potassium regulation, the major homeostatic controls for calcium are not centered solely in
the kidneys but involve the kidneys, bone and the gastrointestinal tract.
IMPORTANCE OF REGULATING PLASMA CALCIUM CONCENTRATION:
• Critical to so many physiological functions
o Ca is an important signal molecule
o Ca is part of the IC cement that holds cells together at tight junctions
o Ca is a cofactor in the coagulation cascade
o Plasma Ca concentrations affect the excitability of neurons
• Ca follows the principle of mass balance
o Total body Ca is all the Ca in the body, distributed among all 3 compartments
o Intake is the Ca ingested & absorbed in the small intestine
o Output is the Ca loss from the body, primarily occurring in the kidneys
Total Body Ca = Intake – Output
Main Effector Sites for Calcium Homeostasis:
• Total body Ca is distributed among 3 compartments of the body
1. Extracellular Fluid
i. Half bound to plasma proteins/molecule
ii. Other half diffuses across membranes through open Ca channels
2. Intracellular Ca
i. Lowest concentration here
ii. EC gradients favor movement of Ca into cytosol when Ca channels open
3. EC matrix (bone)
i. Largest Ca reservoir in the body
Plasma calcium concentration is regulated by parathyroid hormone, and this hormone exerts its
influence at all of these three effector sites. Vitami3 D also plays an active role in calci