Normal Calcium Physiology

As a signal molecule, Ca²⁺ is essential to synaptic activity and muscular activity. Specifically, when Ca²⁺ moves from one compartment to the other, like the ECF to the cytoplasm, it creates what is known as a Ca²⁺ signal. This signal can initiate exocytosis of vesicles which can lead to a plethora of other reactions. In muscle cells, Ca²⁺ calls for the contractions of muscle fibers. In neurons, the Ca²⁺ signal leads to the release of neurotransmitters into the synaptic cleft. Though these are the most discussed functions in physiology, Ca²⁺ also serves a purpose as part of the tight junctions, a cofactor in coagulations, and an active chemical in determining neuron excitability. For example, in hypocalcemia, a low Ca²⁺ level in the plasma increases Na⁺ permeability, which increases membrane potential, and depolarize neurons making them easily excitable to the point where they induce tetany in respiratory muscles, leading to asphyxiation. On the other end is hypercalcemia, which will be discussed more in-depth throughout this website.

1. Parathyroid Hormone (PTH): This is released by the parathyroid glands behind the thyroid gland. These glands contain a G protein-coupled receptor known as Ca²⁺ - sensing receptor (CaSR). These sensors, as their name suggests, detect how much serum Ca²⁺ is present. If there is a decrease in the ion amount, then they signal the release of PTH. PTH increases Ca²⁺ by stimulating calcium resorption from the bones, augmenting the reabsorption of calcium in the kidneys, and by indirectly augmenting the intestinal calcium reabsorption.

2. Calcitriol: This chemical is more famously known as vitamin D₃. Calcitriol increases intestinal reabsorption of calcium. PTH increases the rate of synthesis of calcitriol, which raise Ca²⁺ concentration in the plasma which via negative feedback decreases PTH (and in turn calcitriol) production.

3. Calcitonin: They are a peptide synthesized by the C cells in the thyroid gland. Calcitonin serves to decrease Ca²⁺ concentration in the blood. This action is performed by reducing bone resorption and by facilitating renal Ca²⁺ excretion.