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.
Due to calcium ions having a critical role in physiological functioning, our body has a strict monitoring system in place for this ion. The total calcium ions in the body are distributed between:
The Extracellular fluid: This location is highly concentrated with Ca2+. Around half of the Ca2+ is bound to the proteins of plasma.
The Intracellular fluid: Not very concentrated. Some Ca2+ are free while others are held within the mitochondria and sarcoplasmic reticulum.
The Extracellular Matrix: The major storage of calcium in the human body is the bone. This location is vital to Ca2+ homeostasis [for storage and release].
Major source of calcium in the body comes from ingestion yet, we only absorb a third of the ingested quantity. And finally, calcium is primarily lost via urine after filtration and reabsorption by the kidney.
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.
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.
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.