Calcium: the pathway in the body · Biomarker & Blood Analysis

This page shows the biochemical pathway behind the laboratory value Calcium (intracellular): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Calcium is a mineral, most of which is stored in bone. Inside cells, a brief rise in free calcium acts as a signal that switches enzymes on and off. How much stays in the blood is regulated by parathyroid hormone and calcitriol via gut, bone and kidneys.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Calcium in food → Into the gut cell TRPV6 channel · Calcitriol With small amounts, the channel TRPV6 lets calcium into the cell, and the binding protein calbindin carries it onward. Calcitriol, the active form of vitamin D, controls both. When little calcium is present, the cell forms more of both. Source 1↑ supplies This route is active and regulated: calcitriol determines how many TRPV6 channels and how much calbindin the gut cell forms. Calbindin binds the calcium on the way, so that it does not flood the cell. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of calcium arrives, less calcitriol is formed; the gut cell then forms less TRPV6 and calbindin, and this route contributes less.

   too little — If little calcium is present, calcitriol rises and the gut cell forms more TRPV6 and calbindin. If, on the other hand, little calcitriol is present, this route works only weakly.

   established physiology · Source 1

2. Into the gut cell → Calcium in blood PMCA1b, NCX1 · ATP At the back of the gut cell, the pump PMCA1b and the exchanger NCX1 move calcium into the blood. There, about half is free; the rest is bound to albumin and acids. Parathyroid hormone keeps the free fraction within narrow limits. Source 1↕ both, depending on amount The blood distributes calcium to cells, bone and kidneys. Its free fraction is kept within narrow limits: if it falls, parathyroid hormone and calcitriol make gut, bone and kidneys resupply. established physiology Source 1, 6
   ⚖ When the balance tips

   too much — If free calcium in the blood rises, the sensor CaSR detects it: the parathyroid glands release less parathyroid hormone, and in the loop of Henle the kidneys recover less calcium.

   too little — If free calcium falls, parathyroid hormone rises: the kidneys recover more, bone releases more, and via more calcitriol the gut absorbs more. If little magnesium is present, the release of parathyroid hormone is slowed.

   established physiology · Source 6, 1

3. Calcium in blood → Bone as a store · Parathyroid hormone, Calcitriol By far the largest share of calcium sits in bone. Parathyroid hormone and calcitriol control how much of it passes into the blood and how much is laid down again. When calcium in the blood falls, bone releases more. Source 1↕ both, depending on amount Bone is store and buffer at once: it lays down calcium and, under parathyroid hormone and calcitriol, releases it into the blood again when too little arrives there. established physiology Source 1
   ⚖ When the balance tips

   too much — If there is plenty of calcium in the blood, little parathyroid hormone is circulating; bone then releases less calcium into the blood.

   too little — If little calcium arrives via the gut for a longer time, parathyroid hormone maintains the level in the blood by releasing calcium from bone; the store then becomes smaller.

   established physiology · Source 1

4. Stimulus at cell surface → IP3 as messenger Phospholipase C · PIP2 Phospholipase C splits a particular lipid of the cell membrane. This releases IP3, a small messenger that moves freely through the cell interior. More IP3 opens more channels at the store. Source 2↑ supplies IP3 carries the signal from the cell membrane to the store: it binds to the IP3 receptor of the endoplasmic reticulum and opens it. established physiology Source 2
   ⚖ When the balance tips

   too much — If more IP3 forms, more IP3 receptors open; the released calcium opens neighbouring channels as well, and the signal spreads through the cell as a wave.

   too little — If little IP3 forms, only single channels open; calcium then rises only locally in small bursts.

   established physiology · Source 2

5. IP3 as messenger → Calcium rise IP3 receptor IP3 opens channels on the endoplasmic reticulum, the cell's calcium store. Calcium flows out, and its level inside the cell rises steeply for a short time. In muscle cells, this triggers contraction. Source 2, 3↑ supplies The brief rise is the actual signal: in muscle cells it triggers contraction, in nerve cells the release of messengers. How high, how long and how often it rises determines the response. established physiology Source 2, 3
   ⚖ When the balance tips

   too much — If calcium stays raised for a long time, the mitochondria pick it up; in cell culture and animal models, a sustained overload burdens them and can trigger cell death.

   too little — If calcium rises only weakly, the signal is passed on more weakly: muscle cells contract less, nerve cells release less messenger.

   observed in studies · Source 2, 3

6. Calcium rise → Calmodulin Calcium binds to calmodulin, a small protein. As a result, calmodulin changes shape and can now dock onto other proteins. This makes the calcium signal readable for enzymes. Source 3↑ supplies Calmodulin translates the calcium signal: only when loaded with calcium does it bind to enzymes and ion channels and switch them on. established physiology Source 3
   ⚖ When the balance tips

   too much — If a lot of calcium is present, the binding sites of calmodulin are occupied; more calcium then does not increase its action any further.

   too little — If calcium rises only a little, calmodulin stays largely unloaded and hardly docks onto its target proteins.

   established physiology · Source 3

7. Calmodulin → Kinases, calcineurin The altered calmodulin switches enzymes on: CaM kinases attach phosphate groups, and the phosphatase calcineurin removes them again. This turns the signal into a response of the cell. The height and duration of the signal decide which side prevails. Source 3↕ both, depending on amount Here the signal becomes a response: CaM kinases and calcineurin act against each other, the former attach phosphate, the latter removes it. Which side prevails depends on the height and duration of the signal. established physiology Source 3
   ⚖ When the balance tips

   too much — If the calcium signal lasts a long time, calcineurin stays active; it removes phosphate from the transcription factor NFAT, and NFAT moves into the nucleus and switches on genes.

   too little — If the calcium signal is weak or brief, the kinases and calcineurin are hardly switched on, and the cell's response is weak.

   established physiology · Source 3, 4

8. Calcium rise → Back into the store SERCA pump · ATP The SERCA pump moves calcium back into the endoplasmic reticulum, using ATP. The signal thus ends as quickly as it began. An empty store opens channels in the cell membrane for calcium from outside. Source 2↓ depletes The SERCA pump removes calcium from the cell interior and refills the store. In this way it ends the signal and keeps the store ready for the next rise. established physiology Source 2
   ⚖ When the balance tips

   too much — If SERCA pumps strongly, calcium in the cell interior falls faster and the signal becomes shorter; the store is quickly refilled.

   too little — If little ATP is present or SERCA works slowly, calcium stays longer in the cell interior and the store empties; an empty store opens channels in the cell membrane for calcium from outside via STIM and Orai.

   established physiology · Source 2, 4

9. Back into the store → Out of the cell PMCA, NCX · ATP Pumps and exchangers in the cell membrane move part of the calcium outside. At rest, this keeps the cell's calcium level very low; only then does a rise remain readable as a signal. NCX uses the sodium gradient for this. Source 3, 4↓ depletes Pumps and exchangers remove calcium from the cell and keep the resting level low. The exchanger NCX uses the sodium gradient for this, while the pump PMCA consumes ATP. established physiology Source 2, 3
   ⚖ When the balance tips

   too much — If the pumps move a lot of calcium out, the cell loses calcium to the outside; the store is then refilled from outside via channels in the cell membrane.

   too little — If pumps and exchangers work slowly, for example because little ATP is present or the sodium gradient is shallow, the resting level stays higher and a rise stands out less clearly.

   established physiology · Source 2, 4

Further stations

• Calcium in food — dairy products, water In food, calcium is present as a salt. It dissolves in the stomach and upper small intestine; only dissolved, free calcium can reach the gut wall at all. With large amounts, it also passes between the cells. Source 1↑ supplies Food is the only intake of calcium. How much of it arrives depends on how well it dissolves: only dissolved Ca²⁺ reaches the gut wall. established physiology Source 1
  ⚖ When the balance tips

  too much — If a lot of dissolved calcium arrives, more of it passes through the passive route between the cells; the adjustable route via TRPV6 is then scaled back.

  too little — If little calcium arrives, the body forms more calcitriol, and the active route via TRPV6 and calbindin carries a larger share of absorption.

  established physiology · Source 1

• Stimulus at cell surface — hormone or nerve impulse A hormone or a nerve impulse reaches a receptor on the cell membrane. This is where almost every calcium signal begins. The stimulus switches on phospholipase C. Source 2↑ supplies The stimulus sets the calcium signal in motion: via the receptor, phospholipase C is switched on, or channels in the cell membrane open for calcium from outside. established physiology Source 2
  ⚖ When the balance tips

  too much — If stimuli come in close succession, the calcium rises follow at shorter intervals; the cell reads the strength of the stimulus from the frequency of the spikes.

  too little — If the stimulus is absent, phospholipase C stays inactive, and the calcium inside the cell remains at its low resting level.

  established physiology · Source 2

Cofactors in this pathway

• Vitamin D — As calcitriol, controls the TRPV6 channel and the binding protein calbindin in the gut; this regulates active uptake Source 1In the ORY catalogue as a laboratory value: Vitamin D
• Magnesium — Needed for the release of parathyroid hormone, which controls exchange with bone; it also restrains calcium channels Source 6In the ORY catalogue as a laboratory value: Magnesium
• Sodium — Drives the NCX exchanger, which moves calcium out of the cell in exchange for sodium and so ends the signal Source 2In the ORY catalogue as a laboratory value: Natrium (intrazellulär)
• ATP — Energy carrier of the PMCA and SERCA pumps, which remove calcium from the cytosol and so keep the resting level low Source 2
• Inositol — Building block of PIP2, from which phospholipase C releases the messenger IP3; IP3 opens the calcium store Source 2

What acts on this pathway

• Acid-reducing drugs — Calcium carbonate needs stomach acid to dissolve. In one study, uptake from calcium carbonate was lower under a proton pump inhibitor than without one. Source 5

Sources

1. Christakos S, Li S, De La Cruz J et al. Vitamin D and the intestine: Review and update. J Steroid Biochem Mol Biol 2020 · PubMed 31655181
2. Berridge MJ, Bootman MD, Roderick HL. Calcium signalling: dynamics, homeostasis and remodelling. Nat Rev Mol Cell Biol 2003 · PubMed 12838335
3. Clapham DE. Calcium signaling. Cell 2007 · PubMed 18083096
4. Prakriya M, Lewis RS. Store-Operated Calcium Channels. Physiol Rev 2015 · PubMed 26400989
5. O'Connell MB, Madden DM, Murray AM et al. Effects of proton pump inhibitors on calcium carbonate absorption in women: a randomized crossover trial. Am J Med 2005 · PubMed 15989913
6. de Baaij JH, Hoenderop JG, Bindels RJ. Magnesium in man: implications for health and disease. Physiol Rev 2015 · PubMed 25540137

Related pathways

• Bacterial enteric organisms — Vitamin D, Natrium (intrazellulär)
• Clostridioides difficile — Vitamin D, Natrium (intrazellulär)
• GABA, glutamate and glutamine — Magnesium, Natrium (intrazellulär)
• Creatinine — Magnesium, Natrium (intrazellulär)
• Troponin and NT-proBNP — Magnesium, Natrium (intrazellulär)

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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