Calcium and phosphate are the body’s two main mineral ions. Calcium is used as a signal inside cells and is stored as the mineral of bone, while phosphate is part of bone mineral, of energy metabolism and of many intracellular reactions. Calcium circulates in the extracellular fluid (the fluid outside cells, including the plasma) within narrow limits that keep nerves, muscles and bone working, and even small departures disturb those tissues. Phosphate is not held as tightly: it is buffered in the body and regulated more loosely, so it varies with the diet through the day.
The system that holds them there has two layers. The first is the traffic of the ions themselves: where they are stored, and how the gut and kidney gain and lose them. The second is the set of hormones that adjust that traffic — parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), vitamin D and calcitonin — acting on bone, kidney and intestine.
- Calcium Balance: Distribution, Absorption and Excretion — where the body’s calcium is, the concentration gradient between the cytosol and the extracellular fluid, the plasma fractions and adjusted calcium, intestinal absorption, and renal handling with its modifiers.
- Phosphate Homeostasis — the body’s phosphate distribution, the two forms present at blood pH, intestinal absorption, and renal reabsorption and urinary buffering.
- Parathyroid Hormone (PTH) — the calcium-sensing receptor and the feedback loop it runs, PTH’s synthesis and actions on bone, kidney and calcitriol production, its receptors, and the response to hypocalcemia over time.
- Fibroblast Growth Factor 23 (FGF23) — the bone-derived hormone that promotes phosphate excretion and inhibits calcitriol formation, including its processing, receptor and Klotho co-receptor, and the stimuli that release it.
- Vitamin D and Calcitonin — vitamin D’s two forms, its activation to calcitriol, and calcitriol’s effects on calcium and phosphate, together with calcitonin, the thyroid hormone that lowers plasma calcium.
- Parathyroid Hormone-Related Protein (PTHrP) — the peptide that activates the PTH receptor in many tissues, and the hypercalcemia of malignancy it causes when tumors secrete it.
Read them in the order listed, beginning with calcium balance to see where the body’s calcium is and how it is gained and lost.