Calcium homeostasis is maintained by three primary calciotropic hormones: Parathyroid hormone (PTH), 1,25-dihydroxyvitamin D3 (Calcitriol), and Calcitonin. Follicle-stimulating hormone (FSH) is a gonadotropin regulating gametogenesis.

Standard physiological ion concentrations in mammalian extracellular fluid (plasma/interstitial fluid):
$\text{Na}^+$: $\sim 135-145\text{ mmol/L}$ (140 mmol/L)
$\text{K}^+$: $\sim 3.5-5.0\text{ mmol/L}$ (4 mmol/L)
$\text{Cl}^-$: $\sim 95-105\text{ mmol/L}$ (100 mmol/L)
$\text{Mg}^{2+}$: $\sim 1.5\text{ mmol/L}$.

Antidiuretic hormone (ADH) directly drives facultative water reabsorption by inserting aquaporin-2 water channels in renal collecting ducts (A). Aldosterone promotes $\text{Na}^+$ retention in the distal nephron, which obligatorily drives osmotic water reabsorption (B). Kinins and prostaglandins (PGE2) antagonize ADH and promote natriuresis/diuresis.