Aldosterone

Most important adrenal cortex hormone for the regulation of salt metabolism (mineralocorticoid).

Its biosynthesis is mediated by the adrenocorticotropic hormone (ACTH) from the anterior pituitary. Aldosterone stimulates Na⁺-reabsorption and K⁺-secretion in the salt-transporting cells of the kidney and leads to retention of Na⁺ and increased excretion of K⁺ throughout the body.

The secondary result is water retention, and thus an increase in extracellular volume.

https://en.wikipedia.org/wiki/Aldosterone