Figure 2. Physiology of aldosterone secretion and action. The figure demonstrates the renin-angiotensin-aldosterone (RAA) system and its effects on sodium and potassium homeostasis, and blood pressure. Aldosterone secretion is regulated by decreased blood volume and hyponatremia via activation of the RAA axis, and indirectly, by hyperkalemia. Aldosterone then binds to the mineralocorticoid receptor (MR; aka NR3C2) at the distal convoluted tubules and collecting duct of the kidneys. Upon binding with aldosterone, MR translocate into the nucleus, where it modulates the transcription of the genes that encode the epithelial sodium channel (ENaC). ENaC is a sodium-selective ion channel that plays a crucial role in sodium reabsorption. Aldosterone action results in urinary potassium excretion and sodium reabsorption, and thus, increased blood volume.