REN and hydrops fetalis: In the HF setting, the index event on heart function initiates the activation of compensatory mechanisms, including activation of the sympathetic nervous system (SNS) and the renin–angiotensin aldosterone system (RAAS), which are responsible for maintaining cardiac output through increased salt and water retention via reabsorption through the kidneys, peripheral arterial vasoconstriction, increased contractility, and inflammatory mediators that are responsible for mediating cardiac repair and remodeling.