REN and heart failure: Blood urea nitrogen (BUN) is a major indicator of renal function, and elevated BUN levels suggest impaired renal function, blood volume deficiency, or neurohormonal activation, which may lead to poor prognosis in patients with heart failure through oxidative stress, activation of the sympathetic nervous system (SNS) and renin-angiotensin aldosterone system (RAAS), and activation of the renin-angiotensin aldosterone system (RAAS).