The human body is mostly made up of water, ranging from 75% in infants to 50-60% in adults. Water is divided into intracellular and extracellular fluid compartments. The kidneys and endocrine system work to maintain water balance and regulate fluid levels. The kidneys filter waste from the bloodstream and reabsorb useful substances, while the hormones aldosterone, ADH, and ANP work in concert to regulate sodium and water retention or excretion in response to blood volume and pressure. This precise balance of fluids is essential for cellular function, temperature regulation, cushioning and lubrication throughout the body.