Which organ is primarily responsible for maintaining fluid balance in the body?

The kidneys play a crucial role in maintaining fluid balance in the body by regulating the volume and composition of bodily fluids through processes such as filtration, reabsorption, and secretion. They filter blood to remove waste products and excess substances, which are excreted as urine. The kidneys also manage electrolyte levels, balancing sodium, potassium, and other ions that are vital for hydration and overall cellular function. Through the regulation of water reabsorption, primarily influenced by hormones like antidiuretic hormone (ADH) and aldosterone, the kidneys can either conserve or excrete water, thus directly influencing the body’s fluid balance.

While the heart is vital for pumping blood and the liver has roles in metabolism and detoxification, they do not directly manage fluid balance in the same comprehensive way as the kidneys. The pancreas primarily focuses on regulating blood sugar and digestive processes, thus it's not involved in fluid balance either. Hence, the kidneys are the key organ in preserving the homeostasis of body fluids.