What is the primary method by which kidneys regulate body fluid balance?

The primary method by which kidneys regulate body fluid balance is through urine concentration. The kidneys play a crucial role in maintaining homeostasis, specifically in balancing the volume and concentration of bodily fluids. They achieve this by filtering blood through the nephrons, where they control the reabsorption of water and solutes.

When the body is dehydrated, the kidneys can concentrate urine by reabsorbing more water back into the bloodstream, resulting in less water loss through urine. This process is influenced by hormones like antidiuretic hormone (ADH), which signals the kidneys to retain water. Conversely, when there is excess fluid in the body, the kidneys excrete more dilute urine, helping to reduce overall body fluid levels.

This regulation of urine concentration directly affects blood volume and electrolyte balance, essential for various physiological processes. Therefore, through urine concentration, the kidneys effectively manage body fluid balance.