Why can't the kidneys just excrete the filtered plasma without adjusting its content?

The kidneys cannot simply excrete filtered plasma without adjusting its content because they play a crucial role in maintaining homeostasis, regulating electrolyte balance, acid-base status, and fluid volume in the body. If they merely excreted unaltered plasma, essential nutrients, ions, and water would be lost, leading to imbalances that could disrupt bodily functions. Additionally, the kidneys selectively reabsorb substances based on the body's needs, ensuring that waste products are excreted while retaining vital components. This regulatory function is essential for overall health and stability.