Which process in the kidney involves returning useful substances to the blood?

The process in the kidney that involves returning useful substances to the blood is reabsorption. During reabsorption, the nephron, which is the functional unit of the kidney, selectively absorbs water, ions, glucose, amino acids, and various other essential substances from the filtrate back into the bloodstream. This process occurs primarily in the proximal convoluted tubule and the loop of Henle, with finer adjustments made in the distal convoluted tubule and collecting duct.

Reabsorption is critical for maintaining homeostasis, as it helps regulate the composition of blood and ensures that valuable nutrients are conserved while waste products continue to be excreted. Effective reabsorption is key to preventing excessive loss of nutrients and fluids, thus playing a vital role in overall fluid and electrolyte balance in the body.