What is the primary function of the kidneys?

The primary function of the kidneys is to excrete water and waste products from the body. The kidneys play a crucial role in filtering blood, which involves removing excess substances, toxins, and metabolic waste from the bloodstream. They regulate the body's fluid balance by adjusting the volume of water that is excreted in urine. This filtration process is essential for maintaining homeostasis, as it helps to ensure that the body retains the necessary substances while eliminating unneeded ones.

While the kidneys do produce hormones that are important for regulating various bodily functions and help in blood pressure regulation, their most critical and fundamental role is in waste excretion. The filtration of blood and formation of urine serves to remove metabolic byproducts, such as urea and creatinine, which are created through cellular processes and need to be regularly eliminated to prevent toxicity.

Filtering oxygen from the blood is primarily the function of the lungs and not the kidneys. Therefore, focusing on the kidneys' essential role in excreting waste and maintaining fluid balance provides a clear understanding of their primary function in the human body.