Kidneys are the site of filtrate formation. According to John W. Kimball, a retired biology professor from the University of Harvard, nephrons in the kidneys produce urine by filtering the blood of small impurities and particles.
WebMD states that the kidneys filter the blood numerous times in a single day. They get rid of unwanted substances such as toxins and metabolism waste through filtration over millions of nephrons. Kidneys also maintain the body’s fluid content and control the body's equilibrium, in terms of electrolytes, through endocrine function, as explained by the University of Liverpool and Wikipedia.
According to WebMD, urine is created as a by-product from the filtration process. This urine is stored in the kidney’s pelvis before draining down the ureters to the bladder.