Answer:
The kidney is pretty much an organic filter. As the blood in a body rushes through the arteries, it makes its way around the entire body. When it flows through the renal artery, then that blood will end up at the kidney. The first stage of the kidney process, is to filter out the blood cells and protein molecules by pushing the bodily fluids through the glomerulus (filter) which allows water, glucose, salts, and urea to pass, but does not allow the blood cells and protein to pass, as they are too big. The blood cells and proteins the flow the the capillaries around the renal tubule (the part of the kidney that the rest of the fluid flows through) in order to enter back into the rest of the body through the renal vein. As the remaining fluid filters through the renal tubule, the glucose and most of the the water and salts leave through the capillaries in the renal tubule in order to join the blood in the renal vein and keep it enriched. What is left is some water, salts, urea, and other undesired substances which continue to flow through the renal tubule until they enter the bladder and leave the body in the form of urine.