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There are 3 stages of how the kidney filters urea and other substances out of the blood.
Blood travels through the renal artery and reaches to the glomerulus. The blood contains urea (solid made in the liver where amino acids react with co2 to make urea) ,water,glucose,red blood cells and proteins. The glomurulus is a network of capillaries and is present inside the bowmans capsule. The bowmans capsulle has tiny holes to let small molecules pass through like urea,glucose,water. These small molecules then diffuse into it and go through the convoluted tubules and henlest tube. The larger molecules like the red blood cells and glucose stay in the blood and are taken by the renal vein towards the heart.
2) Selective reabsorption of glucose and excess water
While the fluids flow through the convoluted tubules some useful substances like glucose and the water get reabsorbed back into the blood. Almost all glucose and all water molecules are reabsorbed. This is called selective reabsorption. The amount of water reabsorbed depends on how much the body needs. This is called osmoregulation
3) Removal of excess water in urine
At the end of the nephron the urea and remaining water go to the ureter (it takes urine from kidneys to bladder). The urea dissolves in the water to form urine.