Uric acid requires a tremendous amount of energy to produce it. The removal of nitrogenous waste from the body fluids is tied to osmoregulation , the balance between salts and water. Osmotic uptake of water is through the gills, salt is taken up by active transport through the gills, and they have hypotonic urine. Their urine is isotonic. These organs expend metabolic energy to carry out these functions.
Every vertebrate has a pair of kidneys.
Their functional units are called nephrons . The kidney contains more than a million nephrons, which collect fluid from filtered blood. The renal arteries and renal veins carry blood to and from the kidney. The kidney filters 1600 liters of blood per day, producing 1 liter of urine. The nephron's cup shaped Bowmans Capsule surrounds a knot of blood capillaries called the glomerulus .