Body fluids pass back into the body, nitrogenous wastes empty into the insect's gut. Water is reabsorbed and waste is expelled from the insect. Images from W.H. Freeman and Sinauer Associates, used by permission. ALL vertebrates have paired kidneys. Excretion is not the primary function of kidneys.
Kidneys regulate body fluid levels as a primary duty, and remove wastes as a secondary one.
The urinary system is made-up of the kidneys, ureters, bladder, and urethra. The nephron, an evolutionary modification of the nephridium, is the kidney's functional unit. Waste is filtered from the blood and collected as urine in each kidney. Urine leaves the kidneys by ureters , and collects in the bladder . The bladder can distend to store urine that eventually leaves through the urethra .