Ammonia formation requires the least energy but releases the most water. Urea requires more energy but is less toxic and helps conserve water. Uric acid requires the most energy to produce but can be excreted in an almost solid form, saving water. Animals have diverse excretory systems to remove nitrogenous wastes, including contractile vacuoles, nephridia, Malpighian tubules, and kidneys. The kidneys, each of which drains into a ureter, produce urine.
Ureters drain urine into the urinary bladder for storage.
Urine leaves the body through the urethra. The functional unit of the kidney is the nephron. As the resulting filtrate moves along the renal tubule, its composition is adjusted. Important materials are reabsorbed from the filtrate back to the blood and other substances are secreted into the urine. The adjustment begins in the proximal convoluted tubule.