The kidneys are bean-shaped organs that serve several essential regulatory roles invertebrates. They remove excess organic molecules from the blood, and it is by this action that their best-known function is performed: the removal of waste products ofmetabolism. Kidneys are essential to theurinary system and also serve homeostaticfunctions such as the regulation ofelectrolytes, maintenance of acid–base balance, and regulation of blood pressure(via maintaining the salt and water balance). They serve the body as a natural filter of theblood, and remove water-soluble wastes which are diverted to the bladder. In producing urine, the kidneys excretenitrogenous wastes such as urea andammonium. They are also responsible for the reabsorption of water, glucose, andamino acids. The kidneys also producehormones including calcitriol anderythropoietin. An important enzyme renin is also produced in the kidneys which acts in negative feedback.