The urinary system is also referred to as the excretory system. It should not, however, be confused with the process of “elimination” which is a part of the digestive system. Excretion refers to the removal of toxic substance from the blood that accumulate during cellular processes.
The kidneys have the primary function of filtering these substances and removing them from the blood. Other structures have support roles, such as the ureters which transfer urine to the bladder where it is stored. In some organisms, the urinary system is closely tied to the reproductive system and may share some of the same structures. In humans, for instance, the urethra is the opening for urine and sperm (only in males). In reptiles and birds, the cloaca is an opening for all eliminated substances, including urine, solid wastes, and eggs.