anatomical system renal system Evolution of vertebrate renal anatomy appears quite conservative when compared, for example, to evolution of respiratory and cardiovascular systems in vertebrates. Major anatomical changes in vertebrates kidneys separate those of birds and mammals from kidneys of lower vertebrates. General increase in animal size from fish to mammals is reflected by an increase in total number of nephrons per kidney, rather than by constant change in tubular dimensions.[well established][VHOG] FMA:7159 In humans, the renal system comprises a pair of kidneys, a pair of ureters, urinary bladder, urethra, sphincter muscle and associated blood vessels In various sources such as Encyclopedia Britannica, the excretory and urinary systems are indeed the same system (see wikipedia talk page); we merge two BTO classes here MESH:D014551 The renal system in an anatomical system that maintains fluid balance and contributes to electrolyte balance, acid/base balance, and disposal of nitrogenous waste products.. excretory system galen:UrinaryTract renal or urinary system renal system renal/urinary system systema urinaria systema urinarium urinary system urinary tract organ system subdivision