Kidney Function Introduction and definition of terms: The kidneys are the main organs in the urinary system. They filter waste products out of blood from the renal artery. These are then excreted. Useful solutes are reabsorbed into the blood. They also have a major homeostatic role in the body, and help to control the water content (osmoregulation) and pH of the blood. Homeostasis is the maintenance of a constant internal environment within a living organism. Excretion is the removal