Anatomy and Physiology Resource Site

14.1 - Functions of the Urinary System

Fluid composition balance

The urinary system balances the composition of body fluids (blood, intracellular fluid and extracellular fluid). Each type of body fluid has different solutes whose concentrations are controlled partially by the urinary system.

The urinary system controls body fluid balance by three main processes: filtration, reabsorption and secretion.

Filtration - Blood pressure forces fluid and some solutes (called the filtrate) from the blood into the nephron of the kidneys
Reabsorption - Substances are recovered from the filtrate back into the blood, either by active or passive transport
Secretion - Substances are actively or passively moved from the blood into the filtrate for excretion

Several substances are regulated by excretion from the urinary system.

Acids and bases - pH affects protein function

Nitrogenous wastes - Urea and uric acid are toxic to body tissues

Foreign substances - pharmaceuticals, toxins

Production and secretion by the kidneys

The kidneys produce and activate important chemical signals used to control other body functions.

Erythropoietin - Produced by the kidneys; promotes red blood cell production
Renin - Produced by the kidneys; promotes salt retention and increased blood pressure
Vitamin D - Activated by the kidneys; important in bone and tooth health