The urinary system consists of the kidneys, ureters, bladder, and urethra. The kidneys filter blood to produce urine and are located retroperitoneally in the upper abdomen. Each kidney contains nephrons, the functional filtering units, which are made up of a glomerulus for blood filtration and a tubule for reabsorption and secretion. Urine is produced as it passes through the nephron and is carried from the kidneys by the ureters to the bladder for storage and then expelled through the urethra. Hormones regulate fluid and electrolyte balance to maintain homeostasis.