The urinary system sustains homeostasis by eliminating wastes from the body, regulating blood acidity levels and controlling the levels of metabolites and electrolytes in the blood, such as sodium, potassium and calcium. The urinary system also maintains a stable internal environment by assisting the osmoregulation of blood volume and pressure.
The urinary or renal system includes the kidneys, ureter, urinary bladder and urethra. This organ system filters excess fluid out of blood as well as other substances, such as excess ions and metabolic wastes, which mainly consist of urea and uric acid. These products are filtered out of the blood and combined with water before exiting the body in the form of urine.
The main function of kidneys, the most complex organs of the urinary system, is to sustain homeostasis for optimal cell and tissue metabolism. The renal arteries provide the kidneys with blood, which then leaves through the renal vein. The blood passing through the kidneys is filtered by nephrons, which are made up of a network of blood capillaries known as the glomerulus and the renal tube.
Urine exits the kidneys from this small tube to the ureters and into the bladder. The bladder stores urine until it can be passed through the urethra and out of the body. The kidneys also play a large role in osmoregulation, which is controlled by hormones, such as the antidiuretic hormone or ADH, aldosterone and angiotensin II.
The kidneys regulate the amount of water reabsorbed by the glomerulus in osmoregulation. Osmoreceptors in a region of the brain called the hypothalamus detect water depletion and stimulate ADH release from the pituitary gland to increase the permeability of the kidneys. This allows a large proportion of water to be reabsorbed before it is lost.