The urinary system is a multicomponent organ system mainly consisting of hollow and tube-like structures that are continuous with each other and whose primary function is to produce, collect, transport, store, and expel urine from the body.

This system is also known as the excretory system because kidney-produced urine contains various products and substances, including such by-products as urea and uric acid, drugs, excess body fluid, and excess electrolytes (salts). Overall, the urinary system plays a vital role in maintaining homeostasis and appropriate blood levels of water and electrolytes.

The urinary tract consists of paired kidneys and ureters, a single urinary bladder and a urethra. All these organs can be divided into two groups - the upper urinary tract organs and those forming the lower urinary tract.

• Kidneys - upper urinary tract organs; these are the main organs that filter and modify the blood.
• Ureters - upper urinary tract organs; they conduct and transport urine from the kidneys to the bladder.
• Urinary bladder - lower urinary tract organ; it can also be simply called the bladder; it stores urine.
• Urethra - lower urinary tract organ; discharges urine from the body during urination.

As mentioned above, the urinary system is responsible for a variety of functions and physiological processes. Primarily, this system filters the blood and creates urine. It is also accountable for osmoregulation (fluid and electrolyte balance) and blood volume and pressure regulation via the renin-angiotensin-aldosterone system. Besides that, the urinary system also maintains acid-base (pH) balance in the blood.

The excretory system also eliminates various waste products (such as metabolic and toxic waste products (ammonia and urea)). Also, constant urine production and flow in the upper urinary tract and intermittent elimination from the lower urinary tract ensure that the urinary tract is cleansed and the body is getting rid of the microbes that might have entered it.

And finally, the urinary system participates in hormone production, calcium reabsorption and stimulation of red blood cell production with the help of erythropoietin. Also, the excretory system converts vitamin D to its active form of calcitriol.

Note: Some urinary system organs differ in males and females, and this 3D article also reviews these differences.