The urinary system (also called the renal system) has many functions. First, it removes waste products from the body. It filters blood in the kidneys, thus producing urine. Next, it keeps the acid-base homeostasis in the body by maintaining the balance of minerals, such as sodium, potassium and calcium. Finally, this system regulates blood volume and blood pressure.

The urinary system consists of two kidneys, two ureters, the urinary bladder and the urethra.

The kidneys are pair bean-shaped organs positioned in the abdominal cavity just below the diaphragm. They are covered by tough tissue called the renal capsule and surrounded by peritoneal fat. At top of each kidney there sits an adrenal gland. The place at which the renal artery enters the kidney, and at the same time the renal vein and ureter leave is called the renal hilus. Each kidney consists of the outer portion called the cortex and the inner portion called the medulla. The medulla forms structures called renal pyramids.

The basic unit of each kidney is the nephron, which is of microscopic size. One kidney contains up to 1.5 million nephrons. These structures span both the cortex and medulla. A nephron is the functional unit of the kidneys. It is a place where blood is filtered and urine is formed.

The blood enters the kidney from the renal artery. This artery branches into arterioles and then capillaries which form glomeruli. First, blood enters a glomerulus. There, waste products, water and other substances are filtered out. This filtrate is called primary urine. It passes into the Bowman´s capsule and then into the convoluted tubule. In the convoluted tubules, water and substances that our body needs are reabsorbed. Thus, the final urine is formed and it contains urea, uric acid, inorganic salts and water. Sometimes, there is sugar, blood or pus in urine, which is a sign of an illness. The kidneys produce about 1.5 litre of final urine a day.

The final urine passes out of the kidneys through the ureters to the urinary bladder. The bladder can expand and hold up to 500 ml of urine. Around the exit from the bladder there is a sphincter muscle that prevents urine from escaping. When the bladder is full, the sphincter muscle voluntarily relaxes and we can eliminate urine through the urethra.

The urethra in females is about 4 cm long, while in men it is about 20 cm long. The male urethra goes through the prostate gland and transports also semen.