How is water regulated in urine

Structure and function of the kidneys and urinary tract

The urinary system includes the kidneys, ureters, bladder and urethra.


The Kidneys(ren, plural renis) are two bean-shaped organs that lie above the waist to the left and right of the spine. An adult's kidney is about 10 cm long, 5 cm wide and weighs ~ 150 g. To protect them, the kidneys are embedded in a thick layer of fat.

The kidneys are filter stations, hormone producers and blood pressure regulators. Their main job is to excrete water, minerals and metabolic waste products. The kidneys use the to filter each day Kidney corpuscles (Glomerula) ~ 170 l primary urine from the blood, of which only 1–2 l come from the body as urine (End urine) leave. The remaining fluid is returned to the bloodstream (reabsorption), otherwise you would dry out within less than an hour and lose all of the vital salts. This filtering and resorption takes place in a loop-shaped system of tiny tubes (tubules) at the end of which the concentrated urine is concentrated in the Renal pelvis collects.

Left: Position of the kidney in the abdominal cavity after removal of the digestive organs. Right: Anatomical structure with inflowing and outflowing vessels of the kidney in a longitudinal section. The kidneys are high-performance organs; the entire blood flows through the kidney corpuscles in the kidney cortex 300 times a day. The blood accumulates there and is pressed through the capillaries (Bowman's capsule). However, large molecules such as blood proteins and of course all blood cells stick to the capillary wall and are not lost to the body as a result. The resulting filtrate is the primary urine, which consists of water, blood sugar and dissolved salts (this largely corresponds to blood serum without protein). Every day ~ 170 l of primary urine are produced in the kidney corpuscles. Since it still contains a lot of essential minerals and of course water, this has to be withdrawn from it. This takes place in the renal medulla in the tubule system (tubule = tube) made up of kidney tubules and collecting tubes. The urine is then concentrated and ends up in the urinary bladder as terminal urine. Around 2 liters of terminal urine are excreted through the bladder as urine every day.
Georg Thieme Verlag, Stuttgart

The kidneys make three hormones:

  • Renin, which regulates blood pressure
  • Erythropoietin (EPO), which is needed to make red blood cells
  • Vitamin D3 as a hormone precursor that is important for the calcium balance and thus for bone metabolism.

The kidneys not only intervene in the regulation of blood pressure with the hormone renin. By returning water from the primary urine to the body, they regulate the fluid balance and thus also the blood pressure.

There are many substances in the urine that have to be eliminated. These include end products of metabolism such as creatinine, urea and, in the smallest of additions, ammonia, which gives stale urine its pungent odor. Many of the breakdown products of drugs and toxins are also excreted through the kidneys. In addition, the kidneys ensure the balance in the acid-base balance.

Urinary tract and bladder

The urine produced in the kidneys collects in the renal pelvis and flows continuously through them ureter (Ureter) for bladder (Vesica urinaria), a hollow organ that is surrounded by strong muscles and in which urine is stored for some time. If ~ 250 ml of urine has accumulated, the urge to urinate occurs, whereby the Urinary bladder sphincters (Sphincter) continue to ensure the closure of the bladder, so going to the toilet is not yet absolutely necessary.

Male urinary bladder anatomy. Above: View from above into the pelvis with the urinary bladder and the muscles that lie against the bony pelvic ring and support the bladder. Below: Vertical section through the pelvis with the urinary bladder, urethra, surrounding prostate and first section of the penis.
Georg Thieme Verlag, Stuttgart

The urge to urinate only becomes uncomfortable when the bladder is filled with 350–500 ml of urine. The bladder is usually emptied via a controllable reflex: the Urinary bladder muscle (Detrusor) contracts, at the same time the sphincter relaxes and the urine flows over the urethra (Urethra). In women, the urethra is only 2.5–4 cm short, in men, however, it extends 20–25 cm long through the entire penis. Because of the short urethra, women are more likely to suffer from cystitis. In women, ascending bacteria have a shorter path to the urinary bladder.


Dr. med. André Lauber, Dr. med. Arne Schäffler in: Gesundheit heute, edited by Dr. med. Arne Schäffler. Trias, Stuttgart, 3rd edition (2014). Revision and update: Dr. med. Sonja Kempinski | last changed on at 12:15