Urine is made up of approximately 95% water. The main excreta of human urine are: the urea, O sodium chloride it's the uric acid.

The elimination of urine is done through the urinary system. The organs that make up the urinary system are the kidneys and the urinary tract. At urinary tract comprise the ureter, bladder and urethra.

Our tissues, which receive nourishing substances from the blood, to the blood abandon those toxic chemical compounds that form in them as a result of the complex phenomenon of nutrition. Such substances are harmful and must be disposed of so as not to poison the organism and to endanger life. Most of these products are eliminated by urinary tract work; only a minimal part is eliminated by sweat glands by sweat.

The urinary tract has the task of separate harmful substances from the blood and dispose of them in the form of urine. It consists of the kidneys, which filter the blood and are the true active organs in the work of selecting rejection substances; the renal vials with their ureters, which lead the urine to the bladder; the bladder, which is the urine reservoir; from the urethra, the channel through which urine is led out.

Along with the rejection substances, the urinary tract also filters and eliminates water. The elimination of water is necessary either because the reject substances are dissolved in the plasma, which is mostly water, or because the amount of water in the blood and tissues must be kept constant.

Water comes into the composition of all tissues and the intercellular substance (which fills the spaces between cells): it is the universal constituent of all the "humors" of the organism and has the essential task of serving as the "solvent" of all physiologically active substances. Water enters the body with food and drink; In part it forms in the body itself as a result of the chemical reactions that take place there. Once it has performed its important functions, water must be disposed of: as it had once been the vehicle for nutritive substances, it is now the vehicle for rejection substances.

Our blood contains many substances we do not need and some may even be dangerous - excess water, minerals, dead or altered cells, and residues of cellular activity. Therefore they have to be eliminated.

The components of the urinary system are: two kidneys, two ureters, a urinary bladder and the urethra. The kidneys are the major organs of the urinary system. Located in the abdominal cavity, in the lower back, on either side of the spine and surrounded by fatty tissue, the kidneys are dark red bean-shaped organs. They are about the size of a chicken egg, measuring about 11 cm long and 6 cm wide. They weigh between 115 and 155 grams in women and between 125 and 170 grams in men. The concave side faces the spine and this is where blood vessels enter and exit, of which the renal artery and renal vein are the most important.

The kidneys extract waste products from the blood through millions of small filters called nephrons, which are the functional unit of the kidneys. Each nephron has two main parts: the glomerular capsule (or Bowman's capsule) and the renal tubules. In the figures the renal tubules are identified as proximal contorted tubule, nephew loop (Henle's loop) and distal contorted tubule. Inside the glomerular capsule penetrates a branching arteriole (branching of the renal artery), forming a tangle of capillaries called the renal glomerulus. The glomerular capsule continues into the proximal contorted tubule, which extends into a U-shaped loop called the nephew loop.

From this loop follows another contorted tubule, the distal. The set of these tubules form the renal tubules.

Urine forms in nephrons basically in two stages: glomerular filtration and the renal resorption. It is in the glomerular capsule that glomerular filtration occurs, which consists of extravasation of part of the blood plasma from the renal glomerulus to the glomerular capsule. The leaked liquid is called filtered. This filtrate contains substances useful to the body, such as water, glucose, vitamins, amino acids and various mineral salts. But it also contains toxic or useless substances like urea and uric acid. From the glomerular capsule, the filtrate passes to the renal tubules. The process in which the useful substances in the filtrate are returned to the blood is called renal resorption and occurs in the renal tubules. These useful substances that return to the blood are removed from the filter by the renal tubule cells. From there they pass to the blood capillaries that surround these tubules.

From nephrons, collected waste is sent through the ureters into the bladder. The ureters are two muscular, elastic tubes that protrude from each kidney to the bladder. The bladder is a very elastic, muscular bag with a length of approximately 30 cm where urine (filtered waste) is accumulated. This reservoir is connected to a canal - the urethra - which opens outwards through the urinary meatus, and its base is surrounded by the urethral sphincter, which can remain closed and resist the urge to urinate. Valves between the ureters and the bladder prevent urine from receding.