Urination is the process of formation and excretion of urine. Urinary secretion is an important function of the body - maintaining the constancy of the content of various substances inside the body, and in particular in the blood (water balance, osmotic pressure, pH, etc.), and the excretion of end products of metabolism.
Usually urination is expressed by the volume of urine in ml excreted per day (daily diuresis). During the day, an adult person under normal conditions produces 1000–2000 ml of urine.
Urinary excretion during the day is uneven. The main part of the urinary tract (4/5) falls on the daytime period, the rest - overnight. Sometimes with heart failure, diabetes insipidus, etc., there is a change in these ratios and the main part of the urine falls on the night period (nocturia).
Allocation of urine by the kidneys occurs although unevenly, but continuously.
In the renal glomeruli, filtration of the liquid part of the plasma occurs, except for the co-molecular protein and the formed elements, and the formation of primary urine (150-170 liters per day). The flow of primary urine into the glomeruli is determined by the hydrostatic pressure of the blood in the transporting vessels, as well as by the selective permeability of the glomerular epithelium .Go
In the renal tubules, the processes of reuptake (reabsorption) of glucose, water, sodium, potassium, calcium , chlorine and other substances occur, resulting in the final urine. Reabsorption processes are determined by the laws of osmosis, diffusion and selective permeability of the tubular epithelium.
The main regulator of water reabsorption in the kidneys is antidiuretic hormone (ADH), produced by the anterior nuclei of the hypothalamus and accumulated in the posterior lobe of the pituitary gland. ADH enters the blood every time under the influence of an increase in the osmotic pressure of blood plasma due to the loss of water by the body or its insufficient flow, as well as under the influence of pain, anesthesia, trauma and other factors. In the kidneys, ADH stimulates the secretion of the enzyme hyaluronidase, which destroys hyaluronic acid in the wall structures of the distal nephron (collecting tubes). As a result, their permeability to water increases significantly and water is strongly absorbed from the tubules into the blood. With the abundant flow of water into the body, the secretion of ADH is nada and thus the reabsorption of water in the kidneys is reduced and its excretion in the urine increases.
Tubular processes can also occur in the tubules (for example, urea, with its significant increase in blood).
A decrease in urination (oliguria) can be caused by restricting the introduction of water into the body, an increase in extrarenal excretion of water through the skin, lungs, etc., as well as a decrease in the intake of salts in the body and a decrease in the formation of osmotically active metabolic products. In cases of pathology, there may be a complete cessation of urination (see Anuria) or a significant increase in it (see Polyuria ).
Clinical methods for examining urine are to measure the amount of urine released during a certain time. Special functional tests are also used. For example, the glomerular filtration rate is determined by the administration of inulin or mannitol, as well as by creatinine.
Considering that the concentration of these substances in blood plasma, glomerular filtrate and final urine is equal, the formula is as follows: С · P = VU (С is the filtrate volume, Р is the concentration of inulin in plasma, V is the amount of urine per 1 minute, U is the concentration inulin in urine); hence C = VU / P characterizes the volume of blood plasma that is cleared of inulin in 1 min. (the so-called cleansing is normally 100-120 ml / min).
Tubular secretion is determined by administering dioderast or sodium para-aminogippuric acid. Calculated by the formula S = V · W — Q · P (S is the amount of diode growth released with secreted urine, V is the amount of urine per 1 min., W is the concentration of diorast in the urine, Q is the volume of primary urine, P is the concentration of diode growth in primary urine).