Kidney function and methods of evaluation
Last reviewed: 23.04.2024
All iLive content is medically reviewed or fact checked to ensure as much factual accuracy as possible.
We have strict sourcing guidelines and only link to reputable media sites, academic research institutions and, whenever possible, medically peer reviewed studies. Note that the numbers in parentheses ([1], [2], etc.) are clickable links to these studies.
If you feel that any of our content is inaccurate, out-of-date, or otherwise questionable, please select it and press Ctrl + Enter.
Kidneys play an important role in the body, performing numerous functions. There are different methods for assessing them.
The kidneys perform several functions: the depurative and endocrine, carry out the maintenance of homeostasis.
Homeostasis of the kidney is maintained through volumoregulation (maintenance of blood volume and extracellular fluid), osmoregulation (maintaining a stable concentration of osmotically active substances in the blood and other body fluids), maintaining the constancy of the ionic composition of the blood by regulating the excretion of electrolytes and water and regulating the acid-base state CBS).
The depurational function consists in excretion of the final products of nitrogen metabolism (mainly urea), foreign substances (toxins and drugs) and excess organic substances (amino acid, glucose).
The endocrine function is the production and secretion of kidney enzymes and hormones:
- renin, which plays an important role in the regulation of water-salt balance and blood pressure;
- erythropoietin, stimulating erythropoiesis;
- active form of vitamin D - one of the main regulators of calcium and phosphorus in the body.
Assessment of homeopathic and depurative renal function
To assess the main functions of the kidneys use different methods of research:
- biochemical (determine the serum level of creatinine, uric acid, urea, sodium, potassium and other electrolytes);
- examination of urine;
- special methods, which include primarily methods of clearance (clearance);
- load tests (sample for concentration and dilution of urine, a sample with glucose load, protein, ammonium chloride, etc.);
- radioisotope studies (radioisotope renography, scintigraphy).
Great importance is attached to the determination of the size of the kidneys with the help of ultrasound, the introduction of contrast and isotope compounds, which allows us to determine the tactics of therapeutic measures.
Parameters of primary importance are the level of serum creatinine, the relative density of urine in a single analysis and / or Zimnitsky's trial, and the size of the kidney.
Serum creatinine is the final product of protein metabolism. It is synthesized in the body at a relatively constant rate and is excreted only by the kidneys (mainly by glomerular filtration, which is secreted to a very small extent in the proximal tubules). Its level in physiological conditions depends on the severity of muscle mass. Normally, the serum creatinine concentration is 0.062-0.123 mmol / L). Creatinine clearance is used to determine GFR.
Relative density of urine in a single analysis and / or Zimnitsky's sample more than 1018 g / l testifies to the preserved function of the kidneys.
Normal size of the kidney (length from 10 to 12 cm, width from 5 to 7.5 cm and thickness of 2.5-3 cm) indicate the absence of severe sclerotic processes.
With the development of renal insufficiency, the serum creatinine level exceeds 0.123 mmol / l, the relative density of urine decreases (less than 1018 g / l) and the size of the kidneys decreases. In addition to an increase in creatinine in the blood, renal failure may be associated with a decrease in GFR, an increase in the concentration of uric acid, urea, residual nitrogen, or urea nitrogen in the blood serum. In this situation, the decrease in urinary excretion of urine and creatinine is also important.
Methods for assessing the clearance of individual substances
These methods provide more accurate information on the state of renal function. Quantitatively, the clearance of the substance is the volume of blood (in milliliters), which, when passed through the kidneys per unit time (1 min), is completely cleared of this substance.
The clearance of the substance (X) is calculated by the formula:
C x = (U x x V): P x,
Where C x is the clearance of substance X, U x is the concentration of substance X in the urine, P x is the concentration of substance X in the blood, V is the minute diuresis. The clearance of the substance is expressed in ml / min.
The clearance method is used to calculate GFR, the magnitude of renal plasma flow, and the study of osmoregulatory renal function. The results obtained should be equated to the standard body surface - 1.73 m 2.
In recent years, a number of formulas have appeared that give an opportunity to evaluate GFR, as well as the state of the transport of sodium and potassium in certain segments of the nephron, which is important both for clarifying the localization of the pathological process in the kidneys and for determining the site of action of individual pharmacological preparations.
Study of the self-regulating function of the kidneys
Osmo-regulating function of the kidneys is assessed by their ability to concentrate and dilute the urine. In clinical practice, the following indicators are used to characterize the osmoregulatory function of the kidneys:
- relative density of urine in a single analysis;
- Zimnitsky's trial (determination of fluctuations in the relative density of urine during the day);
- osmolality of whey and urine with calculation of concentration coefficient, excreted fraction of osmotically active substances, clearance of osmotically free water and reabsorption of osmotically free water.
[