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Investigation of the magnitude of renal plasma flow and blood flow
Last reviewed: 23.04.2024
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Kidney blood flow is the volume of blood passing through the kidneys per unit time (1 min). In physiological conditions, the kidneys receive 20-25% of the volume of circulating blood, i.e. The value of renal blood flow in a healthy person is 1100-1300 ml / min.
In terms of 100 g of renal tissue, the blood supply of the kidney is 430 ml / min, which is 6-10 times higher than the blood supply to the heart, brain and other organs. Such a high level of blood supply to the kidneys is determined not by the state of their metabolism, but by the appointment of the kidneys to provide a depurative function.
Blood supply to the kidney is uneven: the cortex accounts for about 80% of the blood flow, the outer zone of the brain substance - about 13%, the inner zone - 3-5% of the blood received per unit time.
In clinical practice, direct and clearance methods are used to determine the magnitude of renal blood flow. These studies are performed with a flowmeter in the presence of direct access to the kidney (in surgical practice) or determine the concentration of the test substance in the renal artery and vein according to the Fick principle.
In the clinic of internal diseases, the clearance of marker substances is used to determine the plasma blood flow, which in the process of transport through the kidneys is not filtered, but, entering the vessels of the cortical substance of the kidney, which wash the proximal segment of the nephron, enter the epithelium of the proximal renal tubules and are subsequently secreted into the nephron lumen . Since the proximal tubules are located in the cortex, the clearance of these substances gives information on the blood supply to only the kidney cortex. Since the marker substances do not enter the red blood cells, the obtained indices reflect only the amount of plasma flowing through the kidney vessels.
[Determination of effective renal plasma flow and blood flow
The clearance of such substances characterizes the effective renal plasma tissues (EPP). To calculate the effective renal blood flow (EPA), the ratio between erythrocytes and blood plasma - hematocrit (Ht) must be taken into account. Accordingly, the value of EIC is calculated by the formula:
EPA = EPP: (1-Ht).
To substances-markers, the clearance of which characterizes EPP, include para-aminoghippuric acid, hippuran and diodone. These methods of research are laborious and rather complicated, for this reason they are rarely used in the clinic. Recently, clearance methods of investigation using radionuclide preparation 1 131- hippuran were widely used to determine renal blood flow . The method is very simple, but requires the observance of special conditions necessary for working with radioactive substances. Normally, the value of EPP is 600-655 ml / min, EPA - 1000-1200 ml / min.
In physiological conditions, renal blood flow decreases with physical stress, nervous excitement, in the course of aging; increases with pregnancy, consumption of large amounts of protein, fever.
In conditions of pathology not related to kidney damage, the decrease in renal blood flow is revealed when:
- acute and chronic circulatory insufficiency: shock, hypovolemia, heart failure;
- acute diseases of the genitourinary system;
- dehydration and electrolyte disorders (hyponatremia, hypokalemia and hypercalcemia);
- with a number of endocrine diseases (pathology of the adrenal glands, hypopituitarism, myxedema).
In renal diseases, the damage to the renal perfusion is caused by damage to the renal vessels (atherosclerosis, thrombosis or embolism of the vessels, systemic vasculitis), a decrease in bcc as a consequence of primary kidney damage (with obstructive nephropathy, nephrocalcinosis, interstitial nephritis), a decrease in the number of active nephrons, papillary necrosis.
Hyperperfusion of the kidneys is observed in the early stages of diabetes, with SLE and hypervolemic variant of HC.
Determination of the filtration fraction
A great importance for the characteristic of renal hemodynamics is attached to the calculation of the filtration fraction, i.e. Fraction of the magnitude of the plasma current, which is filtered in the glomeruli per unit time (1 min). This value is calculated by the formula:
The filtration fraction = (SKFx100) / EPP (%),
Where GFR - the rate of glomerular filtration, EPP - an effective renal plasma flow.
In a healthy person, the filtration fraction is 19-20%. Its decrease characterizes the selective inhibition of the filtration function of the kidneys, a value greater than 20-22% reflects the development of hyperfiltration.
Thus, indirect evidence of hyperfiltration is the depletion of PFR (PFR <5%), the value of the filtration fraction is more than 20-22%.