Decreased glomerular filtration rate

Introduction The amount of filtrate produced by the two kidneys per unit time is called glomerular filtration rate (GFR), which is about 125 ml/min in normal adults. The ratio of glomerular filtration rate to renal plasma flow is called the filtration fraction. The renal plasma flow rate per minute is about 660 ml, so the filtration fraction is 125/660 x 100% 19%. This result indicates that about one-fifth of the plasma flowing through the kidney is filtered into the sac by the glomerulus to produce primary urine. Glomerular filtration rate and filtration fraction are indicators of renal function. Glomerular capillary blood pressure is significantly reduced, renal vasoconstriction, filtration membrane permeability and changes in filtration area can reduce glomerular filtration rate.

Cause

Factors affecting glomerular filtration rate:

First, effective filtration pressure

Changes in any of the three factors that make up the effective filtration pressure can affect the effective filtration pressure and thus the glomerular filtration rate.

The changes of glomerular capillary blood pressure test showed that when the arterial blood pressure fluctuated within the range of 10.7-24.0 kPa (80-180 mmHg), the renal blood flow self-regulation could remain relatively stable, and the glomerular capillary blood pressure did not change significantly. Regarding the mechanism of self-regulation, most people think that when the arterial blood pressure is elevated, the smooth muscle of the wall of the arterial artery is contracted by the stretch and the blood flow resistance is increased, so that the blood flow of the glomerular capillaries is not increased, and the blood pressure is not increased. Elevated, so there is no significant change in effective filtration pressure and glomerular filtration rate; when the arterial blood pressure is lowered, the wall of the arterial wall is dilated, the blood flow resistance is reduced, and the blood flow of the glomerular capillaries is not reduced. Blood pressure did not decrease, so there was no significant change in effective filtration pressure and glomerular filtration rate. This indicates that the regulation of the glomerular filtration function of the body is achieved by self-regulation of renal blood flow to ensure the normal function of the urinary function of the body under physiological conditions. However, if the arterial blood pressure drops below 10.7 kPa (80 mmHg) (such as large blood loss), beyond the renal blood flow self-regulation range, the glomerular capillary blood pressure will decrease accordingly, so that the effective filtration pressure is reduced, glomerular filtration The rate is reduced and oliguria is caused. When the arterial blood pressure drops to 5.3 to 6.7 kPa (40 to 50 mmHg), it can cause anuria. In the advanced stage of hypertension, when the stenosis is caused by the qualitative lesion of the ball artery, the blood pressure of the glomerular capillaries can be significantly reduced, resulting in a decrease in the glomerular filtration rate, resulting in oliguria or even no urine.

Changes in plasma colloid osmotic pressure Human plasma colloid osmotic pressure does not show significant fluctuations under normal conditions. Only when the plasma protein concentration is lowered, the plasma colloid osmotic pressure is decreased, so that the effective filtration pressure and filtration rate of the glomerulus are increased, and the urine volume is increased. For example, the main reason for the increase in urine volume caused by intravenous administration of a large amount of physiological saline is that plasma protein is diluted, plasma protein concentration is lowered, and plasma colloid osmotic pressure is decreased.

Changes in renal capsule pressure are normal under normal conditions. When urinary tract obstruction occurs, such as pyeloliths, ureteral stones or tumor compression, it can cause an increase in the internal sac pressure, which reduces the effective filtration pressure and reduces the filtration rate. In addition, some drugs, such as certain sulfonamides, are easily crystallized in the acidic environment of tubule fluid, or when hemolysis is excessive in some diseases, the drug crystals or hemoglobin can block the renal tubules and cause cystic changes. The increase in pressure results in an effective filtration pressure and a decrease in filtration rate of the glomerulus.

Second, glomerular plasma flow

The glomerulus enters the ball end to the ball end, and the osmotic pressure of the plasma colloid is gradually increased, resulting in an effective filtration pressure reduction. The rate at which the plasma colloid osmotic pressure rises inevitably affects the rate of effective filtration overpressure. The rate at which plasma colloid osmotic pressure rises is closely related to glomerular plasma flow. When the plasma flow rate increases, the colloid osmotic pressure rises slowly, the effective filtration pressure decreases, and the effective length of the glomerular capillary angiogenesis filtrate increases, and the filtration rate increases; on the contrary, the glomerular plasma The flow rate is reduced, the effective length of the glomerular capillary angiogenic filtrate is shortened, and the filtration rate is reduced. Under normal circumstances, due to the renal blood flow self-regulation, glomerular plasma flow can remain relatively stable, only in the human body to exercise vigorously or in the case of large blood loss, severe hypoxia and other pathological conditions, due to increased sympathetic excitation, renal vasoconstriction, When the renal blood flow and glomerular plasma flow are significantly reduced, the glomerular filtration rate is reduced.

Third, the filtration membrane permeability and the change of filtration area

The size of the glomerular filtration membrane permeability can be measured by the molecular weight of the substance it is allowed to pass. Small molecules in plasma can easily pass through various pores on the membrane; however, macromolecular substances such as plasma albumin with a molecular weight of 69,000 are difficult to pass, and there is also a selective blocking effect of prion protein, so it is in the filtrate. The concentration does not exceed 0.2% of the plasma concentration; globulin, fibrinogen, etc. having a molecular weight of more than 69,000 cannot pass through the membrane at all. In addition, hemoglobin having a molecular weight of 64,000 in plasma can be filtered, but it is combined with globin to form a complex form and thus cannot pass. When a large amount of hemolysis occurs, the amount of hemoglobin contained in the blood exceeds the amount of binding to globin. At this time, the hemoglobin not bound to the globin can be filtered and discharged from the urine to form hemoglobinuria.

Under normal circumstances, the filtration membrane permeability is relatively stable, and only changes under pathological conditions affect the composition of urine. For example, glomerular inflammation or hypoxia is often accompanied by proteinuria. In the past, this was thought to be due to the increased permeability of the filtration membrane. Recent studies have found that the filtration membrane permeability is reduced rather than increased at this time. The appearance of proteinuria is caused by the lesion reducing or eliminating the negatively charged prion protein on the filtered membrane, and the isotropic charge repellency of the negatively charged albumin is weakened, which makes the albumin easy to filter. When the lesion causes damage to the filtration membrane, red blood cells can also filter out to form hematuria.

The total area of the glomerular filtration membrane is about 1.5 to 2 m2. Under normal circumstances, all glomeruli are active, and the filtration area remains stable. Under pathological conditions, such as acute glomerulonephritis, glomerular capillary endothelium hyperplasia, swelling, and the basement membrane is also swollen and thickened, causing stenosis or even complete occlusion of the capillary lumen, resulting in a decrease in effective filtration area and a decrease in filtration rate. There is oliguria or even no urine.

an examination

Related inspection

Glomerular filtration fraction glomerular filtration fraction (GFF) glomerular filtration rate (GFR) glomerular function test salivary creatinine

The amount of filtrate produced by the two kidneys per unit time is called glomerular filtration rate (GFR), which is about 125 ml/min in normal adults. The ratio of glomerular filtration rate to renal plasma flow is called the filtration fraction. The renal plasma flow rate per minute is about 660ml, so the filtration fraction is 125/660×100%19%. The amount of raw urine produced by adults every day and night can reach 180L, but the daily urine output is only 1~2L, showing the original urine. About 99% of the water is reabsorbed back into the bloodstream through the tubules and collecting tubes. From the comparison of ingredients, the final urine is also very different from the original urine. For example, the original urine contains glucose, and the final urine is not. The final urine contains more creatinine and ammonia than the original urine. It indicates that the original urine must pass through the role of the renal tubules and collecting ducts in order to generate terminal urine.

Differential diagnosis

Differential diagnosis of glomerular filtration rate reduction:

Abnormal renal function: early onset may be due to decreased glomerular filtration rate, decreased water and sodium retention, and a small number of patients even oliguria. Only a very small number of patients can present with acute renal failure, which is easily confused with acute nephritis.

Rapid glomerulonephritis: In addition to acute nephritic syndrome, it is often characterized by early oliguria, anuria, and a sharp deterioration of renal function. Severe acute nephritis in patients with acute renal failure is difficult to distinguish from the disease, renal biopsy should be done in time to confirm the diagnosis.

Chronic glomerulonephritis: glomerular filtration rate, endogenous creatinine clearance, blood urea nitrogen and creatinine increased, renal function staging mostly compensated or decompensated, phenol red excretion test and urine concentration dilution The function is reduced.

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