24.7 Evaluating Kidney Function.pdf

24.7 Evaluating Kidney Function Determining the effectiveness of kidney function is clinically necessary, given the critical role the kidneys play in regulating concentrations of various substances within the blood and in eliminating wastes and foreign substances. This monitoring is especially cruci...

24.7 Evaluating Kidney Function Determining the effectiveness of kidney function is clinically necessary, given the critical role the kidneys play in regulating concentrations of various substances within the blood and in eliminating wastes and foreign substances. This monitoring is especially crucial while evaluating and treating kidney disease. 24.7a Measuring Glomerular Filtration Rate LEARNING OBJECTIVE 44. Describe how glomerular filtration rate is measured. One way to assess kidney function is to measure the rate filtrate is formed per unit time—that is, the glomerular filtration rate (GFR). To conduct this test, an individual is injected with inulin, a polysaccharide derived from plants that is freely filtered and neither reabsorbed nor secreted in the kidney, so the amount of inulin in the urine is equal to the amount that is filtered. (Inulin should not be confused with the hormone insulin, which is secreted by the pancreas and regulates blood glucose levels.) Enough inulin is injected into a subject to achieve a blood plasma concentration of 1 mg/mL. Urine is collected and measured for volume and concentration of the inulin. Additionally, blood is drawn and the plasma concentration of inulin is measured at given time intervals. Glomerular filtration rate is determined by the following formula: where U = concentration of inulin in urine, V = volume of urine produced per minute, and P = concentration of inulin in plasma. For example, if the concentration of inulin in urine = 125 mg/mL, the volume of urine = 1 mL/min, and the concentration of inulin in plasma = 1 mg/mL, then Page 979 Normal adult GFR is 125 mL/min. A lower glomerular filtration rate indicates a decrease in kidney function, and thus it is more likely that nitrogenous wastes and other unwanted substances are accumulating in the blood. WHAT DID YOU LEARN? 34 What is the purpose of measuring the glomerular filtration rate? 24.7b Measuring Renal Plasma Clearance LEARNING OBJECTIVES 45. Define renal plasma clearance and its importance. 46. Identify the substance that is normally produced within the body that may be measured to estimate the glomerular filtration rate. Another means of assessing kidney function is by measuring renal plasma clearance. The renal plasma clearance test measures the volume of plasma that can be completely cleared of a substance in a given period of time—usually in 1 minute. We may infer from this test whether a substance is reabsorbed or secreted. If a substance (like inulin) is neither reabsorbed nor filtered, its renal plasma clearance is equal to the GFR (125 mL/min). However, if a substance is reabsorbed, its renal plasma clearance is lower than GFR because less of the substance is excreted, or “cleared,” in the urine. For example, the renal plasma clearance of urea is 70 mL/min. If urea is filtered at a rate of 125 mL/min (the normal GFR), and only 70 mL/min is cleared, the rest (55 mL/min) is reabsorbed. In contrast, the renal plasma clearance of glucose is normally 0 mL/min because in a healthy individual 100% of the glucose is reabsorbed and none is excreted in the urine. Substances that are filtered and secreted have renal plasma clearance values higher than the GFR. This occurs because additional amounts of the substance are secreted into the tubular fluid and excreted in the urine. For example, creatinine (a nitrogenous waste formed from creatine breakdown) has a renal plasma clearance of 140 mL/min, indicating that the substance is both filtered and secreted. INTEGRATE CLINICAL VIEW 24.5 Renal Failure, Dialysis, and Kidney Transplant Renal Failure Renal failure refers to greatly diminished or absent renal function caused by the destruction of about 90% of the kidney. Renal failure often results from a chronic disease that affects the glomerulus or the small blood vessels of the kidney, as a result of autoimmune conditions, high blood pressure, or diabetes. Once the kidney’s structures have been destroyed, they do not regenerate or begin functioning again. Thus, the two main treatments are dialysis and kidney transplant. Dialysis The term dialysis (dī-al΄i-sis) comes from a Greek word meaning “to separate agents or particles on the basis of their size.” Two forms of dialysis are commonly used today: peritoneal dialysis and hemodialysis. In peritoneal dialysis, a catheter is permanently placed into the peritoneal cavity, to which a bag of dialysis fluid may be attached externally. A volume of a special dialysis fluid is introduced, and the harmful waste products in the blood are transferred, or dialyzed, into the fluid across the peritoneal membrane. After several hours, the fluid is drained from the peritoneal cavity and replaced with fresh fluid. In hemodialysis, the patient’s blood is cycled through a machine that filters the waste products across a specially designed membrane. The patient must remain stationary for the time it takes to cycle the blood through the dialysis unit while metabolic waste products are removed. Hemodialysis must be performed three or four times a week, and each treatment takes about 4 hours. Kidney Transplant A kidney transplant from a genetically similar person (“matched” for major histocompatability complex; see section 22.4c) may successfully restore renal function. The kidney is generally removed from the donor by a laparoscopic procedure. A recent development for obtaining the kidney is to remove the kidney through the umbilicus (navel, or belly button) after making a single small incision. This decreases the donor’s recovery time from approximately 3 months (with opening of the abdominopelvic cavity) to just under a month. Location of a transplanted kidney in the abdominopelvic cavity. The replacement kidney is attached to an artery and a vein in the inferior abdominopelvic region, where it is relatively easy to establish a vascular connection. The new kidney rests either on the superior surface of or immediately lateral to the urinary bladder. Because a pelvic artery and vein connect the donor kidney to the patient’s blood supply, having the kidney near the bladder means only a short segment of ureter is needed for the bladder connection. The diseased kidney is not removed. Because the transplanted kidney is a foreign tissue, immunosuppressant drugs are regularly administered to suppress the immune system’s activity (see Clinical View 22.5: “Organ Transplants and MHC Molecules”). Page 980 Many medications also have renal plasma clearances higher than the GFR because they are secreted. For this reason, it is important to determine renal plasma clearance to know the amount and timing of drug dosage. The higher the renal plasma clearance, the more often the medications must be given to maintain therapeutic levels. Note that in clinical practice, renal plasma clearance of creatinine can be used to approximate glomerular filtration rate because its clearance is only slightly higher than GFR. Measuring creatinine clearance avoids the need to inject inulin into the patient’s blood. WHAT DID YOU LEARN? 35 What information is gained by measuring the renal plasma clearance for a specific substance (e.g., medication)?