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Glomerular Filtration and Renal Blood Flow
3
GLOMERULAR FILTRATION AND RENAL BLOOD FLOW
O B J E C T I V E S Upon completion of this chapter, the student should be able to answer the following questions: 1. How can the concepts of mass balance be used to measure the glomerular filtration rate? 2. Why can inulin clearance and creatinine clearance be used to measure the glomerular filtration rate? 3. Why is the plasma creatinine concentration used clinically to monitor the glomerular filtration rate? 4. What are the elements of the glomerular filtration barrier, and how do they determine how much protein enters Bowman’s space?
T
he first step in the formation of urine by the kidneys is the production of an ultrafiltrate of plasma across the filtration barrier. The process of glomerular filtration and regulation of the glomerular filtration rate (GFR) and renal blood flow (RBF) are discussed in this chapter. The concept of renal clearance, which is the theoretical basis for the measurements of GFR and RBF, also is presented.
RENAL CLEARANCE The concept of renal clearance is based on the Fick principle (i.e., mass balance or conservation of mass).
5. What Starling forces are involved in the formation of the glomerular ultrafiltrate, and how do changes in each force affect the glomerular filtration rate? 6. What is autoregulation of renal blood flow and the glomerular filtration rate, and which factors and hormones are responsible for autoregulation? 7. Which hormones regulate renal blood flow? 8. Why do hormones influence renal blood flow despite autoregulation?
Figure 3-1 illustrates the various factors required to describe the mass balance relationships of a kidney. The renal artery is the single input source to the kidney, whereas the renal vein and ureter constitute the two output routes. The following equation defines the mass balance relationship: ˙ Pax × RPFa = (Pvx × RPFv ) + (Ux × V)
(3-1)
where Pax and Pvx are concentrations of substance x in the renal artery and renal vein plasma, respectively, RPFa and RPFv are renal plasma flow (RPF) rates in the artery and vein, respectively, Ux is the concentration of x in the urine, and V˙ is the urine flow rate. This relationship permits the quantification of the amount of x excreted in the urine versus the amount returned to the systemic circulation in the renal venous 27
GLOMERULAR FILTRATION AND RENAL BLOOD FLOW
O B J E C T I V E S Upon completion of this chapter, the student should be able to answer the following questions: 1. How can the concepts of mass balance be used to measure the glomerular filtration rate? 2. Why can inulin clearance and creatinine clearance be used to measure the glomerular filtration rate? 3. Why is the plasma creatinine concentration used clinically to monitor the glomerular filtration rate? 4. What are the elements of the glomerular filtration barrier, and how do they determine how much protein enters Bowman’s space?
T
he first step in the formation of urine by the kidneys is the production of an ultrafiltrate of plasma across the filtration barrier. The process of glomerular filtration and regulation of the glomerular filtration rate (GFR) and renal blood flow (RBF) are discussed in this chapter. The concept of renal clearance, which is the theoretical basis for the measurements of GFR and RBF, also is presented.
RENAL CLEARANCE The concept of renal clearance is based on the Fick principle (i.e., mass balance or conservation of mass).
5. What Starling forces are involved in the formation of the glomerular ultrafiltrate, and how do changes in each force affect the glomerular filtration rate? 6. What is autoregulation of renal blood flow and the glomerular filtration rate, and which factors and hormones are responsible for autoregulation? 7. Which hormones regulate renal blood flow? 8. Why do hormones influence renal blood flow despite autoregulation?
Figure 3-1 illustrates the various factors required to describe the mass balance relationships of a kidney. The renal artery is the single input source to the kidney, whereas the renal vein and ureter constitute the two output routes. The following equation defines the mass balance relationship: ˙ Pax × RPFa = (Pvx × RPFv ) + (Ux × V)
(3-1)
where Pax and Pvx are concentrations of substance x in the renal artery and renal vein plasma, respectively, RPFa and RPFv are renal plasma flow (RPF) rates in the artery and vein, respectively, Ux is the concentration of x in the urine, and V˙ is the urine flow rate. This relationship permits the quantification of the amount of x excreted in the urine versus the amount returned to the systemic circulation in the renal venous 27