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Kidney Function
GFR, Tubular Reabsorption, and Secretion
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The kidneys keep the internal environment stable by filtering blood, returning needed substances to the body, and removing wastes into urine. A central measure of kidney function is the glomerular filtration rate, or GFR, which tells how much fluid is filtered into the nephron each minute. After filtration, the tubules carefully reabsorb water and useful solutes such as glucose, sodium, and bicarbonate. They also secrete selected substances such as hydrogen ions, potassium, and many drugs to fine tune body chemistry.
Filtration begins at the renal corpuscle, where pressure across the glomerular capillaries drives fluid into Bowman's space while cells and most proteins stay in the blood. As the filtrate moves through the proximal tubule, loop of Henle, distal tubule, and collecting duct, transport proteins and hormones adjust its composition. Reabsorption reduces the large filtered load to a small final urine volume, while secretion adds extra wastes and ions from blood into the tubular fluid. These processes are tightly regulated by blood pressure, autoregulation, aldosterone, antidiuretic hormone, and acid base balance.
Key Facts
- Normal GFR in a healthy young adult is about 90 to 120 mL/min/1.73 m^2.
- Filtration fraction = GFR / renal plasma flow, and is normally about 0.2.
- Net filtration pressure = P_GC - P_BS - pi_GC, where P_GC is glomerular capillary hydrostatic pressure.
- Filtered load = GFR x plasma concentration.
- Excretion rate = filtration + secretion - reabsorption.
- Creatinine clearance approximates GFR: C = (U x V) / P.
Vocabulary
- Glomerular filtration rate
- The volume of fluid filtered from glomerular capillaries into Bowman's space each minute.
- Tubular reabsorption
- The movement of water or solutes from the tubular fluid back into the blood.
- Tubular secretion
- The movement of substances from peritubular capillaries into the tubular fluid for excretion.
- Clearance
- The virtual volume of plasma completely cleared of a substance per unit time.
- Autoregulation
- The kidney's ability to keep renal blood flow and GFR relatively stable despite changes in arterial pressure.
Common Mistakes to Avoid
- Confusing filtration with reabsorption, which is wrong because filtration moves fluid from blood into Bowman's space while reabsorption moves substances from the tubule back into blood.
- Assuming all filtered substances are excreted, which is wrong because many useful molecules such as glucose and much of the filtered water and sodium are normally reabsorbed.
- Using serum creatinine alone as a direct measure of GFR, which is wrong because creatinine depends on muscle mass, production rate, and tubular handling as well as filtration.
- Forgetting the sign in the excretion equation, which is wrong because secretion increases excretion but reabsorption decreases it, so excretion = filtration + secretion - reabsorption.
Practice Questions
- 1 A patient has a GFR of 100 mL/min and a plasma glucose concentration of 0.9 mg/mL. What is the filtered load of glucose in mg/min?
- 2 Urine creatinine concentration is 120 mg/dL, urine flow rate is 1.5 mL/min, and plasma creatinine is 1.2 mg/dL. Calculate creatinine clearance in mL/min.
- 3 A drug is freely filtered at the glomerulus. Its excretion rate is greater than its filtered load. What does this tell you about tubular transport of the drug, and why?