Renal Physiology 3: Renal Clearance

1. Renal Physiology 3: Renal Clearance Ahmad Ahmeda aahmeda@ksu.edu.sa Cell phone: 0536313454

2. Learning Objectives • Describe the concept of renal plasma clearance. • Use the formula for measuring renal clearance. • Use clearance principles for inulin, creatinine etc. for determination of GFR. • Explain why it is easier for a physician to use creatinine clearance Instead of Inulin for the estimation of GFR. • Describe glucose and urea clearance. • Explain why we use of PAH clearance for measuring renal blood flow.

3. Glomerular Filtration Rate

4. Substances Used to Measure GFR • Inulin, a polymer of fructose, is used in research to precisely measure GFR – Freely filtered into the Bowman’s capsule – Not reabsorbed, secreted or metabolized by the nephron – Non-endogenous, has to be infused intravenously • Assume: – [Inulin]urine = 30 mg/ml – [Inulin]plasma = 0.5 mg/ml – urine flow rate = 2 ml/min • GFR = 120 ml/min or 172.3 L/day

5. Substances Used to Measure GFR • Clinically, creatinine, endogenously released into plasma by skeletal muscle, is used to measure GFR – Not as accurate as inulin as a small quantity is secreted into the proximal tubule – amount excreted > amount filtered – Reasonably accurate measurement of GFR

6. Substances Used to Measure GFR • The usual analytical method for creatinine measurement (alkaline picrate method) also detects substances in the plasma other than true creatinine, leading to increase in plasma creatinine value. • Thus, these two errors usually cancel each other and gives a correct estimate of GFR.

8. Glomerular Filtration Rate • Measurement of creatinine concentration in a urine sample, urine flow rate and plasma creatinine concentration can be used to determine GFR

9. GFR • Only 15 – 20 % of plasma entering glomerulus filtered • Composition of filtrate: Similar to plasma BUT NO large proteins or cells • Determined by filtration barrier: - size: < 20 Å - between 20 – 40 Å depends on charge - electrical charge: -ve charged proteins not filtered (i.e. most plasma proteins) - > 42 Å not filtered

10. Regulation of GFR & RBF • IntrinsicAutoregulation: - Renal vasculature also exhibits a well developed intrinsic ability to adjust its resistance in response to changes in arterial BP and thus to keep BF and GFR essentially constant = autoregulation. - In man, effective over a range of MBP from 75-160mmHg. Below 75mmHg, filtration falls and ceases altogether when MBP = 50mmHg.

11. Regulation of GFR & RBF • If mean arterial P , there is an automatic  in afferent arteriolar constriction, preventing a rise in glomerular pressure. Dilatation occurs if P falls. • Autoregulation is independent of nerves or hormones, occurs in denervated and in isolated perfused kidneys. • 2 mechanisms are responsible for the autoregulation:

12. 1) Myogenic mechanism • normal response of vascular smooth muscle • that is, increased stretch due to pressure rise depolarises the cells, calcium enters and causes a vasoconstriction • well developed in the kidney

13. 2) Tubuloglomerular feedback • [NaCl] dependent mechanism • macula densa cells in JGA detect [NaCl] send signals to afferent arteriole • e.g.  GFR =  [NaCl] filtrate • sensed by JGA  arteriole constricts • (resistance  blood flow) mediator unknown ?? Adenosine/Renin

15. Sympathetic Regulation of GFR • Stimulates vasoconstriction of afferent arterioles. • Preserves blood volume to muscles and heart. • Cardiovascular shock: • Decreases glomerular capillary hydrostatic pressure. • Decreases urine output (UO).

16. Extrinsic Regulation • Neurogenic factors • Sympathetic Nerve Fiber: is the major NF to kidney. Stimulation of sympathetic NF causes renal vasoconstriction and results in decrease of RBF and GFR. • There are some parasympathetic NF to efferent arterioles, most predominantly to juxtamedullary nephrons and sphincters of vasa recta. Stimulation of parasympathtic NF causes renal vasodilation and results in increase in RBF and GFR.

17. Extrinsic Regulation 2) Humoral and pharmacological factors: • Epinephrine, Nor-Epinephrine, Angiotensin II, Prostaglandin (F), and Thromboxane cause renal vasoconstriction and results in decrease in RBF and GFR. • Acetylcholine, Bradykinin, Prostaglandin (D, E, and I), and bacterial pyogens cause renal vasodilation and results in increase in RBF and GFR.

18. Extrinsic Regulation 3) Physiological Stress: cold, deep anesthesia, fright, sever exercise, hypoxia and ischemia stimulate sympathetic NF leading to renal vasoconstriction and decrease in RBF. 4) Posture: RBF increase in supine than sitting than standing. Changing the posture from lying to standing leads to a decrease of about 15% in RBF due to the stimulation of sympathetic NF.

19. Normal Afferent arteriolar constriction Efferent arteriolar constriction Efferent arteriolar dilatation Afferent arteriolar dilatation

20. Plasma Clearance Definition: • The clearance value of a certain substance  (means the vol. of plasma which is cleared from this substance by the kidney (in urine) /min. Calculation: • It is calculated by applying the formula U x V / P where: • (V) = Vol. of urine (ml) /min. • (U) = Conc. of the substance mg/ml urine. • (P) = Conc. of the substance mg/ml plasma.

21. Plasma Clearance Any exogenous substance used in plasma clearance tests should have the following properties: • Stays in the plasma i.e. does not enter the RBC’s. • Does not affect the renal functions. • Not metabolized by the kidney. • Easily measured in plasma & urine. • Non toxic.

22. Renal Clearance • If the substance is freely filtered at the glomeruli and is not reabsorbed, secreted or metabolized in the nephron, then – amount filtered per minute = amount excreted per minute – [sub]urine x urine flow rate = [sub]plasma x glomerular filtration rate

24. Renal Clearance Advantages: • Plasma clearance tests can be used for: • Measurement of the glomerular filtration rate. • Measurement of the renal plasma flow rate (& from there we can calculate the renal blood flow rate). • Determining the renal handling of the different substances; whether or not the substance is reabsorbed or secreted by the renal tubules.

25. Clearance measurements are also used to examine renal management of substances absorbed or secreted by the kidney. For substances secreted by the kidney • GFR.Ps + T = Us.V (T = amount transported) What goes What leaves the into the nephrons nephrons • C = UV/P Cs > Cin Secretion into nephrons is occurring

26. For substances absorbed by the nephrons • GFR.Ps = T + Us.V (T = amount transported) What goes What leaves Into the nephrons the nephrons GFR.Ps - Us.V = T C = UV/P Csubs< Cinulin Absorption from nephrons is occurring Us.V = normally zero for glucose & amino acids.

27. P.A.H.A • Used to measure the RBF. Properties of P.A.H.A: • When present below a certain conc. in the blood • They are completely removed by a single circulation through the kidney. This is because: • They are easily filtered. • They are secreted by renal tubules. • They are not reabsorbed after filtration. 2) Not enter RBC’s or other tissue cells. 3) Not metabolized by tissues. 4) Not toxic. 5) Not adsorbed to the unfiltrated plasma proteins.

28. Urea Clearance Test: • The patient evacuates his bladder, then drinks a glass of water. • After 1 hr  blood & urine samples are taken & he drinks another glass of water. • After 2 hrs  another urine sample is taken. • The urine vol. /min. is calculated. • If it is above 2 ml /min  we get the maximal urea clearance. MC = (U) x (V) / (P) = 75 ml /min. (normally). • If it is below 2 ml /min  we get the standard urea clerance. SC = (U) x (V) / (P) = 54 ml /min. (normally) (U) = Conc. of urea in 1 ml urine. (V) = Vol. of urine /min. (P) = Conc. of urea in 1 ml plasma.

29. Filtered Load • Filtered load – Quantity filtered at the glomerulus per min • Filtered load of S = [S]plasma X GFR • Normal [Glucose]plasma = 100mg/100ml • Filtered load of glucose = 100mg/100ml x 125ml/min = 125mg/ml • [Glucose]plasmaa Filtered load of glucose

30. WHAT GOES INTO THE NEPHRONS • Amount of substance filtered /min • GFR x [Plasma] Substance = TUBULAR (FILTERED) LOAD (UNITS = mg/min) Plus What is secreted into the nephron tubules WHAT COMES OUT OF THE NEPHRONS Tubular load + amount secreted Minus • amount absorbed by nephrons into renal vein.

32. Thanks

33. Renal Threshold • When the plasma concentration of the substance is beyond it  the substance begins to appear in urine. • At this level  the filtered load exceeds the absorptive capacity of the tubules. • Substances of high threshold: glucose, amino acids & vitamins. • Substances of medium threshold: K+ & urea. • Substances of low threshold: phosphate & uric acid. • Substances of no threshold: creatinine, mannitol & inulin.

34. Renal Threshold Notice: • Appearance of glucose in urine before the transport maximum is reached is termed “Splay” and results from: • Nephron variability: “in glomerular size & tubular length”. • Variability in the number of glucose carriers & the transport rate of the carriers.

35. Tubular transport maximum • Definition: It is the maximal amount of a substance (in mg) which can be transported (reabsorbed or secreted) by tubular cells/min.

36. Tubular Transport Maximum • Many substances are reabsorbed by carrier mediated transport systems e.g. glucose, amino acids, organic acids, sulphate and phosphate ions. • Carriers have a maximum transport capacity (Tm)which is due to saturation of the carriers. If Tm is exceeded, then the excess substrate enters the urine. • Glucose is freely filtered, so whatever its [plasma] that will be filtered.

37. Tubular Transport Maximum • In man for plasma glucose up to 10 mmoles/l, all will be reabsorbed. Beyond this level of plasma [glucose], it appears in the urine = Renal plasma threshold for glucose. • (If plasma [glucose] = 15 mmoles/l, 15 will be filtered, 10 reabsorbed and 5 excreted.) • Kidney does NOT regulate [glucose], (insulin and glucagon). Normal [glucose] of 5 mmoles/l, so Tm is set way above any possible level of (non-diabetic) [glucose]. Thus, ensure that all this valuable nutrient is normally reabsorbed. The appearance of glucose in the urine of diabetic patients = glycosuria, is due to failure of insulin, NOT, the kidney.

38. (b) (a) a = Renal Threshold G b = T maximum glucose (TmG)

39. Tubular Transport Maximum • For amino acids, Tm also very high  no urinary excretion occurs. • However, kidney does regulate some substances by means of the Tm mechanism, eg sulphate and phosphate ions. This is because Tm is set at a level whereby the normal [plasma] causes saturation so any  above the normal level will be excreted, therefore achieving its plasma regulation. (Also subject to PTH regulation for phosphate, PTH  reabsorption).