Renal Blood Flow power point prepared by

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1. Renal Blood Flow Blood flow through both kidneys, in an average 70 Kg man, is about 1.1 L/min, or about 22% of the CO. The mechanisms Which regulates renal blood flow are closely linked to the control of GFR and the excretory functions of the kidneys. 1

2. Determinants of Renal Blood Flow Renal blood flow is determined by the pressure gradient across the renal vasculature. Formula is: Renal artery pressure - Renal vein pressure Total renal vascular resistance 2

3. Physiologic Control of Glomerular Filtration and Renal Blood Flow Sympathetic NS Activation Decreases GFR Strong activation can constrict the renal arterioles and decreases renal blood flow and GFR. Moderate or mild sympathetic stimulation has little influence on renal blood flow and GFR. 3

4. Hormone or Autacoid Effect on GFR Norepinephrine ↓ Epinephrine ↓ Endothelin /autacoid/ ↓ Angiotensin II ↔(prevents ↓) Endothelial-derived NO/autacoid/ ↑ Prostaglandins & Bradykinin ↑ 4 Hormonal and Autacoid Control of Renal Circulation

5. Regulation of GFR and Renal blood flow Autoregulation : I ntrinsic to the kidneys The major function is to allow precise control of renal excretion of water and solutes. Maintains relatively constant GFR despite arterial BP fluctuations between 75-160mmHg. 5

6. Macula densa : specialized group of epithelial cells in the distal tubules that come in close contact with the afferent and efferent arterioles . Decreased NaCl => Renin Release . Juxtaglomerular cells are epithelial cells in the walls of the afferent and efferent arterioles and secrete the hormone renin. The juxtaglomerular complex : macula densa cells at initial distal tubule and juxtaglomerular cells in walls of afferent and efferent arterioles. (Picture on z next slide…) 6 Role of Tubuloglomerular Feedback in Autoregulation of GFR (3 structures)

7. Regulation of GFR and Renal blood flow 7

8. Tubuloglomerular Feedback in Autoregulation of GFR /RAAS/ 8

9. Other Factors That Increase Renal Blood Flow and GFR H igh protein intake and increased blood glucose causes increased Na+ reabsorbtion in the proximal tubules along with amino acids and glucose which in turn decreases NaCl delivery to the macula densa . Eg : GFR and renal blood flow increases 20- 30 % 1- 2 hours after high-protein meal. 9

10. NB=Nota Bene ! Prostaglandins oppose vasoconstriction of afferent arterioles, to prevent excessive reductions in GFR and renal blood flow. Under stressful conditions, such as volume depletion or after surgery , the administration of NSAIDs that inhibit prostaglandin synthesis may cause significant reductions in GFR.

11. Renal Tubular Reabsorption and Secretion As the glomerular filtrate enters the renal tubules, it flows sequentially through the proximal tubule, the loop of Henle , the distal tubule, the collecting tubule, and finally, the collecting duct before it is excreted as urine. Along this course, some substances are Reabsorbed or Secreted 11

12. Tubular Reabsorption The Process of returning filtered substances back to the bloodstream . 99% H2O, 100% amino acids & glucose, 99.5% Na+ and 50% urea. Most reabsorption occurs in Proximal tubule. 12

13. Tubular secretion An active transport by which substances are transported from peritubular blood or interstitial fluid into the nephron tubule. It occurs in proximal , distal and collecting tubules. Main purpose is to keep electrolytes, Acid-Base balance & excrete toxins and drugs. Positive ions( K+is secreted and Na+ reabsorbed in exchange). Negative ions ( Cl ¯ and sulfate will be secreted while HCO3¯ will always be retained to keep blood pH. 14

14. Mechanism of Tubular Reabsorption Tubular reabsorption across tubular epithelium into interstisium can be: Transcellular : across the cell membrane Paracellular : b etween the cells(gap junctions) 15

15. Mechanism of Tubular Reabsorption Transport from renal interstisium to peritubular capillaries is by Ultrafiltration (bulk flow) that is mediated by hydrostatic and colloid osmotic forces There is a net reabsorptive force that moves the fluid and solutes from the interstitium into the blood . **************========END=======************* 16