Chapter 25 Urinary System Lecture 16 Part 1: Renal Function Overview Reabsorption and Secretion Marieb’s Human Anatomy and Physiology Ninth Edition Marieb  Hoehn

2 Lecture Overview Tubular reabsorption and secretion Regulation of urine concentration and volume Urea and uric acid excretion Renal clearance Elimination of urine

3 Simplified Overview of Nephron Function Renal corpuscle – provides the raw materials to the nephron for processing PCT – reclaims those substances the body can use; gets rid of some things DCT – gets rid of those substances the body doesn’t want or need; reabsorbs some more Na +, Ca 2+ Collecting duct – provides the OPTION of reclaiming H 2 O or letting it pass out of the body

4 Overview of Renal Function Filtration (based primarily on size) –Occurs exclusively in renal corpuscle –Occurs across the filtration membrane Nutrient and fluid reabsorption –Primarily in the proximal convoluted tubule (PCT) – 65% vol. –Rest occurs in nephron loop, DCT, and collecting tubule Active secretion occurs primarily –PCT –DCT Regulation of final volume and solute concentration of urine –Nephron loops (juxtamedullary nephrons) –Collecting ducts Good overview slide…

5 Overview of Renal Function Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007 You should know what is moved (red arrows indicate the important items), and in what part of the nephron they are moved. Keep in mind: Where Na + goes, H 2 O and Cl - usually follow. **See the Summary Table in your Study Guide for Exam 3 * * * *

6 Tubular Reabsorption and Secretion Reabsorption – reclaiming of substances in filtrate by body (tubule  blood) Secretion – elimination of substances by the body that did not leave the blood at the glomerulus (blood  tubule) Figure from: Hole’s Human A&P, 12 th edition, 2010

7 Tubular Reabsorption Reabsorption by tubular cells is a selective process –Diffusion –Osmosis –Carrier-mediated transport Facilitated diffusion Active transport Co- and counter-transport –Occurs mainly in the proximal convoluted tubules (PCT) –Has a transport maximum, T m, for most substances besides Na + T m is the rate at which solutes can be transported, e.g., 375 mg/min Renal threshold is the plasma level (concentration) above which a particular solute will appear in urine, e.g., 180 mg/dl Peritubular capillaries are well-suited for reabsorption –Low hydrostatic pressure –High degree of permeability –Higher colloid osmotic pressure due to filtration

8 Renal Clearance (C) the rate (ml/min) at which a substance is removed from the plasma; volume of plasma from which the kidneys clear a particular substance in a given time (usually 1 minute) C (ml/min) = U * V / P [U = urine concentration in mg/ml, V = rate of urine formation in ml/min, P = plasma concentration in mg/ml] tests of renal clearance (C x ) inulin clearance test (standard; C inulin = GFR = 125 ml/min) creatinine clearance test (easy to do; C creatinine = 140ml/min) paraminohippuric acid (PAH) test tests of renal clearance are used to calculate glomerular filtration rate Examples: - C x = 125 ml/min (125 ml of ‘x’ is removed from the plasma every min = 100%) - C x = 60 ml/min (some reabsorption of ‘x’ is occurring) - C x = 0 ml/min (complete reasbsorption of ‘x’ is occurring) - C x = 630 ml/min (secretion of ‘x’ is occurring)

9 Reabsorption in PCT Figure from: Saladin, Anatomy & Physiology, McGraw Hill, % of filtrate volume is reabsorbed in the PCT All uric acid, about 50% of urea, and no creatinine is reabsorbed 8 mm Hg  COP Tubular fluid

10 Reabsorption in the PCT SubstanceMechanism of Reabsorption Notes Na + (Cl - )Primary Active TransportNa + reabsorption is the driving force for most other reabsorption H2OH2OOsmosisClosely associated with movement of Na + (Obligatory water reabsorption) GlucoseSecondary Active transportLimited # of molecules can be handled (T m = 375 mg/min); attracts H 2 0 Amino AcidsSecondary Active transportThree different active transport modalities; difficult to overwhelm Other electrolytesSecondary Active transport

11 Sodium and Water Filtration, Reabsorption, and Excretion Because of the large volumes involved, small changes in tubular reabsorption amount to LARGE changes in excretion of Na + and H 2 O (since “water follows salt”) Figure from: Hole’s Human A&P, 12 th edition, 2010

12 Tubular Secretion Tubular secretion (reabsorption in reverse) moves substances from the blood into the tubular lumen (urine) 1.Getting rid of substances not already in filtrate 2.Eliminating undesirable substances reclaimed by passive process, e.g., urea 3.Ridding body of excess K + 4.Controlling blood pH Figure from: Hole’s Human A&P, 12 th edition, 2010

13 Secretion in the PCT and DCT In the DCT potassium ions or hydrogen ions may be secreted in exchange for reabsorbed sodium ions. Reabsorption of Na + in the DCT is increased by the hormone, aldosterone. Other compounds are actively secreted as well, e.g., histamine, ammonia, creatinine, penicillin, phenobarbital. ActiveActive and Passive Figure from: Hole’s Human A&P, 12 th edition, 2010

14 Review Urine formation is a product of –Filtration –Tubular reabsorption –Tubular secretion Tubular reabsorption –Reclaims important substances in the filtrate –Takes place primarily in the PCT –Uses diffusion, osmosis, and carrier-mediated transport

15 Review Tubular secretion –Rids the body of substances that have not been filtered or are present in excess –Takes place throughout the tubules –Is mainly dependent upon active transport