GFR Clearance and Blood Flow Lect 1 PDF

Renal Physiology March 18, 2024 Room 104, Physiology Unit. [email protected] J. Mohan, PhD., Lecturer, Physiology Unit, Dept. of Pre-clinical Sciences Faculty of Medical Sciences, U.W.I., St Augustine. References:  Costanzo, L.S. (2014) Physiology. 5th Edition, Elsevier, Saunders.  Koeppen...

Renal Physiology March 18, 2024 Room 104, Physiology Unit. [email protected] J. Mohan, PhD., Lecturer, Physiology Unit, Dept. of Pre-clinical Sciences Faculty of Medical Sciences, U.W.I., St Augustine. References:  Costanzo, L.S. (2014) Physiology. 5th Edition, Elsevier, Saunders.  Koeppen B.E. & Stanton B.A. (2018). Berne & Levy Physiology. 7th Edition. Mosby, Elsevier.  Hall, J.E. (2020). Guyton and Hall Textbook of Medical Physiology. 14th Edition, Elsevier, Saunders.  Marieb, E. & Hoehn, K. (2010). Human Anatomy & Physiology. 8th Edition, Pearson, Benjamin Cummings.  Stanfield, C.L. & Germann W.J. (2008). Principles of Human Physiology. 3rd Edition, Pearson, Benjamin Cummings. March 18, 2024 Dr J. Mohan 2 Physiology Objectives Objectives 1-5, PBL Booklet Body Fluid Compartments, Osmolarity, Osmolality & Tonicity and Membrane Transport Mechanisms. Costanzo (2014) : Ch 6 : Section on Body Fluids. March 18, 2024 Dr J. Mohan 3 Physiology Objectives 1. Review body fluid compartments with regard to their volume, relative percentages and percentage of body weight. 2. Review the ionic components of the major body fluid compartments. 3. Review osmolarity, osmolality and tonicity with respect to movement between intracellular and extracellular body fluid compartments. 4. Review the various types of membrane transport (active transport, facilitated diffusion etc.) 5. Review the resultant effects on red blood cells which are added to hypotonic and hypertonic solutions. March 18, 2024 Dr J. Mohan 4 Today’s Physiology Objectives 1. List the specific functions of the kidneys. 2. Define the term “Renal Clearance” with the use of equations, and state the renal clearance of various substances e.g. albumin, glucose, inulin, penicillin, PAH. 3. Define glomerular filtration rate (GFR). 4. Describe the forces involved in glomerular filtration and the composition of the glomerular filtrate. 5. Define and quantify the normal filtration fraction. 6. Identify the main methods by which GFR is estimated and justify the use of creatinine clearance over that of inulin in the estimation of GFR. March 18, 2024 Dr J. Mohan 5 Today’s Topics Overview of kidney functions. Glomerular Filtration (GFR). – Brief review of the ultrastructure of the Renal Corpuscle. – Composition of Ultrafiltrate. – Determinants of Ultrafiltration. – Dynamics of Ultrafiltration. – Estimation of GFR- creatinine clearance. – Clinical Importance of GFR. Renal Clearance. March 18, 2024 Dr J. Mohan 6 Functions of the Kidneys March 18, 2024 Dr J. Mohan 7 Functions of the Kidneys Excretory 1. metabolic products and foreign substances e.g. urea, uric acid, creatinine, Hb metabolism, hormone metabolites, drugs Regulatory 1. body fluid osmolality and volumes 2. electrolyte balance 3. acid-base balance Endocrine 1. produce and secrete hormones e.g. renin, active form of Vitamin D (1,25dihydroxycholecalciferol) or calcitriol, erythropoietin March 18, 2024 Dr J. Mohan 8 The Nephron Nephrons are the structural and functional units that form urine, consisting of: – Renal Corpuscle – Proximal Convoluted Tubule – Loop of Henle – Distal Convoluted Tubule – Collecting Duct System March 18, 2024 Figure 32-.3; Koeppen & Stanton, 2010 Dr J. Mohan 9 The Nephron https://open.oregonstate.education/aandp/chapter/25-1-internal-and-external-anatomy-of-the-kidney/ March 18, 2024 Dr J. Mohan 10 Mechanisms of Urine Formation Urine formation and adjustment of blood composition involves three major processes – Glomerular filtration – Reabsorption – Secretion Excretion March 18, 2024 Dr J. Mohan Figure 25-9a; Marieb & Hoehn, 2010 11 Today’s Topics Overview of kidney functions. Glomerular Filtration (GFR). – Brief review of the ultrastructure of the Renal Corpuscle. – Determinants of Ultrafiltration. – Composition of Ultrafiltrate. – Dynamics of Ultrafiltration. – Definition of GFR. – Estimation of GFR- creatinine clearance. – Clinical Importance. Renal Clearance. March 18, 2024 Dr J. Mohan 12 The Nephron Renal Corpuscle : – Glomerulus – a tuft of capillaries associated with a renal tubule – Bowman’s capsule – blind, cup-shaped end of a renal tubule that completely surrounds the glomerulus March 18, 2024 Dr J. Mohan 13 Ultrastructure of the Renal Corpuscle the glomerulus : network of capillaries supplied by the afferent arteriole and drained by the efferent arteriole capillaries covered by epithelial cells called podocytes (form visceral layer of Bowman's capsule) space between the visceral layer and the parietal layer Bowman's space  lumen of the PCT Figure 32-5; Koeppen & Stanton, 2010 March 18, 2024 Dr J. Mohan 14 Ultrastructure of the Renal Corpuscle Figure 25-9a; Marieb & Hoehn, 2010 March 18, 2024 Dr J. Mohan 15 Filtration Membrane Endothelium (capillary) – Fenestration pores ~ 60-70 nm diameter – Glycocalyx in luminal lining covers the fenestrations (-ve charged) – No blood cells Basement membrane – -ve charged glycoproteins – Major filtration barrier for large plasma proteins Epithelium (foot processes of podocytes) – Slit diaphragms & foot processes have – ve charged structural proteins – Supplements the selectivity in permeability of the basement membrane Figure 25-9c; Marieb & Hoehn, 2010 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2681366/#:~:text=These%20fenestrations%20are%20typically%2060,3%E2%80%935%20nm)%20diaphragm. March 18, 2024 Dr J. Mohan 16 Filtration Membrane in Type 2 DM https://www.sciencedirect.com/science/article/pii/S0085253815556767 See myelearning for paper if link does not work OR copy link into browser. March 18, 2024 Dr J. Mohan 17 Determinants of Ultrafiltrate Composition the glomerular filtration barrier determines the composition of the plasma ultrafiltrate it restricts the filtration of molecules on the basis of both size & electrical charge – neutral molecules with a radius < 20 Å are filtered freely – molecules > 42 Å are not filtered – molecules between 20 - 42 Å are filtered to various degrees, depending on their electrical charge e.g. cations Eg: serum albumin : – an anionic protein – molecular radius of 35.5 Å, (i.e. < 42 Å) BUT is filtered poorly (because of negative charges on it) mol wt 66.5 kilo-Daltons March 18, 2024 Dr J. Mohan 18 Figure 26-12; Hall, 2011. Effect of molecular radius and electrical charge of dextran on its filterability by the glomerular capillaries. A value of 1.0 indicates that the substance is filtered as freely as water, whereas a value of 0 indicates that it is not filtered. Dextrans are polysaccharides that can be manufactured as neutral molecules or with negative or positive charges and with varying molecular weights. March 18, 2024 Dr J. Mohan 19 Composition of Glomerular Filtrate first step in the formation of urine is ultrafiltration of plasma by the glomerulus the plasma ultrafiltrate = glomerular filtrate no cellular elements (i.e., RBC, WBC & platelets) nearly protein-free [salts & organic molecules], e.g. glucose and amino acids, is similar to plasma Starling forces drive ultrafiltration across the glomerular capillaries and changes in these forces alter the GFR March 18, 2024 Dr J. Mohan 20 Clinical Note Clinical importance of the negative charges on the filtration barrier in restricting the filtration of plasma proteins : – removal of the negative charges from the filtration barrier proteins filtered solely on the basis of their size – at sizes between 20 - 42 Å, filtration of polyanionic proteins is better than their filtration in the normal state – E.g. in glomerular diseases, the negative charges on the filtration barrier are reduced because of immunological damage and inflammation  filtration of proteins  proteins appear in urine (proteinuria) https://www.niddk.nih.gov/health-information/kidney-disease/glomerular-diseases March 18, 2024 Dr J. Mohan 21 Clinical Note Figure 32.16; Koeppen & Stanton, 2010 March 18, 2024 Dr J. Mohan 22 Case A 6 year-old boy is brought to the hospital emergency room because his urine is dark and his feet are swollen. Two weeks earlier, the child was treated for strep throat with penicillin. His mother admits to stopping the medication after 2 days because she thought he looked better. What is the name of his present condition? In your answer, include : – the layers of the glomerular filtration barrier – 3 characteristics about the composition of the glomerular filtrate https://www.niddk.nih.gov/health-information/kidney-disease/glomerular-diseases https://www.kidney.org/atoz/content/post-infectious-glomerulonephritis-pign March 18, 2024 Dr J. Mohan 23 Fluid Exchange Across Capillaries : Starling Forces March 18, 2024 Dr J. Mohan 24 Fluid Exchange Across Capillaries : Starling Forces March 18, 2024 Dr J. Mohan 25 Fluid Exchange Across Capillaries : Starling Forces March 18, 2024 Dr J. Mohan 26 Starling Forces & Calculating Net Filtration Rate https://www.youtube.com/ watch?v=SCyCDVlEkWQ Link to “The Renal System Lecture Video Resources 2024” Renal YouTube PlayList March 18, 2024 https://www.youtube.com/pl aylist?list=PLFk30doHyXeth5BGjcqJBvqbGyEJ571i Dr J. Mohan 27 Dynamics of Ultrafiltration March 18, 2024 Figure 32.17; Koeppen & Stanton, 2010 Dr J. Mohan 28 Dynamics of Ultrafiltration Can you identify 4 forces that could drive fluid movement across the glomerular capillary wall? Ultrafiltration occurs because the Starling forces drive fluid from the lumen of glomerular capillaries across the filtration barrier  Bowman's space Forces favoring filtration ? Forces opposing filtration? March 18, 2024 Figure 32.17; Koeppen & Stanton, 2010 Dr J. Mohan 29 Dynamics of Ultrafiltration GFR is proportional to the sum of the Starling forces that exist across the capillaries (NFP) multiplied by the ultrafiltration coefficient (Kf) GFR = Kf x Net Filtration Pressure (NFP) GFR = Kf [ (PGC - PBS) – (πGC - πBS) ] where : πBS = 0 GFR = Kf [ (PGC - PBS) – (πGC ) ] March 18, 2024 Dr J. Mohan 30 Dynamics of Ultrafiltration Kf = a measure of the product of the intrinsic permeability of the glomerular capillary to water and the glomerular surface area available for filtration the rate of glomerular filtration is higher in glomerular capillaries than in systemic capillaries – mainly because Kf is approximately 100 times greater in glomerular capillaries than in systemic capillaries – also PGC ~ X2 hydrostatic pressure in systemic capillaries March 18, 2024 Dr J. Mohan 31 Dynamics of Ultrafiltration diseases can lower Kf by : – reducing the number of functional glomerular capillaries (thereby reducing the surface area available for filtration) – increasing the thickness of glomerular capillary membrane and reducing its intrinsic permeability – e.g. chronic uncontrolled HTN & DM increase the thickness of glomerular capillary membrane & eventually damage capillaries so severely that the surface area for filtration is reduced March 18, 2024 Dr J. Mohan 32 Dynamics of Ultrafiltration GFR = Kf x Net Filtration Pressure (NFP) GFR can be altered by changing Kf or by changing NFP (which depends on the algebraic sum of the Starling forces) In normal individuals, the GFR is physiologically regulated by alterations in PGC :  PGC   GFR  PGC   GFR such changes PGC are mediated mainly by changes in : – 1) renal arteriolar pressure (transient change) – 2) afferent arteriolar resistance – 3) efferent arteriolar resistance March 18, 2024 Dr J. Mohan 33 Dynamics of Ultrafiltration Changes in PGC : Constriction of afferent arteriole :  Renal Plasma Flow (RPF);  PGC;  GFR e.g. sympathetic stimulation; high levels AgII Constriction of efferent arteriole :  Renal Plasma Flow (RPF) slightly;  PGC;  GFR e.g. low levels AgII March 18, 2024 Figure 6-11; Costanzo, 2014 Dr J. Mohan 34 Dynamics of Ultrafiltration Ag II constricts both afferent and efferent arterioles, but preferentially constricts efferent arterioles Haemorrhage  higher level of AgII : Pronounced constrictor effect on efferent arteriole Medium constrictor effect on afferent arteriole  Renal Plasma Flow (RPF);  GFR – but a “smaller” fall in GFR i.e. GFR is “protected” GFR is  by the (moderate) afferent arteriolar constriction, but because of the simultaneous (pronounced) efferent arteriolar constriction (which  GFR, the GFR does not fall to the same extent as it would if only afferent arteriolar constriction was present March 18, 2024 Dr J. Mohan 35 Dynamics of Ultrafiltration Changes in πGC – Influenced by arterial plasma colloid osmotic pressure –  [plasma protein]   πGC;  GFR –  [plasma protein] e.g. liver disease   πGC;  GFR Changes in PBS –  PBS   GFR e.g. constriction/blockage of ureter –  PBS   GFR March 18, 2024 Dr J. Mohan 36 Glomerular Filtration Rate GFR – quantity of glomerular filtrate formed each minute in all nephrons of both kidneys – the sum of the filtration rates of all functioning nephrons – index of kidney function – in normal adult male : 90 - 140 mL/min – in normal adult female : 80 - 125 mL/min – so, in 24 hours or 1 day, GFR = 180 L (i.e. 125 mL/min) (vs ~ 4 L/day for all other capillaries in body!) March 18, 2024 Dr J. Mohan 37 Estimation of GFR can be estimated using creatinine clearance test Creatinine – byproduct of skeletal muscle creatine metabolism – freely filtered across the glomerulus into Bowman's space – not reabsorbed, secreted*, or metabolized by the cells of the nephron, therefore... *significantly March 18, 2024 Dr J. Mohan 38 Estimation of GFR the amount of creatinine excreted in urine per minute (excretion rate) = the amount of creatinine filtered at the glomerulus each minute (filtered load) where : PCr = plasma [creatinine] UCr = urine [creatinine] V = urine flow Figure 32.13; Koeppen & Stanton, 2010 March 18, 2024 Dr J. Mohan 39 Estimation of GFR From equation : GFR = U Cr X V _____________ P Cr where : PCr = plasma [creatinine] UCr = urine [creatinine] V = urine flow Figure 32.13; Koeppen & Stanton, 2010 March 18, 2024 Dr J. Mohan 40 Estimation of GFR NB: this equation is the same form as that for clearance (see later) So, “clearance” of creatinine provides a means for determining the GFR in clinical practice Figure 32.13; Koeppen & Stanton, 2010 March 18, 2024 Dr J. Mohan 41 Estimation of GFR Creatinine is not the only substance that can be used to measure GFR Any substance that meets the following criteria can serve as an appropriate marker for the measurement of GFR The substance must: 1. Be freely filtered across the glomerulus into Bowman's space 2. Not be reabsorbed or secreted by the nephron 3. Not be metabolized or produced by the kidney 4. Not alter the GFR (if infused) e.g. Inulin - fructose polymer- its clearance measures GFR (see later) March 18, 2024 Dr J. Mohan 42 Glomerular Filtration Rate Clinical Importance of GFR : – measuring GFR is important when kidney disease is suspected :  GFR may be the first & only clinical sign of kidney disease – knowledge of the patient's GFR is essential in evaluating the severity and course of kidney disease  GFR  kidney disease is progressing  GFR  recuperation – a 50% loss of functioning nephrons reduces the GFR only by about 25%; the decline in GFR is not 50% because the remaining nephrons compensate March 18, 2024 Dr J. Mohan 43 Estimation of GFR Kidney function is usually assessed in the clinical setting by measuring PCr, which is inversely related to GFR BUT GFR must fall substantially before an increase in PCr can be detected clinically Eg: a fall in GFR from 120 to 100 mL/min is accompanied by an increase in PCr from 1.0 to 1.2 mg/dL The increase in PCr might appear small, but the GFR has actually fallen by almost 20% Figure 32.14; Koeppen & Stanton, 2010 March 18, 2024 Dr J. Mohan 44 Estimation of GFR Physiological Changes – e.g. Pregnancy 50% increase GFR – Normal GFR = 125 ml/min – Pregnancy = 188 ml/min – Normal creatinine = 1 mg/dL – Pregnancy = 0.5 mg/dL Figure 32.14; Koeppen & Stanton, 2010 March 18, 2024 Dr J. Mohan 45 Filtration Fraction not all of the plasma coming into the kidneys (renal plasma flow) is filtered ~ 20% of the plasma flowing through the kidneys is filtered through the glomerular capillaries into Bowman’s capsule the fraction of filtered plasma = filtration fraction Filtration Fraction = GFR -------------------RPF ~ 0.20 March 18, 2024 Dr J. Mohan 46 Filtration Fraction Normal Hct = 0.45 i.e. 45% of blood volume = RBC & other 55% = almost all plasma Typical renal blood flow (RBF) = 1.1. L/min Renal Plasma Flow (RPF) = 0.55 x 1.1 L /min = 605 mL / min Typical GFR = 125 mL/min Therefore, of the 605 mL/min that enters the glomeruli via the afferent arteriole, 125 mL/min or 20% filters into Bowman’s space The remaining 480 mL passes via the efferent arterioles into the peritubular capillaries March 18, 2024 Dr J. Mohan 47 Today’s Topics Overview of kidney functions. Glomerular Filtration (GFR). – Brief review of the ultrastructure of the Renal Corpuscle. – Composition of Ultrafiltrate. – Determinants of Ultrafiltration. – Dynamics of Ultrafiltration. – Estimation of GFR- creatinine clearance. – Clinical Importance of GFR. Renal Clearance. March 18, 2024 Dr J. Mohan 48 Renal Clearance Definition : the volume of plasma completely cleared of a substance (x) by the kidneys per unit time Cx = Ux X V _______________ Px ml/min Where : C = clearance (ml/min) [U] x = Urine concentration of substance x (mg/ml) [P] x = Plasma concentration of substance x (mg/ml) V = Urine flow rate per minute (ml/min) March 18, 2024 Dr J. Mohan 49 Renal Clearance Cx = Ux X V _______________ ml/min Px Note : ratio of urinary excretion of substance x to plasma concentration of substance x March 18, 2024 Dr J. Mohan 50 Renal Clearance Cx = Ux X V ---------------------------- ml/min Px Example 1. if Ux = 100 mg/ml; V = 1 ml/min Then excretion rate of x = 100 mg/ml x 1 ml/min = 100 mg/min If Px = 1 mg/ml , then Cx = 100 mg/min _________________________ 1 mg/ml = 100 ml/min March 18, 2024 Dr J. Mohan 51 Renal Clearance Example 2 : In a 24 hour period, 1.44 L of urine is collected from a man receiving an infusion of inulin (the fructose polymer used to estimate GFR). In his urine the [inulin] is 150 mg/ml and in his plasma the [inulin] is 1 mg/ml. What is his inulin clearance? March 18, 2024 Dr J. Mohan 52 Renal Clearance Example 2 : In a 24 hour period, 1.44 L of urine is collected from a man receiving an infusion of inulin (the fructose polymer used to estimate GFR). In his urine the [inulin] is 150 mg/ml and in his plasma the [inulin] is 1 mg/ml. What is his inulin clearance? Express V in ml/min : 1.44 L /24 hours = 1440 ml / 1440 min = 1.0 ml /min U inulin = 150 mg/ml; V= 1.0 ml/min; P inulin = 1 mg/ml Clearance of inulin = 150 mg/ml x 1.0 ml/min _________________________ 1 mg/ml = 150 ml/min March 18, 2024 Dr J. Mohan 53 Renal Clearance Example 3 : A urine sample revealed that 450 ml of urine was collected in 1 hour and this had a concentration of Na+ of 15 mmol/L. The plasma concentration of Na+ was 145 mmol/L. What was the Na+ clearance? March 18, 2024 Dr J. Mohan 54 Renal Clearance Example 3 : A urine sample revealed that 450 ml of urine was collected in 1 hour and this had a concentration of Na+ of 15 mmol/L. The plasma concentration of Na+ was 145 mmol/L. What was the Na+ clearance? Express V in ml/min : 450 ml/60 mins = 7.5 ml/min U Na+ = 15 mmol/L; V= 7.5 ml/min; P Na+ = 145 mmol/L Clearance of Na+ = 15 mmol/L x 7.5 ml/min _________________________ 145 mmol/L = 0.78 ml/min March 18, 2024 Dr J. Mohan 55 Renal Clearance From previous examples, it can be seen that clearance can be calculated for any substance Clearance an vary from zero to ~ 600 ml/min depending on the renal handling of the substance A substance that is not normally filtered across the glomerular capillaries will have a clearance of ~ zero e.g. large plasma protein Renal clearance of glucose = 0; glucose is filtered, unlike the plasma protein, but is completely reabsorbed back into blood Substances that are filtered but partially reabsorbed e.g. Na+, urea, phosphate, Cl- have a clearance higher than zero March 18, 2024 Dr J. Mohan 56 Renal Clearance Recall that the clearance of inulin, the fructose polymer, is used to estimate GFR Like creatinine, (the clearance of which is also used to estimate GFR), inulin is freely filtered across the glomerular capillaries, but is neither reabsorbed nor secreted, therefore its clearance measures GFR Organic acids e.g. para - aminohippuric acid (PAH) is both filtered & secreted, therefore has the highest clearance values Drugs such as penicillin is also secreted, therefore has a high clearance March 18, 2024 Dr J. Mohan 57 Renal Clearance – Other markers for GFR Why is the use of creatinine preferred over that of inulin for the estimation of GFR? – inulin is freely filtered across the glomerular capillaries, but is neither reabsorbed nor secreted, therefore its clearance is the most accurate measure of GFR available (vs creatinine which is secreted to a small extent & therefore slightly overestimates the GFR) – however, the use of inulin clearance is limited in the clinical situation because of several factors : March 18, 2024 Dr J. Mohan 58 Renal Clearance – Other markers for GFR 1. an intravenous injection followed by constant infusion is required 2. complete emptying of the bladder is necessary before the beginning of the clearance period, in order to remove all urine not containing inulin, and again at the end of the period in order to obtain all the urine produced during this period 3. the urine flow must be high so that enough urine may be obtained in a short period of time to permit analysis and to reduce possible errors introduced by urine remaining in the bladder at the beginning and end of the clearance period – March 18, 2024 these requirements can be difficult to meet in a patient with compromised renal function Dr J. Mohan 59 Renal Clearance – Other markers for GFR the use of creatinine clearance overcomes some of these practical problems, but creatinine is secreted into the PT, and certain drugs e.g. cimetidine (organic cations) can inhibit tubular secretion of creatinine, thereby reducing creatinine clearance (but not actual GFR) Otherwise : 1. Creatinine is an end product of protein metabolism and is always present in the blood; its concentration remains fairly constant over a 24 hour period 2. therefore there is no need for an intravenous infusion, and a clearance period can extend for as long as 24 hours so that adequate amounts of urine can be collected and the frequency of bladder emptying is minimised March 18, 2024 Dr J. Mohan 60 Renal Clearance – Other markers for GFR 3. only 1 blood sample is needed and it can be taken during any point in the collection period 4. plasma creatinine alone can be used to follow changes in GFR in a patient with chronic renal disease March 18, 2024 Dr J. Mohan 61