Concentration and dilution of urine lect 4.pdf

Renal Physiology March 25 - April 4, 2024 J. Mohan, PhD. Lecturer, Physiology Unit, Department of Preclinical Sciences Faculty of Medical Sciences, U.W.I., St Augustine. Room 104, Physiology Unit, [email protected] March 25-Apr 4, 2024 Dr J. Mohan 1 References:  Costanzo, L.S. (2022) Physio...

Renal Physiology March 25 - April 4, 2024 J. Mohan, PhD. Lecturer, Physiology Unit, Department of Preclinical Sciences Faculty of Medical Sciences, U.W.I., St Augustine. Room 104, Physiology Unit, [email protected] March 25-Apr 4, 2024 Dr J. Mohan 1 References:  Costanzo, L.S. (2022) Physiology. 7th Edition, Elsevier, Saunders.  Hall, J.E. (2021). Guyton and Hall Textbook of Medical Physiology. 14th Edition, Elsevier, Saunders.  Koeppen B.E. & Stanton B.A. (2010). Berne & Levy Physiology. 6th Edition. Mosby, Elsevier.  Marieb, E. & Hoehn, K. (2010). Human Anatomy & Physiology. 8th Edition, Pearson, Benjamin Cummings. March 25-Apr 4, 2024 Dr J. Mohan 2 Physiology Objectives 1. Describe the role of the kidneys in water balance. 2. Describe the mechanisms that contribute to the formation of the cortico-medullary osmotic gradient (countercurrent multiplication, ADH, urea recycling) 3. Describe the role of the vasa recta in maintaining an osmotic gradient i.e. the countercurrent exchanger mechanism. 4. Describe the transport and permeability characteristics of the tubule including the distal nephron and collecting duct as they relate to the generation of the medullary osmolar gradient. 5. Explain the functions and mechanism of action of antidiuretic hormone (ADH). 6. Explain, with the use of diagrams, how ADH plays an important role in the production of either dilute or concentrated urine. 7. Briefly describe the basis of disorders of ADH production. March 25-Apr 4, 2024 Dr J. Mohan 3 Today’s Topics The role of the kidneys in water balance. The functions and the mechanism of action of ADH. Renal Mechanisms for Concentration and Dilution of Urine. March 25-Apr 4, 2024 Dr J. Mohan 4 Water Balance How much of the body is water anyway? %? – 60% How is water distributed in the body? – ICF 40% – ECF 20% – osmotic equilibrium March 25-Apr 4, 2024 Dr J. Mohan 5 Water Balance Kidneys regulate water balance ; usually the major route for water output Table 34-1; Koeppen & Stanton, 2010 March 25-Apr 4, 2024 Dr J. Mohan 6 Water Balance water loss from sweating, defecation, and evaporation from the lungs and skin can vary with environmental conditions or during pathological conditions, but loss of water by these routes cannot be regulated BUT, renal excretion of water is tightly regulated to maintain whole-body water balance i.e. water intake = water loss intake > loss  positive water balance intake < loss  negative water balance March 25-Apr 4, 2024 Dr J. Mohan 7 Water Balance  intake  kidneys lose water :  volume of dilute urine i.e. urine that is hypo-osmotic with respect to plasma e.g. 50 mOsm/kg H2O; 18 L/day  intake or  loss  kidneys conserve water :  volume of concentrated urine i.e. urine that is hyper-osmotic with respect to plasma e.g. 1200 mOsm/kg H2O; 0.5 L/day March 25-Apr 4, 2024 Dr J. Mohan 8 ADH controls Urine Concentration ADH alters renal water excretion independently of the rate of solute excretion this results in the regulation of plasma osmolarity and Na+ concentration March 25-Apr 4, 2024 Dr J. Mohan 9 Today’s Topics The role of the kidneys in water balance. The functions and the mechanism of action of ADH. Renal Mechanisms for Concentration and Dilution of Urine. March 25-Apr 4, 2024 Dr J. Mohan 10 Antidiuretic Hormone (ADH) small peptide : 9 aa synthesized in neuroendocrine cells mainly in the supraoptic nucleus (SON) of the hypothalamus synthesized hormone packaged in granules that are transported down the axon of the cell and stored in nerve terminals located in the posterior pituitary Figure 34-1; Koeppen & Stanton, 2010 March 25-Apr 4, 2024 Dr J. Mohan 11 Posterior Pituitary Lobe March 25-Apr 4, 2024 Dr J. Mohan 12 Secretion of ADH Secretion of ADH by the posterior pituitary regulated by 2 primary physiological regulators : – osmotic osmolarity of the body fluids (MOST sensitive) – hemodynamic volume & pressure of the vascular system Other factors that can alter ADH secretion include : – nausea () (recall nausea is a symptom of  plasma osmolarity), so this may further worsen the  plasma osmolarity – atrial natriuretic peptide () – angiotensin II () – drugs nicotine  ethanol  March 25-Apr 4, 2024 Dr J. Mohan 13 Osmotic Control of ADH Secretion small changes in the osmolarity of body fluids (1%) can alter secretion of ADH  body fluid osmolarity  osmoreceptors in anterior hypothalamus sense changes in body fluid osmolarity (shrinking)  send signals to ADH-synthesizing cells located in the SON of the hypothalamus  synthesis & secretion of ADH ADH is rapidly degraded in plasma, circulating levels can be reduced to zero within minutes after secretion is inhibited  ADH system can respond rapidly to fluctuations in body fluid osmolality March 25-Apr 4, 2024 Dr J. Mohan 14 Osmotic Control of ADH Secretion steep slope indicates sensitivity of the system set point of the system is the plasma osmolarity value at which ADH secretion begins to increase ; set point varies among individuals and is genetically determined normal range set point : 275 to 290 mOsm/kg H2O physiological factors can also change the set point,eg. changes in BVol & BP March 25-Apr 4, 2024 Dr J. Mohan The effect of changes in plasma osmolality on circulating ADH levels Figure 34-2a; Koeppen & Stanton, 2010 15 Hemodynamic Control of ADH Secretion a decrease in blood volume or pressure also stimulates secretion of ADH  BVol or BP  low-pressure (left atrium and large pulmonary vessels) and the high-pressure (aortic arch and carotid sinus) receptors  afferent fibers of the vagus & glossopharyngeal nerves to the brainstem (solitary tract nucleus of the medulla oblongata) signals to the ADHsecreting cells of the SON of hypothalamus March 25-Apr 4, 2024 Dr J. Mohan 16 Hemodynamic Control of ADH Secretion the sensitivity of the baroreceptor system is < that of the osmoreceptors: a 5% to 10% decrease in BVol or BP is required before ADH secretion  substances alter the secretion of ADH through their effects on BP e.g. bradykinin & histamine, BP   ADH secretion; NE   BP ADH Figure 34-2b; Koeppen & Stanton, 2010 March 25-Apr 4, 2024 Dr J. Mohan 17 Actions of ADH on the Kidneys 1. Primary action of ADH on the kidneys is to increase the water permeability of the principal cells of the late distal tubule and collecting duct 2. ADH increases the permeability of the medullary portion of the collecting duct to urea  an increase in reabsorption of urea and an increase in the osmolarity of the medullary interstitial fluid 3. ADH stimulates reabsorption of NaCl by the thick ascending limb of loop of Henle, distal tubule and collecting duct  helps to maintain hyperosmotic medullary interstitium necessary for water reabsorption (when concentrating urine) March 25-Apr 4, 2024 Dr J. Mohan 18 Mechanism of action of ADH on the Principal Cell in the Late DT & CD Figure 34-3, Koeppen & Stanton, 2010 March 25-Apr 4, 2024 Dr J. Mohan 19 Mechanism of action of ADH on the Principal Cell in the Late DT & CD  permeability of the collecting duct to water ADH binds to V2 receptor on the basolateral membrane of the principal cell of late DT/CD receptor is coupled to adenylyl cyclase via a stimulatory G protein (Gs),  increases intracellular levels of cAMP  intracellular cAMP activates protein kinase A (PKA)  insertion of vesicles containing aquaporin-2 (AQP2) water channels into the apical membrane of the cell +  synthesis of more AQP2 March 25-Apr 4, 2024 Dr J. Mohan 20 Mechanism of action of ADH on the Principal Cell in the Late DT & CD with the removal of ADH, these water channels are reinternalized into the cell, and the apical membrane is once again impermeable to water this shuttling of water channels into and out of the apical membrane provides a rapid mechanism for controlling permeability of the membrane to water because the basolateral membrane is freely permeable to water as a result of the presence of AQP3 and AQP4 water channels, any water that enters the cell through apical membrane water channels exits across the basolateral membrane, thereby resulting in net absorption of water from the tubule lumen March 25-Apr 4, 2024 Dr J. Mohan 21 Test Your Understanding Draw a flow diagram showing the effect of water deprivation on plasma osmolarity urine volume urine osmolarity March 25-Apr 4, 2024 Dr J. Mohan 22 Role of ADH in the Regulation of Body Fluid Osmolarity Response to an increase in plasma osmolarity (water deprivation) Figure 6-37; Costanzo, 2022 March 25-Apr 4, 2024 Dr J. Mohan 23 Role of ADH in the Regulation of Body Fluid Osmolarity Response to a decrease in plasma osmolarity (water drinking) Figure 6-38; Costanzo, 2022 March 25-Apr 4, 2024 Dr J. Mohan 24 Disorders in ADH production Central diabetes insipidus/pituitary diabetes insipidus – Head trauma, brain neoplasms/infections – Inadequate release of ADH – circulating levels are low – Excretion of large volumes of dilute urine (polyuria) – Body fluids become concentrated -  serum osmolarity,  serum [Na+] – To maintain normal body fluid osmolarity, the individual must intake large volumes of water (polydipsia) – Can be corrected by exogenous ADH- ADH analogue – dDAVP (l-deamino-8-D arginine vasopressin) March 25-Apr 4, 2024 Dr J. Mohan 25 Disorders in ADH production Nephrogenic diabetes insipidus – ADH secretion by the posterior pituitary is normal – Principal cells unresponsive to ADH (defect in G protein or adenylate cyclase or aquaporin 2 channel, various tubule-interstitial diseases that disrupt the medulla or distal nephrons and make the kidney seem insensitive to ADH) or drugs e.g. lithium; demeclocycline (affects adenylate cyclase cAMP) – Excretion of large volumes of dilute urine (polyuria) – Increased plasma osmolarity stimulates elevated secretion of ADH – Circulating ADH levels higher than normal, but ineffective – Thirst mechanism normal, serum Na+ near normal; patients who do not have access to water or who cannot communicate thirst e.g. infants, elderly develop hypernatremia due to extreme dehydration. Hypernatremia may cause neurologic symptoms, e.g neuromuscular excitability, confusion, seizures, or coma March 25-Apr 4, 2024 Dr J. Mohan 26 Mechanism of action of ADH on the Principal Cell in the Late DT & CD Figure 34-3, Koeppen & Stanton, 2010 March 25-Apr 4, 2024 Dr J. Mohan 27 Disorders in ADH production Syndrome of inappropriate ADH secretion (SIADH) – Infections/neoplasms of brain, antitumor drugs, pulmonary diseases, lung carcinoma. – Excessive release of ADH – Retain water & body fluids become progressively hypoosmotic – Urine is more hyperosmotic than expected based on the low body fluid osmolarity i.e. inappropriately concentrated for the serum osmolarity – Treated with an ADH antagonist e.g. demeclocycline or water restriction March 25-Apr 4, 2024 Dr J. Mohan 28 Case Study A case of diabetes insipidus? A 45 year old woman is admitted to the hospital following a head injury. She has polyuria and polydipsia. During a 24 hour period in the hospital she produces 10 L of urine containing no glucose. She is placed on water restriction for further evaluation. The next day, her serum osmolarity is 330mOsm/L, her serum [Na+] is 164 mEq/L and her urine osmolarity is 70 mOsm/L. She is treated with l-deamino-8-D arginine vasopressin (dDAVP) by nasal spray. Within 24 hours of treatment, her serum osmolarity is 295 mOsm/L and her urine osmolarity is 620 mOsm/L. Questions 1.Where is ADH synthesised and secreted? 2.What effect does water restriction have on ADH secretion? How does this, in turn, affect serum osmolarity, serum [Na+] and urine osmolarity? 3.How is this different in this patient? 4.What is the condition that she has? 5.What are the main types of this condition? 6.Why does she have polyuria and polydipsia? 7.What is the cellular mechanism by which ADH works? March 25-Apr 4, 2024 Dr J. Mohan 29 Today’s Topics The role of the kidneys in water balance. The functions and the mechanism of action of ADH. Renal Mechanisms for Concentration and Dilution of Urine. March 25-Apr 4, 2024 Dr J. Mohan 30 Urine Concentration and Dilution the kidneys keep the solute load of the body fluids constant (300 mOsm/L) by regulating urine concentration and volume regulation of urine concentration & volume is possible because of an osmotic gradient in the interstitial fluid of the kidneys from the renal cortex to the medulla (300 mOsm to 1200 mOsm/Kg H20) the osmotic gradient originates from 2 processes : 1) countercurrent multipication and 2) urea recycling March 25-Apr 4, 2024 Dr J. Mohan 31 Osmotic gradient in the Renal Medulla Cortex Medulla Figure 25.15; Marieb & Hoehn, 2010 March 25-Apr 4, 2024 Dr J. Mohan 32 Countercurrent Mechanisms March 25-Apr 4, 2024 Dr J. Mohan 33 Countercurrent Mechanisms countercurrent mechanisms : – occurs when fluid flows in opposite directions in two adjacent segments of the same tube connected by a hairpin turn interaction between filtrate flow through the ascending and descending limbs of the long loops of Henle of juxtamedullary nephrons (countercurrent multiplier) blood flow through the ascending and descending portions of the vasa recta blood vessels (countercurrent exchanger) March 25-Apr 4, 2024 Dr J. Mohan 34 Countercurrent Multiplier : LoH Countercurrent multiplication : function of the LoH helps to deposit NaCl in the deeper regions of the kidney, thereby forming the osmotic gradient from cortex to medulla, through a repeating 2 step process the first step : “single effect” the 2nd step : flow of tubular fluid March 25-Apr 4, 2024 Dr J. Mohan 35 Countercurrent Multiplier : LoH Step 1 : “single effect” : the reabsorption of NaCl by the thick ascending limb of the LoH thick ascending limb of the LoH is impermeable to water, so water is not reabsorbed along with NaCl  dilutes the tubular fluid in the ascending limb the NaCl increases the osmolarity of the surrounding interstitial fluid descending limb is water permeable, so water flows out of this limb, until the osmolarity of the tubular fluid increases to that of adjacent interstitial fluid therefore, as a result of the “single effect”, the osmolarity of the ascending limb decreases and the osmolarities of the interstitial fluid and descending limb increase March 25-Apr 4, 2024 Dr J. Mohan 36 Countercurrent Multiplier : LoH Step 2: Flow of tubular fluid : as new fluid enters the descending limb, an equal volume of fluid must leave the ascending limb and enter the distal tubule the new fluid that enters the descending limb has an osmolarity of 300 mOsm/L because it is coming from the PT at the same time, the high osmolarity fluid in the descending limb (created by the single effect) is pushed down towards the bend of the LoH the gradient of osmolarity is beginning (100 mOsm/L) March 25-Apr 4, 2024 Dr J. Mohan 37 Countercurrent Multiplier : LoH Step 3: single effect again : NaCl reabsorbed out of the ascending limb and deposited in interstitial fluid and water remains behind in the ascending limb the osmolarity of the interstitial fluid and descending limb increases, increasing the gradient of osmolarity that was established in previous steps the osmolarity of the ascending limb decreases further March 25-Apr 4, 2024 Dr J. Mohan 38 Countercurrent Multiplier : LoH Step 4: flow of fluid again : new fluid with an osmolarity of 300 mOsm/L enters the decending limb from the PT which displaces fluid from the ascending limb as a result of this fluid shift, high osmolarity fluid in the descending limb is pushed downward towards the bend of the LoH the gradient of omolarity is now larger than in Step 2 (150 mOsm/L) March 25-Apr 4, 2024 Dr J. Mohan 39 Countercurrent Multiplier : LoH Figure 6-39; Costanzo, 2022 March 25-Apr 4, 2024 Dr J. Mohan 40 Countercurrent Multiplier : LoH 315 315 415 415 415 590 590 315 315 415 415 415 590 590 300 115 115 215 215 215 390 390 600 1200 March 25-Apr 4, 2024 Dr J. Mohan 41 Countercurrent Multiplier : LoH NB: the constant difference in filtrate concentration (200 mOsm) between the 2 limbs of the LoH and between the ascending limb and the interstitial fluid at any transverse level – created by the power of the NaCl pumps in the ascending limb. – but a 200 Osm gradient would not be enough to allow excretion to a very concentrated urine – countercurrent flow in the ascending and descending limbs allows the LoH to multiply these small changes in concentration into a gradient change along the vertical length of the loop that is 900 mOsm (1200-300 mOsm) March 25-Apr 4, 2024 Dr J. Mohan 42 Countercurrent Multiplier : LoH the basic 2 steps are repeated until the full cortex-medulla osmotic gradient is established each repeat of the 2 steps increases or “multiplies” the gradient March 25-Apr 4, 2024 Dr J. Mohan 43 Urea Recycling : Inner Medullary CD 2nd process that contributes to establishing the osmotic gradient – urea recycling in the inner medullary collecting duct 1. in the cortical and outer medullary CD, ADH increases water permeability, but does not affect urea permeability. Water is therefore reabsorbed from the cortical and medullary CD’s, but urea remains behind in the tubular fluid 2. the concentration of urea in the tubular fluid increases 3. in the inner medullary CD, ADH increases both water and urea permeability 4. in the presence of ADH, the inner medullary CD is permeable to urea which diffuses down its concentration gradient into the interstitial fluid Urea that would have other wise been excreted, is recycled to the inner medulla where it is added to the osmotic gradient March 25-Apr 4, 2024 Dr J. Mohan 44 Urea Recycling : Inner Medullary CD Freely filtered 100% 110% Half reabsorbed 50% ADH –sensitive urea transporter An amount = previously reabsorbed is secreted into LoH 50% % of filtered load of urea remaining in tubule Urea Uniporters Figure 6-40; Costanzo, 2022 March 25-Apr 4, 2024 Dr J. Mohan 45 Countercurrent Exchanger : Vasa Recta The vasa recta – capillary networks that supply blood to the renal medulla – 2 special features of renal medullary blood flow that contribute to the preservation of the high solute concentrations in this part of the kidney : 1. the medullary blood flow is low (