REGULATION OF WATER AND ELECTROLYTE BALANCE (II) WATER, SODIUM & POTASSIUM BALANCE. Eamonn O Connor Trinity College Dublin

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1 LECTURE 8: REGULATION OF WATER AND ELECTROLYTE BALANCE (II) WATER, SODIUM & POTASSIUM BALANCE Eamonn O Connor Trinity College Dublin Lecture Outline 1 Water balance Sodium balance Potassium balance 1

2 Water Balance 2 Osmolarity and the movement of water Water reabsorption in the proximal tubule Establishment of the medullary osmotic gradient Role of the medullary osmotic gradient in water reabsorption in the distal tubule and collecting duct Exchange of Water 3 Intake Gastrointestinal tract (2.2 L/day) Metabolism (0.3 L/day) Output Insensible loss & Sweating (0.9 L/day) Gastrointestinal tract (0.1 L/day) Kidneys (1.5 L/day) Osmosis Water diffuses down concentration gradient Water moves from low solute to high solute Water reabsorption follows solute reabsorption 2

3 Factors Affecting Water Balance 4 Kidneys can only minimize fluid loss Intake is required in order to add fluids Stanfield 4 th Ed: Figure 19.3 Osmolarity of Fluids 5 Osmolarity of body fluids = mosm Kidneys compensate for changes in osmolarity of extracellular fluid by regulating water reabsorption Water reabsorption: Passive based on osmotic gradient Proximal tubules 70% filtered water is reabsorbed Not regulated Distal tubules and Collecting ducts Most remaining water is reabsorbed 20% distal tubules 10% collecting ducts Regulated by ADH 3

4 6 Water Reabsorption in the Proximal Tubule Stanfield 4 th Ed: Figure 19.5 Medullary Osmotic Gradient for Water Reabsorption 7 Osmolarity of interstitial fluid of renal medulla varies with depth Lower osmolarity near cortex Greater osmolarity near renal pelvis Gradient critical to water reabsorption 4

5 The Medullary Osmotic Gradient 8 Osmotic gradient established by the Loop of Henle Stanfield 4 th Ed: Figure 19.6 Counter-Current Multiplier 9 Peritubular fluid Tubular fluid Cortex Fluid Medulla 400 No osmotic gradient (mosm) Na + Cl K + H 2 O H 2 O Na + Cl K Fluid enters tubule Active transport of Na +, Cl, K + ions into medullary interstitial fluid increases osmolarity Water moves out of descending limb by osmosis Iso-osmotic state in descending limb; osmotic difference between descending and ascending limbs More fluid enters tubule, pushing fluid through by bulk flow Active transport of Na +, Cl, K + ions into medullary interstitial fluid increases osmolarity H 2 O H 2 O H 2 O Osmotic gradient established (mosm) Water moves out of descending limb by osmosis Iso-osmotic state in descending limb; osmotic difference between descending and ascending limbs More water enters tubule and process continues System is in steady state Stanfield 4 th Ed: Figure

6 Result of Counter-Current Multiplier 10 Fluid in proximal tubule = mosm Fluid in descending limb osmolarity increases as it descends Osmolarity = interstitial fluid Osmolarity > ascending limb Fluid in ascending limb osmolarity decreases as it ascends Osmolarity < interstitial fluid, descending limb Fluid in distal tubule = 100 mosm Cortical interstitial fluid = mosm Medullary interstitial fluid Increases from cortex to renal pelvis Vasa Recta: Countercurrent Exchanger 11 Stanfield 4 th Ed: Figure

7 12 Water Reabsorption in Distal Tubules & Collecting Ducts Dependent on osmotic gradient established by counter-current multiplier Dependent on epithelium permeability to water Water permeability dependent on water channels Aquaporin-3: always present in basolateral membrane Aquaporin-2: present in apical membrane only when ADH present in blood Effects of ADH on Principal Cells 13 Stanfield 4 th Ed: Figure

8 Regulation of ADH Release 14 ADH = posterior pituitary hormone Released from neurosecretory cells originating in hypothalamus Primary stimulus for release Increased osmolarity (osmoreceptors) Other stimuli Increase blood pressure (baroreceptors) Increased blood volume (volume receptors) Increase in ECF Osmolarity/Decrease in BP 15 Stanfield 4 th Ed: Figures & 12 8

9 Sodium Balance 16 Hypernatremia = high plasma sodium Hyponatremia = low plasma sodium Sodium primary solute in ECF Critical for normal osmotic pressure Critical to function of excitable cells Renal Handling of Sodium Freely filtered Reabsorbed in proximal tubule, distal tubule, and collecting duct No secretion Reabsorption regulated by aldosterone and ANP Reabsorption regulated at principal cells of distal tubule and collecting duct Sodium Reabsorption 17 Proximal tubule: Coupled to the reabsorption of other solutes Distal tubule: Coupled to the secretion of K + and H + Stanfield 4 th Ed: Figure

10 Aldosterone & Sodium Reabsorption 18 Aldosterone increases sodium reabsorption Steroid hormone Secreted from adrenal cortex Acts on principal cells of distal tubules and collecting ducts Increases number of Na+/K+ pumps on basolateral membrane Increases number of open Na+ and K+ channels on apical membrane Effects of Aldosterone 19 Stanfield 4 th Ed: Figure

11 Renin-Angiotensin-Aldosterone System 20 [NA + ] tubular fluid ( MAP) Stanfield 4 th Ed: Figure Angiotensin II: Effect on MAP 21 Angiotensin II Systemic arterioles Adrenal cortex Posterior pituitary Hypothalamic neurons Vasoconstriction Aldosterone secretion ADH secretion Thirst stimulation Kidneys Sodium reabsorption in late distal tubules and collecting ducts Water reabsorption in late distal tubules and collecting ducts Extracellular fluid osmolarity Plasma volume MAP Stanfield 4 th Ed: Figure

12 Atrial Natriuretic Peptide (ANP) 22 Secreted by atrial cells in response to distension of atrial wall Increases GFR Dilation of afferent arteriole Constriction of efferent arteriole Decreases sodium reabsorption by closing sodium channels in apical membrane Overall effect: increased sodium excretion Potassium Balance 23 Potassium crucial to function of excitable cells Hyperkalemia = high plasma potassium Hypokalemia = low plasma potassium Renal Handling of Potassium Ions Glomerulus freely filtered Proximal tubules reabsorbed Distal tubules and collecting ducts reabsorbed and secreted Potassium secretion in distal tubules and collecting ducts is regulated Aldosterone regulates principal cells 12

13 Proximal Tubule: Potassium Reabsorption 24 Stanfield 4 th Ed: Figure 19.19a Distal & Collecting Tubule: Potassium Secretion 25 Stanfield 4 th Ed: Figure 19.19b 13