Fluid & Electrolyte Balance

1. Fluid & Electrolyte Balance

2. Fluid Balance • homeostatic value-must be maintained • food & water are taken in • what is not needed is excreted • body is in constant flux • must be a balance between amount of water gained & amount lost • Ideally-should cancel each other out • digestive system-major source of water gain • urinary system-primary system for fluid removal

3. Electrolyte Balance • homeostatic value-must be maintained • electrolytes-Cl, Na, K, etc. are ingested everyday • water & sodium regulation are integrated defending body against disturbances in volume & osmolarity • K imbalance • trouble with cardiac & muscle functioning • Calcium imbalances • problems with exocytosis, muscle contraction, bone formation & clotting • H & HCO3- balance • determines pH or acid-base balance

4. Maintaining Fluid & Electrolyte Balance • homeostasis depends on integrationof respiratory, cardiovascular, renal & behavioral systems • primary route for excretion of water & ions-kidneys • essential for regulating volume & composition of fluids • lungs remove H+ & HCO3- by excreting CO2 • behavioral mechanisms • thirst & salt appetite aid in fluid & electrolyte balance

5. Osmolarity • number of soluteparticles dissolved in 1liter of water • reflected in solution’s ability to produce osmosis& alter osmotic propertiesof a solvent • depends onlyon number of non penetrating solute particles in solution • 10 molecules of Na+ has same osmotic activity as 10 glucose or 10 amino acid molecules in same amount of fluid

6. Osmolarity • important to maintain water balance since water can cross most membranes freely • water balance determines osmolarity • as osmolarity of ECF (extra cellular fluid) changeswater moves into or out of cells changing intracellular volumes & cell function • excess water intakeosmolarity decreaseswater moves into cells swell • Na intake (osmolarity increases) water moves out of cellsshrink • changes in cell volume impairs cell function • swelling • may cause ion channels to open • changing membrane permeability

7. Water • major constituent of body • all operations need water as diffusion medium • to distribute gas, nutrients & wastes • distributed differently among various body compartments • 63-65%-intracellular fluid (ICF) • 35- 37%-extracellular fluid (ECF) • ECF-composed of three parts • interstitial or tissue fluid-25% • plasma-8% • transcellular fluid-2% • miscellaneous fluids such as CSF, synovial fluid, etc.

9. Water Balance • obtained when daily gains & losses are equal • average intake and loss-2.5L each day • Gains • metabolism (200ml/day) • preformed water-food & drink • Losses • about 1.5L each day lost via urine • 200ml elmininated with feces • 300 ml is lost during breathing • 100 ml in sweat • 400ml in cutaneous transpiration • water that diffuses through epidermis & evaporates • output through breath & cutaneous transpiration is insensible water loss

11. Regulation of Intake • Intake-governed mostly by thirst • Dehydration • reduces blood volume & blood pressure • raises blood osmolarity • Detected by thirst center • hypothalamus • salivate lessdry mouth sense of thirst • ingest water • cools & moistens mouth • rehydrates blood • distends stomachinhibits thirst

12. Regulation of Output • only way to control water output significantly is through urine volume • kidneys cannot completely prevent water loss or replace lost water or electrolytes • changes in urine volume are usually linked to adjustments in sodium reabsorption • where sodium goes water follows • ADH is one way to control urine volume without sodium • ADHcollecting ducts synthesize aquaporins (water channels) water can diffuse out of ductwater reabsorbed

13. Electrolytes • participate inmetabolism • determinemembrane potentials • affectosmolarityof body fluids • major cations • Na, K, Ca & H • major anions • Cl, HCO3 & P • intracellular fluid contains more K+ • extracellular fluid has more Na+ & Cl-

14. Sodium • crucial role in water & electrolyte balance • involved in excitability of neurons & muscle cells (resting membrane potentials) • majorsolute in extracellular fluid • determines osmolarity of extracellular fluids

15. Sodium Balance • need about 0.5 grams of sodium each day • typical American consumes 3-7 g/day • kidneys regulate Na+ levels • hormonal mechanisms control Na concentrations • Aldosterone • primary role • ADH • ANP

16. ADH • NaCl added to body increased osmolarityADH (vaopressin) secretion & thirst increased • thirstdrink • osmolarity decreases • ADHkidneys • conserves water by concentrating urine • increased water reaborption increases BP • returned to normal with cardiovascular reflexes

18. Aldosterone • Na regulation also mediated by aldosterone • steroid hormone produced by adrenal cortex • stimuli-more closely tied to blood volume & pressure & osmolarity than Na • Hyponatremia & hyperkalemiaadrenal cortexaldosterone • Hypotension reninaldosterone secretion

19. Aldosterone • tells kidneys to reabsorb Na in distal tubule & collecting ducts • primary target-last 3rd of distal tubule • increases activity of Na-K ATPase • target cell-principal cell • Apical membranes of P cells have Na & K leak channels • Aldosterone enters by simple diffusion  combines with membrane receptors Na channels increase time they remain open • as intracellular Na increasesNa-K ATPase speeds up transport of Na into ECFnet result-rapid increase of Na reaborption that does not require synthesis of new channels or ATPase proteins • slower phase of actionnewly made channels & pumps inserted into epithelial cell membranes

20. Renin-Angiotensin-Aldosterone • primary signal for aldosterone release-angiotensin II • component of renin-angiotensin system • kidneys sense low blood pressure triggers specialized cells-juxtaglomerular cells (JG cells) in afferent arterioles to produce renin •  angiotensinogen angiotensin I  angiotensin II by ACE-angiotensin converting enzyme-found in lungs & on endothelium of blood vessels

21. Renin-Angiotensin-Aldosterone Path • Angiotensin IIadrenal cortex aldosteronedistal tubule reabsorbs Na • ADH secretion is also stimulatedwater reabsorption increases • because aldosterone is also acting to increase Na reabsorption, net effect-retention of fluid that is roughly same osmolarity as body fluids • net effect on urine excretion- decrease in amount of urine excreted, with lower osmolarity • Aldosteronemore NaCl reabsorbed in DCT & collecting ductsreduces filtrate osmolarity

22. Renin-Angiotensin-Aldosterone • stimuli that begin renin pathway- related directly or indirectly to blood pressure • JG cells are directly sensitive to pressure & respond to low pressure by releasing renin • sympathetic neurons are activated by cardiovascular control center when blood pressure dropsJG cellsrenin release • paracrine feedback from macula densa cells in distal tubule stimulate renin release • if fluid flow in distal tubule is highmacula densaNO-nitric oxideinhibits renin release • GFR or BP lowfluid flow low macula densa cellsNO loweredJG cellsrenin released

23. Sodium & Blood Pressure • Na reaborption does not directly raise blood pressure • retention helps stimulate fluid intake & volume expansion which increases blood volume& blood pressure

24. Angiotensin & Blood Pressure • Angiotensin II has other effects on blood pressure • increases it directly & indirectly through 4 pathways • activates angiotensin II receptors in brainincreases vasopressin secretionfluid retained in kidneys constricts blood vessels • Angiotensin II serves to stimulate thirstexpands blood volume & increases blood pressure • Vasoconstriction-also stimulated by angiotensin II increases blood pressure without changing blood volume • angiotensin II activates receptors in cardiovascular control centerincreases sympathetic output to heart & blood vesselsincreases cardio output & vasoconstriction  increases blood pressure

25. ANP • Na also regulated by ANP • atrial natriuretic peptide • peptide hormone made by heart atrial cells • released when walls of atria are stretched • ANP enhances Na excretion & urinary water loss • increases GFR by making more surface area available for filtration decreases Na & water reabsorption in collecting ducts • indirectly inhibits renin, aldosterone & vasopressin release

26. K Balance • most abundant cation of ICF • must be maintained within narrow range • changes affect resting membrane potentials • decreased Khypokalemiaresting membrane potential becomes more negative • increased Khyperkalemiamore K inside celldepolarization • Hypokalemiamuscle weakness • more difficult for hyperpolarized neurons & muscles to fire action potentials • very dangerous • respiratory & heart muscle might fail • Hyperkalemia • more dangerous of two situations • depolarization of excitable tissues make them more excited initiallycells unable to repolarize fully • become less excitableaction potentials smaller than normal may lead to cardiac arrhythmias

27. Sodium & Water Balance • Na & water reabsorption are separately regulated in distal nephron • water does not automatically follow Na reabsorption here • vasopressin (ADH) must be present • proximal tubule • water reabsorption automatically follows Na reaborption

29. Acid-Base Balance • water must be strictly monitored to keep it at a certain pH • not too acidic or too alkaline • metabolism depends on functioning enzymes • very sensitive to changes in pH • pH changes also disrupt stability of cell membranes • alter protein structure • normal pH range 7.35 - 7.45 • neutral side

30. pH • measurement of hydrogen ion concentration • lower pH indicates higher hydrogen concentration-higher acidity • higher pH indicates lower hydrogen concentration-higher alkalinity • pH-below 7.35-acidosis • pH-above 7.45-alkalosis • Strong acids dissociate readily in water giving up H which lowers pH • Weak acids ionized slightly • keep most of hydrogen bound • bases accept hydrogen ions • strong base has strong tendency to bind hydrogen ions • raises pH • weak base binds less hydrogen ions • less effect on pH • HNO2      H+  +  NO2 HNO2     H+  +  NO2

31. Disruptions of Acid-Base Balance • pH imbalances produce problems that can be life threatening • intracellular proteins comprising enzymes, membrane channels, etc • very sensitive to pH • functions of proteins depend on 3-d shape can become altered by pH changes • must balance gain & loss of H ions

32. Compensations for Acid-Base Imbalances • Buffers • first line of defense • always present • attempt to suppress changes in H+ • Kidneys • change in rate of hydrogen ion secretion by renal tubules • greatest effect • requires days to take effect • Lungs • can have rapid effect • cannot change pH as much as urinary system • change pulmonary ventilation-expel or retaining carbon dioxide

33. Chemical Buffers • any substance that can bind or release H ions such that they dampen swings in pH • three major chemical buffer systems of body • Bicarbonate System • Phosphate System • Protein System

34. Carbonic Acid-Bicarbonate Buffer System • most important extracellular buffer system • CO2 + H2OH2CO3 H+ + HCO3-__ • add H equation shifts to leftmore HCO3 made increases CO2 & H2O

35. Phosphate Buffer System • important in buffering ICF & urine • H2PO4H + HPO4 • H + HPO4 H2PO4

36. Protein Buffer System • involves amino acids accepting or releasing H+ •  pH: COOH  COO- + H+ •  pH: NH2 + H+ NH3 + amino group accepts H

37. Respiratory Compensation • change in respiratory rate directly affects carbonic acid-HCO3 buffer system • any change in PCO2 affects H ion & HCO3 concentrations • increasing or decreasing rate of respiration alters pH by lowering or raising PCO2 • PCO2 increasespH decreases • PCO2 decreasespH increases • excess CO2 ventilation increases to expel more • low CO2 ventilation is reduced

38. Renal Compensation • slower than buffers or lung compensation • changes rate of H & HCO3 secretion or reabsorption in response to changes in pH • directly-excretes or reabsorbs H ions • indirectly-changes reabsorption or excretion of HCO3 • during times of acidosis renal tubule secretes H+ into filtrate • HCO3- & K+ blood pH increases • pH levels-secretion of H ions decreased & bicarbonates not reclaimed

39. Disorders of Acid-Base Balance • Acidosis • low pHneurons less excitableCNS depressionconfusion & disorientation comadeath • Alkalosis • high pHneurons hyperexcitable numbness & tinglingmuscle twitches tetanus • Acid-base imbalances fall into two categories • Respiratory • Metabolic

40. Respiratory Acidosis • respiratory system cannot eliminate all CO2 made by peripheral tissues • accumulates in ECF lowers its pH • primary symptom of hypercapnia-respiratory acidosis • typical cause • Hypoventilation-low respiratory rate

41. Respiratory Alkalosis • uncommon • usually due to hyperventilation (plasma PCO2 decreases) • can be modulated by breathing into paper bag & rebreathing exhaled CO2

42. Metabolic Acidosis • due to drop in blood bicarbonate levels drop • lost due to renal dysfunction • lost through severe diarrhea • due to accumulation of non-volatile acids-organic acid • Lactic acidosis • Ketoacidosis • generation of large amount of ketone bodies • occurs during starvation & diabetes • may also be caused by impaired ability to excrete H ions at kidneys or by severe HCO3 loss as occurs during diarrhea or overuse of laxatives

43. Metabolic Alkalosis • HCO3 ions become elevated • Rare • can be due to non respiratory loss of acid • excessive intake of alkaline drugs • excessive vomiting causes a loss of HCl.

45. Compensations for Decreased pH

46. Compensations for Increased pH