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Chapter 27

Chapter 27. Fluid, Electrolyte, and Acid-Base Homeostasis. Fluid Compartments. The fluid compartments of the body are all contained in either the intracellular compartment or the extracellular compartment

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Chapter 27

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  1. Chapter 27 Fluid, Electrolyte, and Acid-Base Homeostasis

  2. Fluid Compartments • The fluid compartments of the body are all contained in either the intracellular compartment or the extracellular compartment • Intracellularfluidis all fluid contained inside cells, and comprises 2/3 of all body fluids • Extracellular fluid is all fluid outside the cells. 1/3 of all body fluid is contained in the extracellular compartment

  3. Fluid Compartments • Intracellular fluid (ICF) – about two thirds by volume • Extracellular fluid (ECF) – consists of: • Plasma • Interstitial fluid (IF) – fluid in tissue spaces • Other ECF – lymph, CSF, synovial fluid, serous fluid etc.

  4. Fluid Compartments • Babies are more “wet” than adults, with water composing about 80% of total body mass Water makes up 55–80% of total body mass (depending on age and sex)

  5. Composition of Body Fluids • Extracellular fluids (ECFs)are similar (except for the high protein content of plasma) • Sodium is the chief cation, chloride is the major anion • Intracellular fluid • Potassium is the chief cation, phosphate is the chief anion • Three times protein content than plasma • Sodium and potassium concentrations in ECF & ICF are opposite due: • Cell membrane Na+/ K+ ATPase pump

  6. Fluid movement between plasma & interstitial fluid • Exchange occurs across capillary membranes • At the arterial end, net HP is more (fluid flows out) • At the venous end of a bed, net COP is more ( fluid flows in) • Any leakage of fluid from the blood is picked up by lymphatics & returned to the blood HP 35 HP16

  7. Fluid movement between intracellular fluid & interstitial fluid • Exchanges between IF & ICF occur across plasma membrane- depend on membrane permeability • Water moves according to osmotic gradients; from low osmolarity to high osmolarity (from more water to less water)

  8. Fluid movement between compartments: Fluid movement after fluid intake: when you drink water, water enters your blood from the digestive system  plasma osmolarity decreases Water moves out of plasma to become part of the interstitial fluid, and then moves from the interstitial fluid into cells Reverse when dehydrated

  9. Fluid Balance • Normal fluid intake is through: • Ingestion of liquids and moist foods (2300mL/day) • Metabolic synthesis of water during cellular respiration and dehydration synthesis (200mL/day) • Normal fluid loss is through: • The kidneys (1500mL/day) • Evaporation from the skin (600mL/day) • Exhalation from the lungs (300mL/day) • In the feces (100mL/day)

  10. Fluid Balance • Fluid intake and output (I & O) are usually balanced on a daily basis, despite the fact that intake of water and electrolytes are rarely proportional • The kidneys excrete excess water through dilute urine,or retain water through concentrated urine

  11. Regulation of water intake- the thirst mechanism • The hypothalamic thirst center is stimulated: • By increases in plasma osmolarity- even a small increase (by stimulating osmoreceptors in the hypothalamus) • thirst increases water intake – osmolarity becomes normal • decreased salivary secretions- sensory input relayed from receptors in mucous membranes to thirst center • decreased blood pressure (significant decrease) • renin released from kidney- in response angiotensin II is formed which stimulates the thirst center

  12. Fluid Intake

  13. Regulation of Water Output • Antidiuretic hormone (ADH) plays a major role in directly regulating water loss in the collecting ducts of the kidneys • Rise in osmolarity stimulates hypothalamic osmoreceptors -triggers ADH release • Significant decrease in blood volume/BP also triggers ADH release, but rise in osmolarity more potent stimulus • ADH increases permeability of the collecting ducts to water by insertion of aquaporins into the principal cells – water reabsorbed-producing a concentrated small volume urine- water retained in the body • When ADH levels low- water in CDs not reabsorbed- producing dilute urine

  14. Mechanisms and Consequences of ADH Release

  15. Regulation of Water Output • Water output also regulated by: • Aldosterone: promotes urinary Na+ reabsorption (followed by water by osmosis) & decrease urine output • Atrial natriuretic peptide (ANP) • promotes excretion of Na+ followed by water excretion-increases urine output • Angiotensin II- decreases GFR- decreases urine output

  16. Edema • Edema occurs when excess interstitial fluid collects, causing swelling in the tissues. Edema occurs anytime filtration exceeds reabsorption • The most important causes of edema are: • increased blood pressure (increased blood hydrostatic pressure) • an increase in the capillary permeability • a decrease in COP (decreased plasma proteins) • an obstruction in lymphatic drainage

  17. Electrolytes • Water is the universal solvent • Solutes classified into: • Nonelectrolytes–do not dissociate in solution e.g. glucose, lipids, urea • Electrolytes –dissociate into ions in solution e.g. salts, acids, bases • Electrolytes have greater osmotic power and cause fluid shifts ( because more number of particles in solution) • Electrolytes expressed in milliequivalents per liter (mEq/L) • Sodium - 136-146 mEq/L • Potassium - 3.5-5.0 mEq/L

  18. Sodium • Most abundant extracellular cation • accounts for most of osmolarity of ECF • Sodium salts account for 90-95% of all solutes in ECF • Regulation of Na-water balance is linked to BP & blood volume regulation

  19. Regulation of Sodium Balance: Aldosterone • Sodium reabsorption in the kidney • 65% of sodium in filtrate is reabsorbed in the proximal tubules • 25% is reabsorbed in the loops of Henle • When aldosterone levels are high, all of remaining Na+ can be reabsorbed in DCT & CDs-water follows if tubule permeability has been increased with ADH • When aldosterone is inhibited- no more Na reabsorbed in DCT & CDs • ANP: decreases Na reabsorption- causing sodium loss followed by water loss in urine

  20. Chloride • Cl- is most prevalent extracellular anion • Regulation: • In the kidney negatively charged chloride passively follows the positively charged Na+ • Helps balance anions in different compartments e.g. chloride shift across red blood cells with HCO3 ions • It plays a role in forming HCl in the stomach

  21. Potassium • K+ is the most abundant cation in intracellular fluid • Exchanged for H+ across cells to help regulate pH of body fluids • Helps establish resting membrane potential & repolarize nerve & muscle cells • Regulation: mainly by aldosterone which stimulates principal cells to increase K+ secretion into the urine • Abnormal plasma K+ levels adversely affect cardiac and neuromuscular function

  22. Clinical Application • Hyperkalemia-high K+ concentration • Can be caused by crush injury, hemolytic anemia's, (K+ released from ruptured cells) • Can cause death by abnormal cardiac rhythms • Hypokalemia- low K+ concentration • Can be caused by excessive vomiting, diarrhea • Nerve & muscle cells become less excitable, can cause muscle paralysis

  23. Bicarbonate (HCO3-) • Forms the blood acid-base buffer system with carbonic acid. • Concentration increases as blood flows through systemic capillaries due to CO2 released from metabolically active cells • Concentration decreases as blood flows through pulmonary capillaries and CO2 is exhaled • Kidneys are main regulator of plasma levels • Intercalated cells of collecting ducts generally reabsorb HCO3-, but can excrete excess in the urine if levels high

  24. Calcium • 98% located in bones and teeth. • Important role in blood clotting, neurotransmitter release, muscle contraction • Regulated by parathyroid hormone: • 1.Stimulates osteoclasts to release calcium from bone • 2. Increases production of calcitriol (VitD)- which promotes Ca++ absorption from GI tract • 3.Increases reabsorption of Ca in kidneys

  25. Phosphate • Present as calcium phosphate salts in bones and teeth, and in phospholipids, ATP, DNA and RNA • Is most important intracellular anion and acts as buffer of H+ inside cells and in urine • Regulation: plasma levels are regulated by parathyroid hormone • resorption of bone releases phosphate • in the kidney, PTH increase phosphate excretion, lowering blood phosphate • Calcitriol increases GI absorption of phosphate

  26. Clinical Application • Individuals at risk for fluid and electrolyte imbalances include: • those dependent on others for fluid and food needs • those undergoing medical treatment involving intravenous infusions, drainage, suction, and urinary catheters • those receiving diuretics • individuals with burns, and those with altered states of consciousness

  27. Acid-Base Balance • Normal pH of blood • Arterial blood is 7.4 • Venous blood is 7.35 • Alkalosis– arterial blood pH rises above 7.45 • Acidosis– arterial pH drops below 7.35

  28. Sources of Acids • Produced from metabolic wastes: • e.g., lactic acid from anaerobic cellular respiration • e.g., phosphoric acid from nucleic acid metabolism • e.g. ketoacids from fat metabolism • Regulated by kidney through reabsorption and elimination of HCO3- and H+ • Volatile acid: • Carbonic acid produced when carbon dioxide combines with water • CO2 + H2O  H2CO  H+ + HCO3- • Regulated by respiratory system through respiratory rate

  29. pH Regulation • Concentration of hydrogen ions in blood is regulated by: • Chemical buffer systems – act within seconds • The respiratory system– acts within 1-3 minutes • Renal mechanisms– require hours to days to effect pH changes

  30. Strong acids – dissociate completely-release all their H+ in water Weak acids – dissociate partially in water- act as buffers Strong bases – dissociate easily in water and quickly tie up H+ Weak bases – accept H+ more slowly (e.g., HCO3¯)- act as buffers Chemical Buffer Systems- General Concepts

  31. Chemical Buffer Systems • A chemical buffer consists of a weak acid and a weak base • Resist pH changes when a strong acid or base is added by converting strong acids or bases into weak acids & weak bases • Three major chemical buffer systems • Bicarbonate buffer system • Phosphate buffer system • Protein buffer system

  32. Carbonic acid-Bicarbonate Buffer System • Bicarbonate buffer system is a mixture of carbonic acid (H2CO3) a weak acid , and bicarbonate (HCO3 ) a weak base • This system is the most important ECF buffer (blood, tissue fluids) • The HCO3 ion levels in ECF are regulated by the kidney, the H2CO3 levels by the lungs

  33. Bicarbonate Buffer System- contd. • If there is an excess of H+ due to a strong acid the buffer system acts by: • H+released by strong acid combine with the HCO3ions to form carbonic acid; a weak acid which then replaces the strong acid H+ + HCO3¯ H2CO3 • Therefore the pH of the solution decreases only slightly • If there is a shortage of H+ due to a strong base which ties up H+: • the carbonic acid dissociates into H+ and bicarbonate ions • The weak bicarbonate base replaces the strong base

  34. Chemical Buffer Systems • Phosphate buffer system • Important intracellular buffer, but also acts to buffer acids in the urine • Works the same way as the carbonic acid- bicarbonate system • Protein buffer system • Buffer in cells & in plasma • Hemoglobin is an important buffer which binds H+ released from H2CO3 formed during transport of CO2 • Albumin is main protein buffer in plasma

  35. Respiratory Regulation of pH: Exhalation of Carbon Dioxide • Respiratory system acts by changing the rate and depth of breathing, CO2 is exhaled or retained, and blood pH is corrected • CO2 formed by cell respiration enters RBCs & is converted to HCO3¯for transport in plasma CO2 + H2O  H2CO3 H+ + HCO3¯ • Normally released H+ are buffered by Hb • An increase in CO2 , increases H+ concentration, thus lowers the pH (makes body fluids more acidic) • An decrease in CO2 , decreases H+ concentration, thus raises the pH (makes body fluids more alkaline)

  36. Respiratory Regulation of pH: Exhalation of Carbon Dioxide • Changes in rate & depth of breathing can alter the pH within minutes: • An increase in the rate and depth of breathing causes more carbon dioxide to be exhaled, lowering pCO2 levels in blood; thereby increasing pH. • A decrease in respiration rate and depth means that less carbon dioxide is exhaled, increasing CO2 levels in blood causing the blood pH to fall.

  37. pH & rate & depth of breathing interact by a negative feedback loop: • Low pH detected by chemoreceptors in medulla, carotid & aortic bodies • Increases rate & depth of breathing; more CO2 exhaled: blood pH goes back up to normal

  38. Need for Renal Mechanisms • Chemical buffers prevent changes in pH, but they cannot eliminate acids & bases from the body • The body produces nonvolatile acids such as lactic acids, uric acid, ketone bodies etc, unlike volatile acid H2CO3, they cannot be removed by lungs- have to be removed by the kidneys • Therefore although slow acting the ultimate acid-base regulatory organs are the kidneys

  39. Acid-Base Balance • Kidneys control acid base balance by excreting: • Acidic urine– reducing the amount of acid in ECF • Alkaline urine- removing base in urine • The kidneys regulates acid base balance mainly by reabsorption of HCO3¯ & secretion of H+: • The kidneys prevent the loss of filtered HCO3¯conserving this important buffer • They secrete H+ to get rid of excess acid • Both the PCT & collecting ducts secrete H+ and reabsorb HCO3¯

  40. Kidney excretion of H+ -PCT • Within PCT cells H+ and bicarbonate ion produced • Na+/H+ antiporters used to secrete H+ while bicarbonate reabsorbed into peritubular capillaries • H+ secreted into the tubular fluid, combines with filtered bicarbonate to form CO2 & water • H+ secreted but not actually excreted in urine

  41. Kidney excretion of H+ in Collecting ducts • Withinintercalated cells H+ and bicarbonate ion are produced • The intercalated cells have proton pumps (H+ ATPases) that secrete H+ into the tubular fluid, while HCO3– reabsorbed • H+ secreted is actually excreted in urine; can create very acidic urine • If pH of blood too alkaline other intercalated cells can secrete HCO3– and reabsorb H+

  42. Respiratory Acidosis and Alkalosis • Respiratory system itself the cause of pH imbalance; caused by changed levels of pCO2 • Respiratory acidosis: • Arterial blood pCO2 above 45mmHg • Occurs when a person breathes shallowly, or gas exchange is hampered by diseases such as pneumonia, emphysema, pulmonary edema • CO2 accumulates in blood-rise in pCO2 causes fall in pH • Respiratory alkalosis: • Arterial blood pCO2 below 35mmHg • CO2 is eliminated faster than it is produced- pH becomes alkaline • Common result of hyperventilation- stress, panic, stroke • Renal compensation can help keep pH within normal range by controlling H+ secretion& HCO3 reabsorption

  43. Metabolic Acidosis and Alkalosis • Metabolic pH imbalances – from disturbances in plasma HCO3¯ • Include pH imbalances except those caused by abnormal blood carbon dioxide levels • Metabolic Acidosis • Low bicarbonate levels , low pH • Causes- • excessive loss of bicarbonate ions in diarrhea • accumulation of lactic acid, keto acids in diabetic crisis • kidney failure- failing kidney unable to excrete H+ • Respiratory compensation through hyperventilation may bring pH into normal range

  44. Metabolic Acidosis and Alkalosis • Metabolic Alkalosis • Rising blood pH and bicarbonate levels indicate metabolic alkalosis • Typical causes are: • Vomiting of the acid contents of the stomach • Intake of excess base (e.g., from antacids) • Gastric suctioning • Respiratory compensation through hypoventilation may bring pH into normal range

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