Chapter 26 Fluid, Electrolytes, and Acid/Base Balance Lecture 17 Marieb’s Human Anatomy and Physiology Marieb w Hoehn Chapter 26 Fluid, Electrolytes, and Acid/Base Balance Lecture 17 Slides 1-15; 80 min (with review of syllabus and Web sites) [Lecture 1] Slides 16 – 38; 50 min [Lecture 2] 118 min (38 slides plus review of course Web sites and syllabus)

Lecture Overview Overview Fluid (water) balance Electrolyte balance Compartments Body fluid composition Intercompartmental fluid shifts Electrolyte balance Acid-base balance Buffer systems Acidosis and alkalosis

Overview Our survival depends upon maintaining a normal volume and composition of Extracellular fluid (ECF) Intracellular fluid (ICF) Ionic concentrations and pH are critical Three interrelated processes Fluid balance (How does water move from one place to the other? ) Electrolyte balance (What is an electrolyte?) Acid-base balance (What is normal pH?)

Water Content of the Human Body Of the 40 liters of water in the body of an average adult male: - one-third (15L) is extracellular - two-thirds (25L) is intracellular Because of decreased bone mass and decreased fat mass, infants are up to about 73% water by weight. The elderly have about 45% body water. In a normal adult male (70Kg), there is about 60% body water, while the normal adult female has about 50% body water (due to increased % of body fat). Figure from: Hole’s Human A&P, 12th edition, 2010

Fluid Compartments Figure from: Hole’s Human A&P, 12th edition, 2010 ‘Compartments’ commonly behave as distinct entities in terms of ion distribution, but ICF and ECF osmotic concentrations are identical (about 290-300 mOsm/L). Why?

Osmolarity Amount of solute Osmolarity = Volume of H2O

Osmolarity and Milliequivalents (mEq) Recall that osmolarity expresses total solute concentration of a solution Osmolarity (effect on H2O) of body solutions is determined by the total number of dissolved particles (regardless of where they came from) The term ‘osmole’ reflects the number of particles yielded by a particular solute (milliosmole, mOsm, = osmole/1000) 1 mole of glucose (180g/mol) 1 mole of NaCl (58g/mol) Osmolarity = #moles/L X # particles yielded An equivalent is the positive or negative charge equal to the amount of charge in one mole of H+ A milliequivalent (mEq) is one-thousandth of an Eq Number of Eq = molecular wt. / valence -> 1 osmole of particles -> 2 osmoles of particles

Body Fluid Ionic Composition ECF major ions: - sodium, chloride, and bicarbonate ICF major ions: - potassium, magnesium, and phosphate (plus negatively charged proteins) Figure from: Hole’s Human A&P, 12th edition, 2010 You should know these chemical symbols and charges (valences) of ions

Important Principles of Fluids and Electrolytes Homeostatic mechanisms respond to changes in the ECF, not the ICF No receptors directly monitor total fluid or electrolyte balance, but do this indirectly through plasma volume and osmotic concentrations Cells cannot move water molecules by active transport (“Water follows salt”) Content of water or electrolytes Will rise if dietary gains exceed losses Will fall if dietary losses exceed gains

Movement of Fluids Between Compartments Figure from: Hole’s Human A&P, 12th edition, 2010 Water moves between mesothelial surfaces: peritoneal, pleural, and pericardial cavities as well as the synovial membranes. It also moves between the blood and CSF and through the fluids of the eye and ear Net movements of fluids between compartments result from differences in hydrostatic and osmotic pressures

Fluid (Water) Balance Figure from: Hole’s Human A&P, 12th edition, 2010 * urine production is the most important regulator of water balance (water in = water out)

Water Balance and ECF Osmolarity Regulation of water intake increase in osmotic pressure of ECF → osmoreceptors in hypothalamic thirst center → stimulates thirst and drinking (water! ) Regulation of water output Obligatory water losses (must happen) insensible water losses (lungs, skin) water loss in feces water loss in urine (min about 500 ml/day) increase in osmotic pressure of ECF → ADH is released concentrated urine is excreted more water is retained LARGE changes in blood vol/pressure → Renin and ADH release

Fluid Imbalance Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Dehydration and Overhydration Dehydration (removing only H2O) osmotic pressure increases in extracellular fluids water moves out of cells osmoreceptors in hypothalamus stimulated hypothalamus signals posterior pituitary to release ADH urine output decreases Overhydration (adding only H2O) osmotic pressure decreases in extracellular fluids water moves into cells osmoreceptors inhibited in hypothalamus hypothalamus signals posterior pituitary to decrease ADH output urine output increases Severe thirst, wrinkling of skin, fall in plasma volume and decreased blood pressure, circulatory shock, death ‘Drunken’ behavior (water intoxication), confusion, hallucinations, convulsions, coma, death

Electrolyte Balance Electrolyte balance is important since: Figure from: Hole’s Human A&P, 12th edition, 2010 Electrolyte balance is important since: It regulates fluid (water) balance Concentrations of individual electrolytes can affect cellular functions Na+: major cation in ECF (plasma: 136-142 mEq/L; Avg ≈ 140) K+: major cation in ICF (plasma: 3.8-5.0 mEq/L; Avg ≈ 4.0)

Regulation of Osmolarity Figures from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Recall: [Na+]  Osmolarity Osmolarity is regulated by altering H2O content ** Osmolarity = Amt of solute / volume of H2O

Fluid Volume Regulation and [Na+] Volume is regulated by altering Na+ content Figures from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Estrogens are chemically similar to aldosterone and enhance NaCl absorption by renal tubules Glucocorticoids can also enhance tubular reabsorption of Na+ Primary mechanisms of Na+ regulation: intake, aldosterone, adh, natriuretic peptides, and steroids (estrogen, glucocorticoids).

Summary Table of Fluid and Electrolyte Balance Condition Initial Change Initial Effect Correction Result Change in OSMOLARITY (**Corrected by change in H2O levels)  H2O in the ECF  Na+ concentration,  ECF osmolarity  Thirst →  H2O intake  ADH →  H2O output  H2O in the ECF  Na+ concentration,  ECF osmolarity  Thirst →  H2O intake  ADH →  H2O output Change in VOLUME (**Corrected by change in Na+ levels)  H2O/Na+ in the ECF  volume,  BP Renin-angiotensin:  Thirst  ADH  aldosterone  vasoconstriction  H2O intake  Na+/H2O reabsorption  H2O loss  H2O/Na+ in the ECF  volume,  BP Natriuretic peptides:  Thirst  ADH  aldosterone  H2O intake  Na+/H2O reabsorption  H2O loss You should understand this table

Potassium Balance Figure from: Hole’s Human A&P, 12th edition, 2010 Potassium loss generally occurs via the urine. The rate of tubular secretion of K+ varies with: Changes in the [K+] in the ECF Changes in pH Aldosterone levels Remember that Na+ can be exchanged for H+ or K+ in the nephron tubules

Calcium Balance [Ca2+] in ECF is about 5 mEq/L Figure from: Hole’s Human A&P, 12th edition, 2010 [Ca2+] in ECF is about 5 mEq/L Recall that calcitonin is more important in children whose osteoclasts release approx. 5 mg per day whereas in adults it is only about 0.8 gm.

Strengths of Acids and Bases Strong acids ionize more completely and release more H+ Weak acids ionize less completely and release fewer H+ (**allows them to act as buffers) Strong bases ionize more completely and bind more H+ Weak bases ionize less completely and bind fewer H+

Sources of Hydrogen Ions Figure from: Hole’s Human A&P, 12th edition, 2010 Volatile acids – can leave solution and enter the atmosphere, e.g., H2CO3 Fixed acids – do not leave solution; once produced they remain in body fluids until eliminated at the kidneys, e.g., Sulfuric and phosphoric acids Organic acids – participants in, or by-products of, aerobic metabolism. E.G., lactic acid, ketone bodies Some H+ is also absorbed from the digestive tract

Regulation of Hydrogen Ion Concentration 1. chemical acid-base buffer systems (physical buffers) first line of defense can tie-up acids or bases, but cannot eliminate them act in seconds 2. respiratory excretion of carbon dioxide a physiological buffer (can eliminate excess acid indirectly via CO2) minutes 3. renal excretion of hydrogen ions a physiological buffer (can eliminate excess metabolic acids directly, e.g., keto-, uric, lactic, phosphoric) hours to a day

Acid-Base Buffer Systems Bicarbonate System the bicarbonate ion converts a strong acid to a weak acid carbonic acid converts a strong base to a weak base an important buffer of the ECF (~ 25 mEq/L) H+ + HCO3- ↔ H2CO3 ↔ CO2 + H2O Strong acid Weak acid Phosphate System the monohydrogen phosphate ion converts a strong acid to a weak acid the dihydrogen phosphate ion converts a strong base to a weak base H+ + HPO4-2 ↔ H2PO4- Strong acid Weak acid

Acid-Base Buffer Systems Protein Buffer System ICF, plasma proteins, Hb Most plentiful and powerful chemical buffer system COOH group releases hydrogen ions when pH rises NH2 group accepts hydrogen ions when pH falls - Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Respiratory Excretion of Carbon Dioxide Figure from: Hole’s Human A&P, 12th edition, 2010 A physiological buffer system

Renal Excretion of Hydrogen Ions Figure from: Hole’s Human A&P, 12th edition, 2010 *The kidney is most powerful and versatile acid-base regulating system in the body

Buffering Mechanisms in the Kidney Note that secretion of H+ relies on carbonic anhydrase activity within tubular cells Net result is secretion of H+ accompanied by the (1)retention of HCO3- (2) Ability of kidneys to eliminate H+ is dependent upon buffers in the urine. If these buffers were not present, the kidneys could secrete less than 1% of the 100 mEq/L of H+ produced every day. The secretion of H+ can continue only down to a pH of tubular fluid of about 4.0 – 4.5. After this, H+ would flow back into the tubule cells as fast as they are pumped out. Buffers (mainly carbonic acid-bicarbonate, ammonia, and phosphate) help keep the pH of the tubular fluid above this pH ‘cutoff’. Production of new HCO3- Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Summary of Acid-Base Balance Figure from: Hole’s Human A&P, 12th edition, 2010 (Seconds) Know this slide! (Minutes) (Hours-Days)

Acidosis and Alkalosis If the pH of arterial blood drops to 6.8 or rises to 8.0 for more than a few hours, survival is jeopardized Classified according to: Whether the cause is respiratory (CO2), or metabolic (other acids, bases) Whether the blood pH is acid or alkaline Figure from: Hole’s Human A&P, 12th edition, 2010

Acidosis Nervous system depression, coma, death Respiratory acidosis Figure from: Hole’s Human A&P, 12th edition, 2010 (hypopnea) Respiratory acidosis Metabolic acidosis Nervous system depression, coma, death

Alkalosis Nervousness, tetany, convulsions, death Figure from: Hole’s Human A&P, 12th edition, 2010 The expected change in pH with respiratory alkalosis can be estimated with the following equations: Acute respiratory alkalosis: Change in pH = 0.008 X (40 – PCO2) Chronic respiratory alkalosis: Change in pH = 0.017 X (40 – PCO2) Respiratory alkalosis Metabolic alkalosis Nervousness, tetany, convulsions, death

Acidosis and Alkalosis What would be the indications of acidosis and alkalosis in terms of changes in pH and PCO2? pH and HCO3-? How would the body try to compensate for Acidosis Respiratory Metabolic Alkalosis See Handout: Marieb, Human Anatomy & Physiology, Pearson, 2004

Flow chart for Acidosis/Alkalosis Three things to check: 1) pH – 7.35-7.45 2) pCO2 – 35-45 mm Hg 3) HCO3- - 22 – 26 mEq/L pH   acidosis alkalosis  pCO2  HCO3-  pCO2 HCO3- respiratory metabolic respiratory metabolic Norm HCO3- Norm pCO2 Norm HCO3- Norm pCO2 HCO3-  pCO2  HCO3-  pCO2 No Comp Comp No Comp Comp No Comp Comp No Comp Comp

Review There are two major fluid compartments of the body Intracellular About 2/3 of body’s fluid Includes the fluid within cells Major ions: K+, Mg2+, PO43-, Proteins Extracellular About 1/3 of body’s fluid Includes interstitial fluid, plasma, lymph, and transcellular fluid Major ions: Na+, Cl-, HCO3-

Review There are two major forces affecting movement of fluid between compartments Hydrostatic Pressure Osmotic Pressure Fluid balance Amount of water you take in is equal to the amount of water you lose to the environment Intake of water in food/drink is the most important source of fluid Kidney regulation of water is the most important regulator of water loss

Review Electrolyte balance Balance: Gains and losses of every electrolyte are equal Electrolyte balance is important because It directly affects water balance Electrolyte concentrations affect cell processes Na+ (aldosterone, ADH, ANP) Increased [Na+ ] in ECF -> ↑ ADH, ↑ ANP Decreased [Na+ ] in ECF ->  ADH, ↑ aldosterone K+ ([K+] in plasma, aldosterone) Increased [K+ ] in ECF -> increased secretion, ↑ aldosterone Decreased [K+ ] in ECF -> decreased secretion,  aldosterone,

Review Electrolyte balance (cont’d) Acid-base balance Ca2+ (PTH, calcitriol, calcitonin) Increase in ECF -> calcitonin promotes bone deposition Decrease in ECF -> PTH , calcitriol ↑ intestinal absorption ↑ bone resorption  Ca2+ secretion, ↑ PO43- secretion Acid-base balance Production of H+ is exactly offset by the loss of H+ Major mechanisms of maintaining acid-base (chemical) buffer systems: HCO3-, PO43-, protein respiratory excretion of carbon dioxide renal excretion of hydrogen ions

Review Acidosis (pH < 7.35) Alkalosis (pH > 7.45) Compensations Excessive H+ in the plasma Respiratory acidosis Metabolic acidosis Alkalosis (pH > 7.45) Insufficient H+ in the plasma Respiratory alkalosis Metabolic alkalosis Compensations

Concentration Range (mEq/L) Review Electrolyte Concentration Range (mEq/L) Typical Value (mEq/L) Na+ 136 - 142 140 K+ 3.8 - 5.0 4.0 Ca2+ 4.5 – 5.8 5.0 Cl- 96 - 106 105 HCO3- 24 - 28 25