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This 13 page Class Notes was uploaded by Jess Graff on Wednesday May 4, 2016. The Class Notes belongs to BMS 508 at University of New Hampshire taught by Mary Katherine Lockwood, PhD in Spring 2016. Since its upload, it has received 19 views. For similar materials see Human Anatomy and Physiology II in Biological Sciences at University of New Hampshire.
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Date Created: 05/04/16
BMS 508.03 5/4/2016 Chapter 27 Electrolyte and Acid Base Balance Electrolyte Balance • Homeostatic Mechanisms • A rise in blood volume elevates blood pressure • A drop in blood volume lowers blood pressure • Monitor ECF volume indirectly by monitoring blood pressure • Baroreceptors at carotid sinus, aortic sinus, and right atrium • Hyponatremia • Body water content rises (overhydration) • ECF Na concentration <136 mEq/L • Hypernatremia • Body water content declines (dehydration) + • ECF Na concentration >145 mEq/L • ECF Volume • If ECF volume is inadequate: • Blood volume and blood pressure decline • Renin–angiotensin system is activated • Water and Na losses are reduced • ECF volume increases • Plasma Volume • If plasma volume is too large: • Venous return increases • Stimulating release of natriuretic peptides (ANP and BNP) • Reducing thirst • Blocking secretion of ADH and aldosterone • Salt and water loss at kidneys increases • ECF volume declines • Potassium Balance • 98% of potassium in human body is in ICF • Cells expend energy to recover potassium ions diffused from cytoplasm into ECF • Processes of Potassium Balance • Rate of gain across digestive epithelium • Rate of loss into urine • Potassium Loss in Urine • Is regulated by activities of ion pumps • Along distal portions of nephron and collecting system + + • Na from tubular fluid is exchanged for K in peritubular fluid • Are limited to amount gained by absorption across digestive epithelium (about 50–150 mEq or 1.9–5.8 g/day) + • Factors in Tubular Secretion of K + • Changes in K concentration of ECF • Changes in pH • Aldosterone levels • Changes in Concentration of K in ECF+ • Higher ECF concentration increases rate of secretion • Changes in pH • Low ECF pH lowers peritubular fluid pH • H rather than K is exchanged for Na in tubular fluid • Rate of potassium secretion declines • Aldosterone Levels + • Affect K loss in urine + + • Ion pumps reabsorb Na from filtrate in exchange for K from peritubular fluid • High K plasma concentrations stimulate aldosterone • Calcium Balance • Calcium is most abundant mineral in the body • A typical individual has 1–2 kg (2.2–4.4 lb) of this element • 99 percent of which is deposited in skeleton 2+ • Functions of Calcium Ion (Ca ) • Muscular and neural activities • Blood clotting • Cofactors for enzymatic reactions • Second messengers • Hormones and Calcium Homeostasis • Parathyroid hormone (PTH) and calcitriol • Raise calcium concentrations in ECF • Calcitonin • Opposes PTH and calcitriol • Calcium Absorption • At digestive tract and reabsorption along DCT • Is stimulated by PTH and calcitriol • Calcium Ion Loss • In bile, urine, or feces • Is very small (0.8–1.2 g/day) • Represents about 0.03 percent of calcium reserve in skeleton • Hypercalcemia 2+ • Exists if Ca concentration in ECF is >5.5 mEq/L • Is usually caused by hyperparathyroidism • Resulting from oversecretion of PTH • Other causes • Malignant cancers (breast, lung, kidney, bone marrow) • Excessive calcium or vitamin D supplementation • Hypocalcemia • Exists if Ca 2+ concentration in ECF is <4.5 mEq/L • Is much less common than hypercalcemia • Is usually caused by chronic renal failure • May be caused by hypoparathyroidism • Undersecretion of PTH • Vitamin D deficiency • Magnesium Balance • Is an important structural component of bone • The adult body contains about 29 g of magnesium • About 60% is deposited in the skeleton • Is a cofactor for important enzymatic reactions • Phosphorylation of glucose • Use of ATP by contracting muscle fibers • Is effectively reabsorbed by PCT • Daily dietary requirement to balance urinary loss • About 24–32 mEq (0.3–0.4 g) • Magnesium Ions (Mg ) 2+ • In body fluids are primarily in ICF 2+ • Mg concentration in ICF is about 26 mEq/L • ECF concentration is much lower • Phosphate Ions (PO 43–) • Are required for bone mineralization • About 740 g PO 43 is bound in mineral salts of the skeleton • Daily urinary and fecal losses about 30–45 mEq (0.8–1.2 g) 3 – • In ICF, PO 4 is required for formation of high-energy compounds, activation of enzymes, and synthesis of nucleic acids • In plasma, PO 43 is reabsorbed from tubular fluid along PCT • Plasma concentration is 1.8–3.0 mEq/L • Chloride Ions (Cl ) – • Are the most abundant anions in ECF • Plasma concentration is 100–108 mEq/L • ICF concentrations are usually low • Are absorbed across digestive tract with Na + • Are reabsorbed with Na by carrier proteins along renal tubules • Daily loss is small 48–146 mEq (1.7–5.1 g) Acid-Base Balance • Acid–Base Balance • pH of body fluids is altered by addition or deletion of acids or bases • Acids and bases may be strong or weak • Strong acids and strong bases • Dissociate completely in solution • Weak acids or weak bases • Do not dissociate completely in solution • Some molecules remain intact • Liberate fewer hydrogen ions • Have less effect on pH of solution • Carbonic Acid • Is a weak acid • In ECF at normal pH: • Equilibrium state exists H 2O «3H + HCO 3– • The Importance of pH Control • pH of body fluids depends on dissolved: • Acids • Bases • Salts • pH of ECF • Is narrowly limited, usually 7.35–7.45 • Acidosis • Physiological state resulting from abnormally low plasma pH • Acidemia plasma pH <7.35 • Alkalosis • Physiological state resulting from abnormally high plasma pH • Alkalemia plasma pH >7.45 • Acidosis and Alkalosis • Affect all body systems • Particularly nervous and cardiovascular systems • Both are dangerous • But acidosis is more common • Because normal cellular activities generate acids • Types of Acids in the Body • Fixed acids • Organic acids • Volatile acids • Fixed Acids • Are acids that do not leave solution • Once produced they remain in body fluids • Until eliminated by kidneys • Sulfuric acid and phosphoric acid • Are most important fixed acids in the body • Are generated during catabolism of: • Amino acids • Phospholipids • Nucleic acids • Organic Acids • Produced by aerobic metabolism • Are metabolized rapidly • Do not accumulate • Produced by anaerobic metabolism (e.g., lactic acid) • Build up rapidly • Carbonic Acid • A volatile acid that can leave solution and enter the atmosphere • At the lungs, carbonic acid breaks down into carbon dioxide and water • Carbon dioxide diffuses into alveoli • Carbon Dioxide • In solution in peripheral tissues: • Interacts with water to form carbonic acid • Carbonic acid dissociates to release: • Hydrogen ions • Bicarbonate ions • Carbonic Anhydrase • Enzyme that catalyzes dissociation of carbonic acid • Found in: • Cytoplasm of red blood cells • Liver and kidney cells • Parietal cells of stomach • Other cells • CO a2d pH • Most CO in2solution converts to carbonic acid • Most carbonic acid dissociates • PCO2 is the most important factor affecting pH in body tissues • PCO2 and pH are inversely related • When CO lev2ls rise: + • H and bicarbonate ions are released • pH goes down • At alveoli: • CO 2iffuses into atmosphere • H and bicarbonate ions in alveolar capillaries drop • Blood pH rises • Mechanisms of pH Control • To maintain acid–base balance: • The body balances gains and losses of hydrogen ions • And gains and losses of bicarbonate ions + • Hydrogen Ions (H ) • Are gained • At digestive tract • Through cellular metabolic activities • Are eliminated • At kidneys and in urine • At lungs • Must be neutralized to avoid tissue damage • Acids produced in normal metabolic activity • Are temporarily neutralized by buffers in body fluids • Buffers • Are dissolved compounds that stabilize pH + • By providing or removing H • Weak acids + • Can donate H • Weak bases + • Can absorb H • Buffer System • Consists of a combination of: • A weak acid • And the anion released by its dissociation • The anion functions as a weak base • In solution, molecules of weak acid exist in equilibrium with its dissociation products • Three Major Buffer Systems • Protein buffer systems • Help regulate pH in ECF and ICF • Interact extensively with other buffer systems • Carbonic acid–bicarbonate buffer system • Most important in ECF • Phosphate buffer system • Buffers pH of ICF and urine • Protein Buffer Systems • Depend on amino acids + • Respond to pH changes by accepting or releasing H • If pH rises: • Carboxyl group of amino acid dissociates • Acting as weak acid, releasing a hydrogen ion • Carboxyl group becomes carboxylate ion • At normal pH (7.35–7.45) • Carboxyl groups of most amino acids have already given up their H + • If pH decreases: • Carboxylate ion and amino group act as weak bases • Accept H + • Form carboxyl group and amino ion • Carboxyl and amino groups in peptide bonds cannot function as buffers • Other proteins contribute to buffering capabilities • Plasma proteins • Proteins in interstitial fluid • Proteins in ICF • The Hemoglobin Buffer System • CO d2ffuses across RBC membrane • No transport mechanism required • As carbonic acid dissociates: • Bicarbonate ions diffuse into plasma • In exchange for chloride ions (chloride shift) • Hydrogen ions are buffered by hemoglobin molecules • Is the only intracellular buffer system with an immediate effect on ECF pH • Helps prevent major changes in pH when plasma P is rising or falling CO2 • The Carbonic Acid–Bicarbonate Buffer System • Carbon dioxide • Most body cells constantly generate carbon dioxide • Most carbon dioxide is converted to carbonic acid, which dissociates into H and a bicarbonate ion • Is formed by carbonic acid and its dissociation products • Prevents changes in pH caused by organic acids and fixed acids in ECF • The Carbonic Acid–Bicarbonate Buffer System • Cannot protect ECF from changes in pH that result from elevated or depressed levels of CO 2 • Functions only when respiratory system and respiratory control centers are working normally • Ability to buffer acids is limited by availability of bicarbonate ions • Bicarbonate ion shortage is rare • Due to large reserve of sodium bicarbonate • Called the bicarbonate reserve • The Phosphate Buffer System • Consists of anion H PO 2a w4ak acid) • Works like the carbonic acid–bicarbonate buffer system • Is important in buffering pH of ICF • Limitations of Buffer Systems • Provide only temporary solution to acid–base imbalance • Do not eliminate H ions • Supply of buffer molecules is limited • Maintenance of Acid–Base Balance + • For homeostasis to be preserved, captured H must: • Be permanently tied up in water molecules • Through CO remo2al at lungs • Be removed from body fluids • Through secretion at kidney + • Requires balancing H gains and losses • Coordinates actions of buffer systems with: • Respiratory mechanisms • Renal mechanisms • Respiratory and Renal Mechanisms • Support buffer systems by: • Secreting or absorbing H + • Controlling excretion of acids and bases • Generating additional buffers • Respiratory Compensation • Is a change in respiratory rate • That helps stabilize pH of ECF • Occurs whenever body pH moves outside normal limits • Directly affects carbonic acid–bicarbonate buffer system • Increasing or decreasing the rate of respiration alters pH by lowering or raising the P CO2 • When P CO2 rises: • pH falls • Addition of CO dr2ves buffer system to the right • When P CO2 falls: • pH rises • Removal of CO driv2s buffer system to the left • Renal Compensation + – • Is a change in rates of H and HCO secretio3 or reabsorption by kidneys in response to changes in plasma pH • The body normally generates enough organic and fixed acids each day + to add 100 mEq of H to ECF • Kidneys assist lungs by eliminating any CO that: 2 • Enters renal tubules during filtration • Diffuses into tubular fluid en route to renal pelvis • Hydrogen Ions • Are secreted into tubular fluid along: • Proximal convoluted tubule (PCT) • Distal convoluted tubule (DCT) • Collecting system
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