TOPIC 11 Fluid & Electrolyte Balance

Biology 221 Anatomy & Physiology II TOPIC 11 Fluid & Electrolyte Balance Chapter 27 pp. 1041-1066 E. Lathrop-Davis / E. Gorski / S. Kabrhel

Fluid Compartments • Intracellular fluid (ICF) = fluid within the cell • Extracellular fluid (ECF) • plasma - fluid matrix of blood • interstitial fluid (IF) - fluid matrix of tissues Fig. 27.1, p. 1041

Composition of Body Fluids • Water = universal solvent • Solutes • nonelectrolytes = not ionized; with or without electrical charge • polar (hydrophilic) compounds such as carbohydrates, proteins that dissolve in water; have charged areas; and • lipids and other nonpolar (hydrophobic) solutes that do not (e.g., O2, CO2) • electrolytes = particles that ionize in water to form anions (- charge) and cations (+ charge)

Electrolytes • Inorganic salts (e.g., NaCl, NaHCO3, MgCl2, KCl, CaCO3) – do not form H+ or OH- when they dissociate • HCO3- = bicarbonate; CO3= = carbonate • NaHCO3  Na+ + HCO3- • CaCO3  Ca2+ + CO3= • Acids– dissociate to form H+ and an anion  lower pH • Inorganic acids (e.g., HCl) • Organic acids (e.g., H2CO3, amino acids, lactic acid, fatty acids, ketone bodies)

Electrolytes • Bases – dissociate to form OH- and cation (e.g., NaOH), or accept H+ (e.g, NH3)  raise pH • Inorganic bases - e.g., NaOH, NH3 • NaOH: NaOH  Na+ + OH- • NH3: NH3 + H+ NH4+ • Organic bases - e.g., nitrogen bases of DNA and RNA

ECF versus ICF • ECF: higher in Na+, Cl-, HCO3- • ICF: higher in K+, HPO42-, Mg2+, protein Fig. 27.2, p. 1043

Water Balance • Sources: • ingested water (from food or liquids) • metabolic water (from aerobic respiration in mitochondria) • glucose + 6 O2 6 CO2 + 6 H2O • Losses: • urine (60%)* • sweat • lungs • feces • skin See also Fig. 27.3, p. 1044 Fig. 27.4, p. 1044

Water Balance: Regulating Intake Thirst response by hypothalamus • Thirst stimulated by: • dry mouth (sensation carried to hypothalamus) • increased osmolality of ECF in hypothalamus • Results in urge to drink liquids Fig. 27.5, p. 1045

Water Balance: Obligatory Output Obligatory water loss – not controlled • Loss through lungs – air humdified during inhalation (necessary for gas exchange); some water lost with exhalation • Some always lost through feces - • diarrhea – irritation of GI tract decreases residence time  less reabsorption  greater loss through feces • Loss across skin – not completely water tight

Water Balance: Regulating Output • Sweat – controlled for body temperature regulation, not fluid balance • Urine output – controlled for water balance, electrolyte balance, pH balance and blood pressure

Regulating Urine Output: ADH Antidiuretic hormone • Protein hormone secreted by posterior pituitary in response to: • impulses from hypothalamus, which responds to increased osmolality of ECF (resulting in increased osmolality of IF in hypothalamic cells) • presence of aldosterone in plasma • Increases water permeability of collecting ducts • Water follows osmotic gradient back into plasma facultative water reabsorption Fig. 27.7, p. 1049

Regulating Urine Output: Aldosterone • Steroid hormone secreted by zona glomerulosa of adrenal cortex • Increases Na+ reabsorption in DCTs and CDs • Reabsorption of Na+ adds to osmotic gradient in IF  water follows by osmosis obligatory water reabsorption Fig. 27.8, p. 1050

Regulating Urine Output: Diuretics Enhance urinary output (decrease water reabsorption) • alcohol – inhibits ADH secretion • caffeine and most other drugs – inhibit Na+ reabsorption

Disorders of Fluid Balance • Dehydration – water loss exceeds water intake over a period of time; solute concentration gets too high • Hypotonic Hydration – cells have too much water (concentrations of cellular solutes becomes too dilute) due to excessive water intake or renal insufficiency Fig. 27.6, p. 1046

Disorders of Fluid Balance: Edema See Topics 4 - 6 Accumulation of fluid in IF caused by: • increased BP – which increases movement of fluid from the plasma into the IF • decreased lymphatic drainage • inflammation – caused by histamine and other chemicals resulting in vasodilation and increased capillary permeability • decreased blood proteins (due to decreased liver function, protein malnutrition, loss of proteins at glomerulus) – resulting in lower blood osmotic pressure to draw fluid back into blood

Electrolyte Balance • Electrolytes = charged particles • dissociate to form cations (+ charge) and anions (- charge) • include salts, acids, bases • Salts • ionic compounds that form cations and anions other than H+ and OH- (hydroxide) • sources: foods, fluids (e.g., sodas), small amounts from metabolism

Electrolytes: Salt Losses • perspiration in hot environment – hotter environment means more sweat  more salts lost with water • normal loss with feces • GI upset increases loss • diarrhea – shorter retention time  less reabsorption  greater loss of salts as well as water • vomiting loss of salts as well as water • urine – point of control for most important electrolytes

Important Electrolytes: Na+ & K+ • Na+ (sodium): main cation, accounts for 90-95% of all solutes in ECF • most important electrolyte in creating significant osmotic pressure • essential to neural and muscular function • K+ (potassium) • important to neuron and muscle function function due to its influence on membrane potential (repolarization, hyperpolarization) • also influences acid-base balance (to be discussed shortly)

Important Electrolyte: Ca2+ • important to neuron and muscle function • maintaining correct Na+ permeability of neuronal membranes • exocytosis of neurotransmitter • muscle contraction (all types) • action potential in autorhythmic cardiac cells • other functions of Ca2+ • clotting factor IV • important constituent of bone

Other Important Ions • Mg2+ (magnesium) • enzyme cofactor for carbohydrate and protein metabolism • important component of bone • Cl- (chloride): main anion; follows Na+

Control of Selected Ions: Sodium Aldosterone • steroid hormone secreted by zona glomerulosa of adrenal cortex • secreted in response to • high K+, low Na+ • angiotensin II (renin-angiotensin pathway) • ACTH from adenohypophysis • increases active reabsorption of Na+ from DCT and CD (without aldosterone, little Na+ is reabsorbed from DCT or CD) Fig. 27.8 p. 1050

Control of Selected Ions: Sodium ADH • released by neuro-hypophysis (synthesized in hypothalamus) in response to increased Na+ detected by hypothalamus increases water reabsorption to decrease plasma osmolality • affects concentration by not total amount of solute Fig. 27.7, p. 1049

Control of Selected Ions: Sodium Atrial natriuretic peptide (ANP; a.k.a., atrial natriuretic factor) • released in response to elevated BP • blocks reabsorption of Na+ (by decreasing aldosterone release) • blocks ADH secretion • inhibits renin release by kidney Fig. 27.10, p. 1052

Control of Selected Ions: Sodium • Estrogens • steroids produced by ovaries and zona reticularis of adrenal cortex • enhance Na+ reabsorption • Glucocorticoids • steroids produced by zona fasciculata of adrenal cortex • enhance Na+ reabsorption

Control of Sodium: Disorders • hyponatremia = decreased blood Na+ • neurological dysfunction (brain swelling; mental confusion, irritability, convulsions, progresses to coma; muscular twitching) • systemic edema (less osmotic pressure in plasma) • hypernatremia = increased blood Na+ • thirst • CNS dehydration leading to confusion, lethargy, progressing to coma • increased neuromuscular irritability leading to twitching and convulsions

Control of Selected Ions: Potassium • Regulated at CDs in cortex of kidney – K+ secretion tied to Na+ reabsorption • Most important factor in regulation is K+ concentration in plasma • increased K+ directly stimulates cells of CDs • excess of K+ causes K+ to move into CD cells  secretion into filtrate • Aldosterone – stimulates active secretion of K+

Control of Potassium: Disorders • hypokalemia = decreased blood K+ • cardiac arrhythmias • muscular weakness • alkalosis (due to action of kidney) • hypoventilation (to compensate for alkalosis) • mental confusion • hyperkalemia = increased blood K+ • nausea, vomiting, diarrhea • at slightly elevated levels causes tachycardia; at severely elevated levels causes bradycardia; cardiac arrhythmias, depression and arrest • skeletal muscle weakness and flaccid paralysis

Control of Selected Ions: Calcium Parathyroid hormone (PTH) – increases plasma Ca2+ • parathyroid glands secrete PTH in response to decreased plasma Ca2+ • PTH acts on • gut – stimulates uptake by epithelial cells (works by increasing Vita. D formation in kidney) • bones – stimulates osteoclasts, inhibits osteoblasts • kidney – acts on DCT to increase active reabsorption of Ca2+ (also inhibits PO42- reabsorption to maintain balance between Ca2+ and PO42-)

Control of Selected Ions: Calcium Calcitonin from thyroid • secreted by parafolliclar cells of thyroid in response to increased plasma Ca2+ • generally, thought to be only really important during youth when bones are being remodeled • stimulates osteoblasts, inhibits osteoclasts in bone • actions of calcitonin result in decreased plasma Ca2+

Control of Calcium: Disorders • Hypocalcemia – decreased blood calcium • tingling in fingers, tremors, convulsions, tetany • depressed cardiac function • “bleeder’s disease” • Hypercalcemia – increased blood calcium • bone wasting • kidney stones • nausea, vomiting • cardiac arrhythmias and arrest • depressed respiration • coma

Control of Selected Ions: Magnesium • Second most abundant intracellular cation • Important cofactor of enzymes involved in protein and carbohydrate metabolism • Important component of bone • Reabsorption inhibited by PTH

Control of Selected Ions: Anions • Cl- is major anion • follows Na+ actively or passively in PCT, DCT and CD to balance charge • Most others passively reabsorbed; involves membrane proteins for transport • hence, transport maxima exist for most anions • any concentration in excess of transport maximum is excreted in urine

Acid-Base Balance • pH – measure of H+ concentration in a liquid • pH of distilled water (i.e., neutral) = 7.0 • Normal values • arterial blood = average pH 7.4 • venous blood and interstitial fluid = pH 7.3 • intracellular fluid (ICF) = pH 7.0 (neutral, but more acidic than other compartments) • Protein function depends on H+ concentration

Acid-Base Balance • Acids – dissociate to form H+ and an anion • addition of H+ lowers pH • metabolic sources of acids: • anaerobic respiration (produces lactic acid) • protein catabolism (produces amino acids and keto acids) • fat metabolism (produces fatty acids and ketone bodies) • Bases – dissociate to form OH- and a cation, or sequester H+ (e.g., NH3) • removal of H+ or addition of OH- raises pH

Strength of Acids/Bases • Refers to ability to ionize (i.e., dissociate to form H+ or OH-) • Strong acids/bases – • dissociate readily and completely • are usually inorganic (e.g., HCl, KOH, NaOH, NH3) • lead to large changes in pH when added to unbuffered solutions Fig. 27.11, p. 1056

Strength of Acids/Bases (con’t) • Weak acids/bases – • do not completely ionize (i.e., some of the molecular form remains) • are usually organic (e.g., H2CO3, NaHCO3, amino acids, fatty acids) • only change pH slightly when added to unbuffered solutions Fig. 27.11, p. 1056

Acid-Base Balance: Chemical Buffer Systems • Act quickly • Involve exchange of strong acid/base for weak one • Three major chemical buffer systems • Bicarbonate buffer system • Phosphate buffer system • Protein buffer system

Chemical Buffers: Bicarbonate • Most important chemical buffer present in ECF; also buffers ICF • HCl + NaHCO3 --> H2CO3 + NaCl • NaOH + H2CO3 --> NaHCO3 + H2O • Carbonic acid (H2CO3) levels affected by carbon dioxide availability (ventilation) • Bicarbonate (NaHCO3) levels regulated by kidney

Chemical Buffers: Phosphate • very important in ICF and urine • HCl + Na2HPO4 NaH2PO4 + NaCl • NaOH + NaH2PO4 Na2HPO4 + H2O

Chemical Buffers: Protein • very important in ICF • also important in plasma • based on presence of acidic (donate H+)and basic (accept H+) side chains of amino acids • reduced hemoglobin = hemoglobin without O2 • reduced hemoglobin takes on H+ (decreases free H+ increases pH)

Acid-Base Balance: Respiratory System • Acts more slowly (minutes) • Respiratory compensation changes in ventilation make up for metabolic pH changes • CO2 + H2O  H2CO3 H+ + HCO3- • decreased pH (higher H+) stimulates medulla to increase ventilation  increases loss of CO2  less carbonic acid  pH increases (less H+) • increase pH (lower H+) decreases stimulation of medulla decreased ventilation  decreases loss of CO2 CO2 accumulates  more carbonic acid

Acid-Base Balance: Renal Compensation • Makes up for changes due to respiratory problems • Kidney excretes or conserves HCO3-, depending on needs of body to make up for changes in respiration that cause pH imbalances • if pH decreases, HCO3- is conserved • if pH increases, HCO3- is excreted

Acid-Base Balance: Renal Compensation • Kidney excretes H+ • if pH decreases, H+ is excreted • if pH increases, H+ is conserved • Only kidney rids body of metabolic acids other than H2CO3 • H+ competes with K+ for removal (hyperkalemia can lead to decreased pH)

Acid-Base Disorders • Changes in pH result from respiratory or metabolic causes • Factors that would cause increased pH if uncompensated = alkalosis • Factors that would cause decreased pH if uncompensated = acidosis

Disorders of Acid-Base Balance: Alkalosis • Any condition that may lead to alkalemia (increase in arterial pH above 7.45) • Respiratory Alkalosis • caused by hyperventilation • excessive loss of CO2 less H2CO3 • Metabolic Alkalosis • loss of H+ through vomiting • constipation (retention of HCO3-) • K+ depletion (stimulates secretion of H+) • excess aldosterone secretion (reabsorption of Na+ tied to loss of H+)

Effects of Alkalemia • Increased cardiac irritability and arrhythmias • Compensatory hypoventilation (if cause is metabolic) • Vascular changes • Vasodilation • Spasm of coronary arteries • Decreased cerebral blood flow • Seizures • Increased blood lactate (lactic acid) • Hypokalemia and hypocalcemia

Disorders of Acid-Base Balance: Acidosis • Any condition that may lead to physiological acidosis (decrease in arterial pH below 7.35) • Respiratory Acidosis–retention of CO2 • hypoventilation • impairment of lung function

Disorders of Acid-Base Balance: Acidosis (con’t) • Metabolic acidosis • loss of HCO3- in diarrhea (decreased reabsorption time) • renal disease (failure of kidney to excrete H+) • excess alcohol intake (ethanol  acetic acid) • high K+ in ECF (competes with H+ for excretion; either one is countertransported with Na+) • lactoacidosis – build-up of lactic acid after heavy exercise or prolonged hypoxia • ketoacidosis – generation of ketone bodies due to improper glucose metabolism (starvation or diabetes mellitus)

Effects of Acidemia • Increased pulmonary resistance leading to pulmonary edema • Decreased cardiac function (bradycardia, ventricular fibrillation) • Vascular changes (venoconstriction, arterial dilation) • Hyperkalemia • Insulin resistance • Coma

Respiratory Compensation • For metabolic acid-base disorders • Metabolic alkalosis results in compensatory hypoventilation • Metabolic acidosis results in compensatory hyperventilation

TOPIC 11 Fluid & Electrolyte Balance