1. Chapter 22 Fluid, Electrolyte and Acid-Base Balance
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Fluid Compartments
Total body water = 55-60% of lean body mass
Remainder: solid parts of bone, muscles, tendons
Major compartments (3): ICF, IF, plasma
Intracellular fluid (ICF): inside cells= 2/3
Extracellular Fluid (ECF): outside cells = 1/3
Interstitial fluid (IF): 80% of ECF
Includes: lymph; cerebrospinal, synovial, pericardial, pleural and peritoneal fluids; fluid in eyes and ears
Blood plasma: 20% ECF
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Fluid Compartments
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5. Barriers Between Compartments
Plasma membrane: between ECF and ICF
Blood vessel walls: between plasma and interstitial fluid
Fluid balance correct distribution of water & solutes
Water redistributes rapidly by osmosis
Thus fluid balance depends on solute (electrolyte) balance
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Fluid Balance
Fluid balance requires
Appropriate total volume of body fluid
Appropriate distribution of water and solutes
Fluid balance depends on solute (electrolyte and nonelectrolyte) balance
Fluids and electrolytes are closely linked
Water redistributes rapidly by osmosis
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7. Fluid Balance Interactions Animations
Water and Fluid Flow
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Water Gain and Loss
Gain: ingestion + metabolic reactions
Ingestion (food and drink): 2300 mL/day
Metabolism: 200 mL/day
Gain should = loss
Daily intake = daily output. Both 2500 mL/day
Loss: skin, lungs, kidneys, GI tract
Kidneys: ~1500 mL/day
Skin: sweat evaporates ~600 mL/day
Lungs: 300 mL/day; more if fever
GI tract: ~100 mL/day; more if diarrhea
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Water Balance
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Regulation of Gain
Thirst center in hypothalamus
~2% dehydration will cause  BP
Increase in body osmolality  dry mouth  thirst
Hormonal responses
High osmolality  hypothalamus releases ADH  water retention by kidneys
 BP  renin released from kidney  angiotensin II  aldosterone  water retention by kidneys
Sensation of thirst may be decreased, especially in elderly
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Regulation of Gain
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12. Regulation of Salt and Water Loss
Urinary NaCl loss mainly determines body fluid volume
Na+ = main solute in ECF determining osmosis
Fluid intake varies so loss must vary also
ANP, angiotensin II and aldosterone regulate
ADH regulates water loss
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13. Regulation of Salt and Water Loss
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Movement of Fluid
ICF and ECF are normally at the same osmolality
Water moves freely
 interstitial fluid osmolality  cell swelling and vice versa
Most often due to Na+ change
ADH responds rapidly: prevents significant cell change
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15. Electrolytes in Body Fluids
Functions of electrolytes
Confined to compartments; control osmosis
Help maintain acid-base balance
Carry electrical currents
Serve as cofactors for enzymes
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16. Electrolyte Distribution
Electrolyte content of ICF and ECF differ significantly
ICF: K+ major cation; protein, HPO42-: anions
ECF: Na+ major cation; Cl- major anion
Na+/K+ pump maintains the cation difference
The two ECF fluids are similar
Electrolytes in plasma similar to those in IF
One difference: plasma contains more protein than interstitial fluid (IF)
Colloid osmotic pressure (due largely to plasma proteins) “holds onto” fluid in capillaries
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17. Electrolyte Distribution
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Other Electrolytes
K+ high in ICF, low in ECF
Regulated by aldosterone
Mg2+ and SO42- high in ICF, low in ECF
Ca2+ high in ECF, low in ICF
Regulated in plasma (PTH, calcitriol, and calcitonin)
Bones serve as Ca2+ reservoir
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Acid- Base Balance
Input: diet, products of metabolism
Such as lactic acid, ketones
Output
Lungs: exhale CO2
Kidney: can eliminate H+ or HCO3-
Regulatory mechanisms
Buffers: fastest but incomplete
Respiratory responses: fast but incomplete
Renal responses: slowest but compete elimination
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1: Buffer Systems
Protein in cells or plasma
Carboxyl and amino groups of amino acids
Hemoglobin (protein) in red blood cells
Carbonic acid-bicarbonate
Especially important in plasma
CO2 + H2O  H2CO3 ↔ HCO3- + H+
Phosphate
H2PO4-  H+ + HPO42-
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21. 2: Exhalation of Carbon Dioxide
H+ + HCO3- ↔ H2CO3 ↔ CO2 + H2O
Decrease of CO2 ↔ decrease of H+
Increase of CO2 ↔ increase of H+
Change of rate and depth of ventilation rapidly alters plasma pH
Negative feedback loop regulates
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22. 2: Exhalation of Carbon Dioxide
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3. Renal Responses
Kidney Excretion of H+
Slow but only way to actually eliminate acid or base
Secrete H+ and replace with HCO3-
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Imbalances
Acidosis: arterial blood pH < 7.35
 Depresses CNS
Below pH 7.0 can be fatal
Alkalosis: arterial blood pH > 7.45
 Overexcitation of CNS
Muscle spasms, convulsions
Compensation
Respiratory or renal mechanisms
Respiratory very rapid; renal slower
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Aging
Decrease in control of water and electrolyte balance can lead to pH problems
Decreases in respiratory and renal functioning
Decreased capacity to sweat
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