Lecture 12 fluid, electrolyte and acid base balance

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Chapter 22
Fluid, Electrolyte and
Acid-Base Balance

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End of Chapter 22
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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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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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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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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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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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Electrolytes in Body Fluids
 Functions of electrolytes
1. Confined to compartments; control osmosis
2. Help maintain acid-base balance
3. Carry electrical currents
4. Serve as cofactors for enzymes

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Electrolyte Distribution
 Electrolyte content of ICF and ECF differ
significantly
 ICF: K+
major cation; protein, HPO4
2-
: 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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Electrolyte Distribution

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Other Electrolytes
 K+
high in ICF, low in ECF
 Regulated by aldosterone
 Mg2+
and SO4
2-
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
1.Buffers: fastest but incomplete
2.Respiratory responses: fast but incomplete
3.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+
+ HPO4
2-

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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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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