REGULATION OF FLUID AND ELECTROLYTE BALANCE

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REGULATION OF FLUID AND ELECTROLYTE BALANCE

• Body Fluids and Fluid Compartments
• Body Fluid and Electrolyte Balance fluid and
  electrolyte homeostasis

• Why do we care about this?
• ECF volume
• Osmolarity

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Water Steady State

• Amount Ingested Amount Eliminated

• Pathological losses
• vascular bleeding (H20, Na)
• vomiting (H20, H)
• diarrhea (H20, HCO3-).

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Electrolyte (Na, K, Ca) Steady State

• Amount Ingested Amount Excreted.
• Normal entry Mainly ingestion in food.
• Clinical entry Can include parenteral
  administration.

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Electrolyte losses

• Renal excretion
• Stool losses
• Sweating
• Abnormal routes e.g.. vomit and diarrhea

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Body Fluids and Fluid Compartments

• The percentage of total body water 45-75
• Intracellular compartment
• 2/3 of body water (40 body weight)
• Extracellular compartment
• 1/3 of body water (20 body weight)
• the blood plasma (water4.5 body weight)
• interstitial fluid and lymph (water15 body
  weight)
• transcellular fluids e.g. cerebrospinal fluid,
  aqueous humor (1.5 BW)
• Distribution of substances within the body is NOT
  HOMOGENEOUS.

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Body Water Distribution

• Individual variability (lean body mass)
• 55 - 60 of body weight in adult males
• 50 - 55 of body weight in adult female
• 42 L For a 70 Kg man.

PLASMA WATER
RBC
ECF
5
3 L
20
14 L
CELL WATER
INTERSTITIAL FLUID COMPARTMENT
40
28 L
15
10 L
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Electrochemical Equivalence

• Equivalent (Eq/L) moles x valence
• Monovalent Ions (Na, K, Cl-)
• 1 milliequivalent (mEq/L) 1 millimole
• Divalent Ions (Ca, Mg, and HPO42-)
• 1 milliequivalent 0.5 millimole

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Solute Overview Intracellular vs. Extracellular

• Ionic composition very different
• Total ionic concentration very similar
• Total osmotic concentrations virtually identical

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Summary of Ionic composition
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Net Osmotic Force Development

• Semipermeable membrane
• Movement some solute obstructed
• H2O (solvent) crosses freely
• End point
• Water moves until solute concentration on both
  sides of the membrane is equal
• OR, an opposing force prevents further movement

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Osmotic Pressure (?)

• The force/area tending to cause water movement.

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Glucose Example
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Osmotic Concentration

• Proportional to the number of osmotic particles
  formed Osm/L moles x n (n, of particles in
  solution)
• Assuming complete dissociation
• 1mole of NaCl forms a 2 osmolar solution in 1L
• 1mole of CaCl2 forms a 3 osmolar solution in 1L
• Physiological concentrations
• milliOsmolar units most appropriate
• 1 mOSM 10-3 osmoles/L

e.g. 1 M NaCl 2 M Glu in Osm/L
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Principles of Body Water Distribution

• Body control systems regulate ingestion and
  excretion
• constant total body water
• constant total body osmolarity
• Osmolarity is identical in all body fluid
  compartments (steady state conditions)
• Body water will redistribute itself as necessary
  to accomplish this

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Intra-ECF Water RedistributionPlasma vs.
Interstitium

• Balance of Starling Forces acting across the
  capillary membrane
• osmotic forces
• hydrostatic forces

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Intracellular Fluid Volume

• ICFV altered by changes in extracellular fluid
  osmolarity.
• ICFV NOT altered by iso-osmotic changes in
  extracellular fluid volume.
• ECF undergoes proportional changes in
• Interstitial water volume
• Plasma water volume

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Primary Disturbance Increased ECF Osmolarity

• Water moves out of cells
• ICF Volume decreases (Cells shrink)
• ICF Osmolarity increases
• Total body osmolarity remains higher than normal

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Primary Disturbance Decreased ECF Osmolarity

• Water moves into the cells
• ICF Volume increases (Cells swell)
• ICF Osmolarity decreases
• Total body osmolarity remains lower than normal.

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Plasma Osmolarity Measures ECF Osmolarity

• Plasma is clinically accessible
• Dominated by Na and the associated anions
• Under normal conditions, ECF osmolarity can be
  roughly estimated as POSM 2 Nap 270-290
  mOSM

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SOLUTIONS USED CLINICALLY FOR VOLUME REPLACEMENT
THERAPY

• Isotonic Solutions --gt n.c. ICF
• Hypertonic Solutions --gt Decrease ICF
• Hypotonic --gt Increase ICF

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Type of solutions

• Saline solutions
• Come in a variety of concentrations hypotonic
  (eg., 0.2), isotonic (0.9), and hypertonic (eg.
  5).
• Dextrose in Saline
• Glucose is rapidly metabolized to CO2 H2O
• The volume therefore is distributed
  intracellularly as well as extracellularly
• Again available in various concentrations
• Used for simultaneous volume replacement and
  caloric supplement
• Dextran, a long chain polysaccharide
• Solutions are confined to the vascular
  compartment and preferentially expand this
  portion of the ECF

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Body Fluid and Electrolyte Balance

• Water input and output
• The role of the kidneys in maintaining balance of
  water and electrolytes
• The regulation of body water balance
• thirst sensation
• control of renal water excretion by ADH

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• Thirst centers in the hypothalamus
• relay information to the cerebral cortex where
  thirst becomes a conscious sensation
• controls the release of ADH
• Stimuli for thirst sensation
• Baroreceptors and stretch receptors as detectors
  
• impulses sent to the thirst control centers in
  the hypothalamus
• Effect of ADH (vasopressin)

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Factors affecting ADH release
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• Sodium balance
• The kidneys - the major site of control of
  sodium output
• Influence of dietary input on appropriate
  changes in sodium excretion by the kidneys
• Effector mechanisms include changes in
• glomerular filtration rate
• plasma aldosterone levels
• peritubular capillary Starling forces

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• renal sympathetic nerve activity
• intrarenal blood flow distribution
• plasma atrial natriuretic factor (ANF
• Effects of aldosterone
• The renin-angiotensin system
• release of renin
• action of renin on the formation of angiotensin
  II
• effects of angiotensin II a.blood pressure b.
  synthesis and release of aldosterone c.
  stimulation of the hypothalamic thirst centers
  d. release of ADH

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Pathway of RAAS
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Principal cells aldosterone
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• Net reabsorption of salt and water by the
  proximal convoluted tubule
• peritubular capillary hydrostatic forces
• colloid osmotic pressure
• Decrease in renal sodium excretion by stimulation
  of renal sympathetic nerves
• Release of Atrial natriuretic peptide (ANP)
• in response to an increase in blood volume
• increase sodium excretion by increasing GFR and
  inhibiting sodium reabsorption

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• Atrial natriuretic peptide

• Decreased blood pressure stimulates renin
  secretion

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• The regulated variable affecting sodium
  excretion - effective arterial blood volume
• Changes in effective arterial blood volume can
  elicit the appropriate renal response by three
  possible mechanisms
• a change in blood volume ? glomerular blood flow
  and capillary pressure ? GFR
• a change in blood volume detected by an
  intrarenal baroreceptor ? release of renin
• a change in blood volume could change
  peritubular capillary Starling forces

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• Other factors affecting sodium excretion
  include
• glucocorticoids
• estrogen
• osmotic diuretics
• poorly reabsorbed anions
• diuretic drugs

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Homeostasissevere dehydration
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• Potassium balance
• Potassium plays a number of important roles in
  the body
• electrical excitability of cells
• major osmotically active solute in cells
• acid-base balance
• cell metabolism
• The kidneys are the major site in control of
  potassium balance

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• Factors affecting the distribution of potassium
  between cells and extracellular fluid include
• activity of the sodium-potassium pump
• acid-base status of body fluids
• availability of insulin
• cellular breakdown due to trauma, infection,
  ischemia, and heavy exercise
• The regulation of plasma potassium by hormones
• insulin
• epinephrine
• aldosterone,

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• Factors affecting potassium excretion include
• intracellular potassium concentration
• aldosterone
• excretion of anions
• urine flow rate