Fluid, Electrolyte and AcidBase Balance

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Fluid, Electrolyte and Acid-Base Balance

• Chapter 27

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Body mass solid vs fluid
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Wheres the water?
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Water content varies with age tissue type

• Infants 73
• Adult male 60
• Adult female 50
• Elderly 45
• Fat has the lowest water content (20).
• Bone is close behind (22 25).
• Skeletal muscle is highest at 65.

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Electrolyte concentrations are calculated in
milliequivalents
mEq/L ion concentration (mg/L) x number of
charges on one ion atomic weight
Na concentration in the body is 3300 mg/L Na
carries a single positive charge. Its atomic
weight is approximately 23. Therefore, in a
human the normal value for Na is 3300 mg/L
143 mEq/L 23
Note One mEq of a univalent is equal to one mOsm
whereas one mEq of a bivalent ion is equal to ½
mOsm. However, the reactivity of 1 mEq is equal
to 1 mEq.
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Relative electrolyte concentrationsPlasma,
Interstitial Fluid ICF
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Sources of intake output
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Regulation of water balance

• It is not so much water that is regulated, but
  solutes.
• osmolality is maintained at between 285 300
  mOsm.
• An increase above 300 mOsm triggers
• Thirst
• Antidiuretic Hormone release

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The Thirst Mechanism
An increase of 2 3 in plasma osmolality
triggers the thirst center of the
hypothalamus. Secondarily, a 10 15 drop in
blood volume also triggers thirst. This is a
significantly weaker stimulus.
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Sodium influence on blood volume
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Sodium regulation
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Dehydration

• Chronic dehydration leads to oliguria.
• Severe dehydration can result in hypovolemic
  shock.
• Causes include
• Hemorrhage
• Burns
• Vomiting
• Diarrhea
• Sweating
• Diuresis, which can be caused by diabetes
  insipidus, diabetes mellitus and hypertension
  (pressure diuresis).

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Hypotonic hydration

• A severe drop in osmolality
• Caused by
• Excessive water intake
• Renal dysfunction
• Major consequence is hyponatremia.
• Hyponatremia results in
• Cerebral edema (brain swelling)
• Sluggish neural activity
• Convulsions, muscle spasms, deranged behavior.
• Treated with I.V. hypertonic mannitol or
  something similar.

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Hypotonic hydration
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A rather lame illustrationYou do remember how
osmosis works, dont you?
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Blood pressure, sodium, and water
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Atrial Naturetic PeptideThe hearts own
compensatory mechanism.
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What a buffer

• Acids are proton donors
• Bases are proton acceptors
• Strong acids bases dissociate completely
• Acid buffer systems are comprised of compounds
  that resist pH changes by accepting protons from
  solutions containing strong acids.
• Base buffer systems accept OH- ions from
  solutions. (Not discussed in the text).

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Chemical Buffer Systems

• KEY CONCEPTS
• 1. The major fluid compartments of the body are
  the extracellular fluid compartment (EFC), which
  includes the plasma and interstitial fluid, and
  the intracellular fluid compartment (IFC). About
  33 of the body fluid is ECF and 67 is ICF.

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Chemical Buffers key concepts

• 2. The body controls the ECF osmolarity and the
  ECF volume through the plasma compartment, even
  though it is only 7 of the total body fluids.

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Chemical Buffers key concepts

• 3. Regulating pH at about 7.4 is important for
  normal cellular function. pH is a log scale of
  relative amounts of H and OH-. The scale runs
  from 0 (acidic) to 14 (basic) with 7 being
  neutral (having an equal amount of H and OH-
  ions). Buffers, such as Hb, other proteins,
  phosphates, and bicarbonates, are substances
  which cushion a solution from abrupt pH changes.

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Chemical Buffers key concepts

• 4. The lung and the distal tubules of the kidney
  are responsible for regulating blood pH. Acidosis
  and Alkalosis occur when pH drifts away from 7.4,
  which results from either a respiratory (e.g.,
  hyperventilation) or metabolic (e.g., excessive
  vomiting) imbalance.

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Buffers
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The 3 primary buffer systems in the body

• Bicarbonate (this is the buffer system in the
  ECF.)
• Phosphate (important buffer in the urine.)
• Protein (this is the buffer system of the
  cytoplasm.)

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The Bicarbonate Buffer System
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pH HomeostasisRespiratorycompensation
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Renal compenstation
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Reabsorption of bicarbonate
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Generation of new bicarbonate from phosphate
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Generation of bicarbonate from glutamine
deamination
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Enjoy