Water, Electrolyte, and Acid-Base Balance

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Water, Electrolyte, and Acid-Base Balance
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Function of Water Most of cellular activities
are performed in water solutions.
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40 TBW
4 TBW
Body Fluid
- makes up 60 of total body weight (TBW) -
distributed in three fluid compartments.
16 TBW
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40 TBW
4 TBW
Fluid is continually exchanged between the three
compartments.
16 TBW
5
40 TBW
4 TBW
Exchange between Blood Tissue Fluid
- determined by four factors capillary blood
pressure plasma colloid osmotic
pressure interstitium Hydrostatic
Pressure Interstitium colloid osmotic pressure
16 TBW
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40 TBW
4 TBW
Exchange between Blood Tissue Fluid
- not affected by electrolyte concentrations   -
Edema water accumulation in tissue fluid
16 TBW
7
40 TBW
4 TBW
Exchange between Tissue Fluid Intracellular
Fluid
- determined by two 1) intracellular osmotic
pressure electrolytes 2) interstitial osmotic
pressure electrolytes
16 TBW
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Water Gain Water is gained from three
sources. 1) food (700 ml/day) 2) drink
voluntarily controlled 3) metabolic water (200
ml/day) --- produced as a byproduct of aerobic
respiration
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Routes of water loss 1) Urine obligatory
(unavoidable) and physiologically regulated,
minimum 400 ml/day 2) Feces -- obligatory water
loss, 200 ml/day 3) Breath obligatory water
loss, 300 ml/day 4) Cutaneous evaporation --
obligatory water loss, 400 ml/day 5) Sweat
for releasing heat, varies significantly
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Regulation of Water Intake - governed by
thirst.
?blood volume and ?osmolarity ? peripheral
volume sensors central osmoreceptors ?
hypothalamus ? thirst felt
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Regulation of Water Output - The only
physiological control is through variations in
urine volume. - urine volume regulated by
hormones
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1) ADH
dehydration ? ?blood volume and/or ?osmolarity
? hypothalamic receptors / peripheral volume
sensors ? posterior pituitary to release ADH ? ?
H2O reabsorption ? Water retention
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2) Atrial Natriuretic Factor
? blood volume ? atrial volume sensors ? atria
to release ANF ? inhibits Na and H2O
reabsorption ? ? water output
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• Dehydration
• - decrease in body fluid
• Causes
• the lack of drinking water
• 2) excessive loss of body fluid due to
• overheat
• diabetes
• overuse of diuretics
• diarrhea

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• Edema
• - the accumulation of fluid in the interstitial
  spaces
• caused by
• increased capillary filtration,
• or
• 2) reduced capillary reabsorption, or
• 3) obstructed lymphatic drainage

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ELECTROLYTE BALANCE
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Electrolytes small ions that carry charges
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Distribution of Electrolytes
Na
K
Ca
Cell
PO4---
Cl-
Extracellular space
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• Sodium Na
• Functions
• - involved in generating action membrane
  potential of cells
• make a major contribution to extracellular
  osmolarity.

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• Regulation of plasma Na
• Aldosterone

Na

• plasma Na
• ?
• ? aldosterone
• ?
• renal Na excretion
• ?
• ? plasma Na

plasma
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• Renin-angiotensin-II
• renin
• ?
• angiotensin-II
• ?
• ? aldosterone
• ?
• ? renal Na excretion
• ?
• ? plasma Na

Na
plasma
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3) ADH increases water reabsorption in kidneys
? water retention ? dilute plasma Na
H2O
Na
plasma
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• Atrial Natriuretic Factor
• inhibits renal reabsorption of Na and H2O and
  the excretion of renin and ADH
• ?
• eliminate more sodium and water
• ?
• ? plasma Na

Na
plasma
Na
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Sodium imbalance hypernatremia plasma sodium
gt 145 mEq/L, hyponatremia plasma sodium
lt 130 mEq/L
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Potassium Functions - the greatest
contributor to intracellular osmosis and cell
volume - determines the resting membrane
potentials - an essential cofactor for protein
synthesis and some other metabolic processes.
K
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• Regulation of Potassium
• by aldosterone
• Aldosterone
• ?
• stimulates K
• secretion by the kidneys
• ?
• ? Plasma K

K
plasma
K
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Potassium Imbalance hyperkalemia (gt 5.5 mEq/L)
hypokalemia (lt 3.5 mEq/L)
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Chloride - makes a major contribution to
extracellular osmolarity - required for the
formation of stomach acid (HCl)
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• Regulation of Cl
• No direct regulation
• indirectly regulated as an effect of Na
  homeostasis. As sodium is retained or excreted,
  Cl passively follows.

Chloride Imbalance hyperchloremia (gt 105
mEq/L) hypochloremia (lt 95 mEq/L).
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Calcium
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Functions of Ca - lends strength to the
skeleton
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Functions of Ca - lends strength to the
skeleton - activates muscle contraction
Ca i
Contraction
Excitation
(Action Potentials)
(shortening)
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Functions of Ca - lends strength to the
skeleton - activates muscle contraction -
serves as a second messenger for some hormones
and neurotransmitters
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Functions of Ca - lends strength to the
skeleton - activates muscle contraction -
serves as a second messenger for some hormones
and neurotransmitters - activates exocytosis of
neurotransmitters and other cellular secretions
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Functions of Ca - lends strength to the
skeleton - activates muscle contraction -
serves as a second messenger for some hormones
and neurotransmitters - activates exocytosis of
neurotransmitters and other cellular secretions -
essential factor in blood clotting.
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Functions of Ca - lends strength to the
skeleton - activates muscle contraction -
serves as a second messenger for some hormones
and neurotransmitters - activates exocytosis of
neurotransmitters and other cellular secretions -
essential factor in blood clotting. -
activates many cellular enzymes
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Dynamics of Calcium
Ca
Ca
Ca
plasma
Ca
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Regulation of calcium 1) parathyroid hormone
(PTH)
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Regulation of calcium 1) parathyroid hormone
(PTH) - dissolving Ca in bones - reducing
renal excretion of Ca
Ca
Ca
plasma
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2) calcitonin (secreted by C cells in thyroid
gland)
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2) calcitonin (secreted by C cells in thyroid
gland) depositing Ca in bones
Ca
Ca
plasma
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3) calcitrol (derivative of vitamin D) -
enhancing intestinal absorption of Ca from food
Ca
Ca
plasma
Ca
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Calcium imbalances hypocalcemia (lt 4.5
mEq/L) hypercalcemia (gt 5.8 mEq/L).
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• Phosphates
• needed for the synthesis of
• ATP, GTP
• DNA, RNA
• phospholipids

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• Regulation of Phosphate
• by parathyroid hormone
• PTH
• ?
• increases renal excretion of phosphate
• ?
• decrease plasma phosphate
•  
• - no real phosphate imbalances

PO4---
plasma
PO4---
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ACID-BASE BALANCE
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Acid An acid is any chemical that releases H
in solution.
Base A base is any chemical that accepts H.
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pH is the negative logarithm of H
concentration, and an indicator of acidity.  
pH - log H
Example H 0.1 ?M 10 7 M
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pH is the negative logarithm of H
concentration, and an indicator of acidity.  
pH - log 10 7
7 log 10 7
Example H 0.1 ?M 10 7 M
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Normal functions of proteins (especially enzymes)
heavily depend on an optimal pH.
pH7.35-pH7.45
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Regulation of acid-base balance
1) Chemical Buffers
2) Respiratory Control of pH
3) Renal Control of pH
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Buffer is any mechanism that resists
changes in pH.
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acid
acid
H2O pH 7.0
Buffer pH 7.0
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base
base
H2O pH 7.0
Buffer pH 7.0
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Chemical Buffers
There are three major buffers in body fluid.
1) The Bicarbonate (HCO3-) Buffer
2) The Phosphate Buffer
3) The Protein Buffer
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2) The Phosphate Buffer System
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• 3) The Protein Buffer System
• more concentrated than either bicarbonate or
  phosphate buffers
• - accounts for about three-quarters of all
  chemical buffering ability of the body fluids.
• The carboxyl groups release H when pH rises
  and amino groups bind H when pH falls.

H
H
NH2-CH2-CH2? CH2-CH2-COOH
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Properties of Chemical Buffers - respond to pH
changes within a fraction of a second. - Bind
to H? but can not remove H? out of the body -
Limited ability to correct pH changes
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Respiratory Control of pH
H? HCO- H2CO3 CO2 H2O
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? pH ? stimulate peripheral/central
chemoreceptors ? ? pulmonary ventilation ? removal
of CO2 and ? pH
H2CO3
H? HCO3-
H2O CO2
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Limit to respiratory control of pH The
respiratory regulatory mechanism cannot remove H
out of the body. Its efficiency depends on the
availability of HCO3- .
H? HCO3- H2CO3
H2O CO2
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• Renal Control of pH
• The kidneys can neutralize more acid or base than
  both the respiratory system and chemical buffers.
• a. Renal tubules secrete hydrogen ions into
  the tubular fluid, where most of it combines with
  bicarbonate, ammonia, and phosphate buffers.
• b. Bound and free H are then excreted in
  urine.

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• The kidneys are the only organs that actually
  expel H from the body. Other buffering systems
  only reduce its concentration by binding it to
  another chemical.
• 3. Tubular secretion of H continues as long as a
  sufficient concentration gradient exists between
  the tubule cells and the tubular fluid.

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• Disorders of Acid-Base Balance
• Acidosis lt pH 7.35 , Alkalosis gt pH 7.45
• Mild acidosis
• depresses CNS, causing
• confusion, disorientation, and coma.
• Mild alkalosis
• CNS becomes hyperexcitable.
• Nerves fire spontaneously and overstimulate
  skeletal muscles.
• - Severe acidosis or alkalosis is lethal.

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Respiratory vs Metabolic Cause Respiratory
acidosis / alkalosis - caused by
hypoventilation or hyperventilation
Initial change
H? HCO- H2CO3
H2O CO2
Emphysema
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Respiratory acidosis / alkalosis - caused
by hypoventilation or hyperventilation
Metabolic acidosis or alkalosis - result
from any causes but respiratory problems
Diabetes ? ? production of organic
acids ? metabolic acidosis
Chronic vomiting ? loss of stomach
acid ? metabolic alkalosis