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Chapter 27
Fluid, Electrolyte, and Acid
Base Homeostasis
Fluid Compartments, Fluid Balance
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Fluid separated by semi-permeable
membranes into compartments
Four different compartments
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Intracellular = inside cells
Extracellular = outside cells
Interstitial = between cells in the tissues
Plasma = fluid portion of blood
Fluid Compartments, Fluid Balance
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Total Body Water (TBW) - 42L,
60% of body weight
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Intracellular Fluid (ICF) - 28L,
67% of TBW
Extracellular Fluid (ECF) - 14L,
33% of TBW
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Interstitial Fluid - 11L, 80% ECF
Plasma - 3L, 20% of ECF
Fluid Compartments, Fluid Balance
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Fluid balance
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When in balance,
required amount of
water present and
proportioned among
various compartments
according to their
needs
Many things freely
exchanged between
fluid compartments,
especially water
Fluid movements by:
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bulk flow (i.e. blood
circulations)
diffusion & osmosis most I overall
Water
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General
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Largest single component of body, 45-75% of
body
Fat is water free so more\less water in body
depending on composition
Water
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Water balance
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Sources for 2500
ml - average daily
intake
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Metabolic Water
Preformed Water
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Ingested Foods
Ingested Liquids
Balance achieved
by output = 2500
ml
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GI tract
Lungs
Skin
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evaporation
perspiration
Kidneys
Regulating Fluid Intake - Thirst
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Wetting mucosa of
mouth (temporary)
Stretching of stomach
Decreased osmolarity
of blood (most I)
Regulation of Fluid Output
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Hormonal control
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AntiDiuretic Hormone (ADH)
Aldosterone
Atrial Natriuretic Peptide (ANP)
Physiologic fluid imbalances
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Dehydration:  blood pressure,  GFR
Overhydration:  blood pressure,  GFR
Hyperventilation - water loss through lungs
Vomiting & Diarrhea - excess water loss
Fever - heavy perspiration
Burns - initial fluid loss
Concentrations of Solutes
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Non-electrolytes
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molecules formed by covalent bonds
do not form ions in solution
Electrolytes
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Molecules formed by ionic bonds
Disassociate into cations (+) & anions (-) in
solutions (acids, bases, salts)
4 I functions in body
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essential minerals
control osmosis of water
maintain acid-base balance
conduct electrical current
Distribution of Electrolytes
Electrolyte Balance
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Aldosterone
Atrial Natriuretic Peptide
Female sex hormones (Estrogen,
Progesterone)
Antidiuretic Hormone
Calcitonin
Parathyroid Hormone
Acid-Base Balance
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Normal metabolism produces H+ (acidity)
Three Homeostatic mechanisms:
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Buffer systems - instantaneous; temporary
Exhalation of CO2 - operates within minutes;
cannot completely correct
Kidney excretion - can completely correct any
imbalance (eventually)
Buffer Systems
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Consist of a weak acid and the salt of that acid
which function as a weak base
Strong acids dissociate more rapidly and easily
than weak acids
Acid-Base Balance
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Carbonic Acid - Bicarbonate Buffer
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Phosphate Buffer
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A weak base
H+ + HCO3- ---> H2CO3 ----> H2O + CO2
NaOH + NaH2PO4 -----> H2O + Na2HPO4
HCl + Na2HPO4 -----> NaCl + NaH2PO4
Protein Buffer
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Most abundant buffer in body cells and plasma
Amino acids have amine group (proton acceptor,
base) carboxyl group (proton donor, acid)
Acid-Base Imbalances
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Acidosis
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High blood [H+]
Low blood pH, <7.35
Alkalosis
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Low blood [H+]
High blood pH, >7.45