fluid, electrolyte & ph lemastm/teaching/bi336/unit 4 - water balance (2).pdf fluid, electrolyte &...

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  • 1

    Fluid, Electrolyte

    & pH Balance

    Cell function depends not only on continuous nutrient supply / waste removal,

    but also on the physical / chemical homeostasis of surrounding fluids

    Body Fluids:

    1) Water: (universal solvent)

    Fluid / Electrolyte / Acid-Base Balance

    Body water varies based on of age, sex, mass, and body composition

    H2O ~ 73% body weight

    Low body fat

    Low bone mass H2O (♂) ~ 60% body weight

    H2O (♀) ~ 50% body weight

    ♀ =  body fat /  muscle mass H2O ~ 45% body weight

  • 2

    Intracellular Fluid (ICF)

    Volume = 25 L (40% body weight)

    Extracellular Fluid (ECF)

    Total Body Water

    Interstitial

    Fluid

    P la

    s m

    a

    Volume = 12 L

    V o

    lu m

    e =

    3 L

    Volume = 15 L (20% body weight)

    Volume = 40 L (60% body weight)

    Fluid / Electrolyte / Acid-Base Balance

    Body Fluids:

    Cell function depends not only on continuous nutrient supply / waste removal,

    but also on the physical / chemical homeostasis of surrounding fluids

    1) Water: (universal solvent)

    Clinical Application:

    The volumes of the body fluid compartments are measured

    by the dilution method

    Fluid / Electrolyte / Acid-Base Balance

    Step 1:

    Identify appropriate marker

    substance

    A marker is placed in

    the system that is distributed

    wherever water is found

    Marker: D2O

    Extracellular Fluid Volume:

    A marker is placed in

    the system that can not cross

    cell membranes

    Marker: Mannitol

    Total Body Water:

    Step 2:

    Inject known volume of

    marker into individual

    Step 3:

    Let marker equilibrate and

    measure marker

    • Plasma concentration

    • Urine concentration

    Step 4:

    Calculate volume of body

    fluid compartment

    Volume = Amount

    Concentration (L)

    Amount:

    Amount of marker injected (mg)

    – Amount excreted (mg)

    Concentration:

    Concentration in plasma (mg / L)

    Note:

    ICF Volume = TBW – ECF Volume

    (mg)

  • 3

    A) Non-electrolytes

    (do not dissociate in solution – neutral)

    Although individual [solute] are different

    between compartments, the osmotic

    concentrations of the ICF and

    ECF are usually identical…

    B) Electrolytes

    (dissociate into ions in solution – charged)

    • Mostly organic molecules (e.g., glucose, lipids, urea)

    • Inorganic salts

    • Inorganic / organic acids

    • Proteins

    Marieb & Hoehn (Human Anatomy and Physiology, 8th ed.) – Figures 26.2

    Fluid / Electrolyte / Acid-Base Balance

    Body Fluids:

    Cell function depends not only on continuous nutrient supply / waste removal,

    but also on the physical / chemical homeostasis of surrounding fluids

    2) Solutes:

    Electrolytes have great

    osmotic power:

    MgCl2  Mg 2+ + Cl- + Cl-

    Fluid / Electrolyte / Acid-Base Balance

    Fluid Movement Among Compartments:

    The continuous exchange and mixing of body fluids are regulated

    by osmotic and hydrostatic pressures

    Intracellular

    Fluid

    Interstitial

    Fluid

    Plasma Hydrostatic Pressure

    Osmotic Pressure

    Osmotic

    Pressure

    Osmotic

    Pressure

    The volume of a particular

    compartment depends on

    amount of solute present

    The shift of fluids between

    compartments depends on

    osmolarity of solute present

    In a steady state,

    intracellular osmolarity

    is equal to

    extracellular osmolarity

    300 mosm

    300 mosm

    300 mosm

  • 4

    Ingested liquids (60%)

    Solid foods (30%)

    Metabolism (10%)

    Water Intake

    2500 ml/day = 0

    Urine (60%)

    Skin / lungs (30%)

    Sweat (8%)

    Water Output

    2500 ml/day

    Feces (2%)

    > 0

    < 0

    Osmolarity rises:

    Osmolarity lowers:

    ICF functions as

    a reservoir

    • Thirst

    • ADH release

    Fluid / Electrolyte / Acid-Base Balance

    Water Balance:

    For proper hydration: Waterintake = Wateroutput

    • Thirst

    • ADH release

     volume /

     osmolarity of

    extracellular fluid

     saliva

    secretion Dry mouth

    Osmoreceptors

    stimulated

    (Hypothalamus)

    Sensation

    of thirst

     osmolarity /

     volume

    of extra. fluid

    Drink

    (-)

    Fluid / Electrolyte / Acid-Base Balance

    Water Balance:

    Thirst Mechanism:

    (-)

  • 5

    Intracellular

    Fluid

    Extracellular

    Fluid

    300 mOsm

    300 mOsm

    Pathophysiology:

    Disturbances that alter solute or water balance in the body can

    cause a shift of water between fluid compartments

    Volume Contraction: A decrease in ECF volume

    Isomotic Contraction

    Diarrhea

    300 mOsm

    Intracellular

    Fluid

    Extracellular

    Fluid

    300 mOsm

    300 mOsm

    Intracellular

    Fluid

    Extracellular

    Fluid

    < 300 mOsm

    300 mOsm

    Intracellular

    Fluid

    Extracellular

    Fluid

    > 300 mOsm

    ECF osmolarity = No change

    Hyperosmotic Contraction

    ECF volume = Decrease

    Water

    deficiency

    < 300 mOsm

    ECF volume = Decrease

    Aldosterone

    insufficiency

    NaCl not reabsorbed

    from kidney filtrate

    Hyposmotic Contraction

    < 300 mOsm > 300 mOsm

    Fluid / Electrolyte / Acid-Base Balance

    ICF volume = No change

    ICF osmolarity = No change

    ECF volume = Decrease

    ICF osmolarity = Increase

    ICF volume = Decrease

    ECF osmolarity = Increase ECF osmolarity = Decrease

    ICF volume = Increase

    ICF osmolarity = Decrease

    Intracellular

    Fluid

    Extracellular

    Fluid

    300 mOsm

    300 mOsm

    Pathophysiology:

    Disturbances that alter solute or water balance in the body can

    cause a shift of water between fluid compartments

    Volume Expansion: An increase in ECF volume

    Isomotic Expansion

    Osmotic IV

    300 mOsm

    Intracellular

    Fluid

    Extracellular

    Fluid

    300 mOsm

    300 mOsm

    Intracellular

    Fluid

    Extracellular

    Fluid

    < 300 mOsm

    < 300 mOsm

    Intracellular

    Fluid

    Extracellular

    Fluid

    > 300 mOsm

    Hyperosmotic Expansion

    Yang’s lunch SIADH

    Greater than normal

    water reabsorption

    Hyposmotic Expansion

    IV bag

    300 mOsm > 300 mOsm

    NaCl

    Water

    intoxication

    IV bags of varying solutes

    allow for manipulation of

    ECF / ICF levels…

    Fluid / Electrolyte / Acid-Base Balance

    ECF osmolarity = No change

    ECF volume = Increase ECF volume = Increase

    ICF volume = No change

    ICF osmolarity = No change

    ECF volume = Increase

    ICF osmolarity = Increase

    ICF volume = Decrease

    ECF osmolarity = Increase ECF osmolarity = Decrease

    ICF volume = Increase

    ICF osmolarity = Decrease

    300 mOsm

  • 6

    Fluid / Electrolyte / Acid-Base Balance

    Acid-Base Balance:

    Acid-base balance is concerned with maintaining a normal

    hydrogen ion concentration in the body fluids

    Costanzo (Physiology, 4th ed.) – Figure 7.1

    [H+] = 40 x 10-9 Eq / L

    pH = -log10 [H +]

    Normal

    [H+]

    pH = -log10 [40 x 10 -9]

    pH = 7.4

    Note:

    Normal range of arterial pH = 7.37 – 7.42

    Compatible range for life

    Problems Encountered:

    1) [H+] and pH are inversely related

    2) Relationship is logarithmic, not linear

    1) Disruption of cell membrane stability

    2) Alteration of protein structure

    3) Enzymatic activity change

    1) Volatile Acids: Acids that leave solution and enter the atmosphere

    H2CO3

    carbonic

    acid

    CO2 + H2O HCO3 - + H+

    2) Fixed Acids: Acids that do not leave solution (~ 50 mmol / day)

    Fluid / Electrolyte / Acid-Base Balance

    Acid-Base Balance:

    Acid production in the human body has two forms:

    volatile acids and fixed acids

    End product of

    aerobic metabolism

    (13,000 – 20,000 mmol / day)

    Products of normal

    catabolism:

    Products of extreme

    catabolism:

    Ingested (harmful)

    products:

    Eliminated via the lungs

    • Sulfuric acid (amino acids)

    • Phosphoric acid (phospholipids)