fluid, electrolyte & ph balance - western oregon …lemastm/teaching/bi336/unit 4 - water...1...
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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
Pla
sm
a
Volume = 12 L
Vo
lum
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 Mg2+ + 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)
• Lactic acid (anaerobic respiration)
• Acetoacetic acid (ketobodies)
Diabetes
mellitus
• B-hydroxybutyric acid
• Salicylic acid (e.g., aspirin)
• Formic acid (e.g., methanol)
• Oxalic acids (e.g., ethylene glycol)
Eliminated via the kidneys
7
Chemical Buffer:
A mixture of a weak acid and its conjugate base or a weak base and
its conjugate acid that resist a change in pH
Fluid / Electrolyte / Acid-Base Balance
Acid-Base Balance:
H+ Gain:
• Across digestive epithelium
• Cell metabolic activities
H+ Loss:
• Release at lungs
• Secretion into urine
Distant from
one another
Brønsted-Lowry Nomenclature:
Weak acid:
Acid form = HA = H+ donor
Base form = A- = H+ acceptor
Weak base:
Acid form = BH+ = H+ donor
Base form = B = H+ acceptor
11.4 L
11.4 L
150 mEq H+ 150 mEq H+
7.44 7.14 7.00 1.84
Robert Pitt:
The Henderson-Hasselbalch equation is used to calculate
the pH of a buffered solution
Fluid / Electrolyte / Acid-Base Balance
Acid-Base Balance:
Henderson-Hasselbalch Equation:
pH = pK + log [A-]
[HA]
pH = -log10 [H+]
pK = -log10 K (equilibrium constant)
[A-] = Concentration of base form of buffer (mEq / L)
[HA] = Concentration of acid form of buffer (mEq / L)
pK is a characteristic value for a buffer pair
(strong acid = pK; weak acid = pK)
Costanzo (Physiology, 4th ed.) – Figure 7.2
Most effective
buffering -1 +1
For the human body, the
most effective physiologic buffers
will have a pK at 7.4 1.0
8
1) HCO3- / CO2 Buffer:
Utilized as first line of defense when H+ enters / lost from system:
Fluid / Electrolyte / Acid-Base Balance
Acid-Base Balance:
The major buffers of the ECF are bicarbonate and phosphate
H2CO3 HCO3- + H+ CO2 + H2O
(HA) (A-)
(1) Concentration of HCO3- normally high at 24 mEq / L
(2) The pK of HCO3- / CO2 buffer is 6.1 (near pH of ECF)
(3) CO2 is volatile; it can be expired by the lungs
The pH of arterial blood can
be calculated with the
Henderson-Hasselbalch equation
pH = pK + log HCO3
-
0.03 x PCO2
pH = 6.1 + log 24
0.03 x 40
pK = 6.1
[HCO3-] = 24 mmol / L
Solubility = 0.03 mmol / L / mm Hg
PCO2 = 40 mm Hg
pH = 7.4
1) HCO3- / CO2 Buffer:
Fluid / Electrolyte / Acid-Base Balance
Acid-Base Balance:
The major buffers of the ECF are bicarbonate and phosphate
Costanzo (Physiology, 4th ed.) – Figure 7.4
Acid-base map:
Shows relationship between the
acid and base forms of a buffer
and the pH of the solution
Isohydric line
(‘same pH’)
Ellipse:
Normal values for
arterial blood
Note:
Abnormal combinations of PCO2
and HCO3- concentration can
yield normal values of pH
(compensatory mechanisms)
9
2) H2PO4- / HPO4
2- Buffer:
Fluid / Electrolyte / Acid-Base Balance
Acid-Base Balance:
The major buffers of the ECF are bicarbonate and phosphate
(HA) (A-)
H2PO4- HPO4
2- + H+
Costanzo (Physiology, 4th ed.) – Figure 7.3
Why isn’t inorganic phosphate the primary ECF buffer?
(1) Lower concentration than bicarbonate (~ 1.5 mmol / L)
(2) Is not volatile; can not be cleared by lung
1) Organic phosphates:
Fluid / Electrolyte / Acid-Base Balance
Acid-Base Balance:
The major buffers of the ICF are organic phosphates and proteins
H+ enters cells via a build up of
CO2 in ECF, which then enters cells,
or to maintain electroneutrality
• ATP / ADP / AMP
• Glucose-1-phosphate
• 2,3-diphosphoglycerate
pKs range from 6.0 to 7.5
2) Proteins:
R – NH2 + H+ R – NH3+ If pH falls
The most significant ICF protein
buffer is hemoglobin
Proteins also buffer H+
in the ECF
A relationship exists
between Ca++, H+,
and plasma proteins A Ca++
Ca++ Ca++
Ca++
H+ H+
Normal pH
Normal circulating Ca++
A Ca++
Ca++
Ca++
Ca++
H+
H+
Acidemia Alkalemia
H+
Hypercalcemia Hypocalcemia
Ca++
R – COOH R – COO- + H+ If pH rises
10
1) Respiratory Regulation:
If H+ begins to rise in
system, respiratory
centers excited
H2CO3
carbonic
acid
CO2 + H2O HCO3- + H+
CO2 leaves
system
H2CO3
carbonic
acid
CO2 + H2O HCO3- + H+
If H+ begins to fall in
system, respiratory
centers depressed CO2 leaves
system
Doubling / halving of areolar ventilation
can raise / lower blood pH by 0.2 pH units
( H; homeostasis
restored)
( H; homeostasis
restored)
Buffers are a short-term fix to the problem; in the long term,
H+ must be removed from the system…
Fluid / Electrolyte / Acid-Base Balance
Maintenance of Acid-Base Balance:
2) Renal Regulation:
A) HCO3- reabsorption:
Process continues
until 99.9% of
HCO3- reabsorbed
Buffers are a short-term fix to the problem; in the long term,
H+ must be removed from the system…
Fluid / Electrolyte / Acid-Base Balance
Maintenance of Acid-Base Balance:
Kidneys are ultimate acid-base
regulatory organs
• Reabsorb HCO3-
• Secrete fixed H+
Costanzo (Physiology, 4th ed.) – Figure 7.5
(brush
border)
(intracellular)
Net secretion of
H+ does not occur
Net reabsorption of
HCO3- does occurs
THUS
pH of filtrate
does not significantly
change
If HCO3- loads reach 40 mEq / L,
transport maximized and
HCO3- excreted.
11
2) Renal Regulation:
B) Secretion of fixed H+:
Buffers are a short-term fix to the problem; in the long term,
H+ must be removed from the system…
Fluid / Electrolyte / Acid-Base Balance
Maintenance of Acid-Base Balance:
Kidneys are ultimate acid-base
regulatory organs
• Reabsorb HCO3-
• Secrete fixed H+
(1)
Excretion of H+
as titratable acid
Costanzo (Physiology, 4th ed.) – Figure 7.6
Titratable Acid:
H+ excreted with buffer
Recall:
Only 85% of phosphate
reabsorbed from filtrate
CO2
New HCO3- replaces
HCO3- that is lost when
CO2 exits blood
Minimum urinary
pH is 4.4
(Maximum H+ gradient
H+ ATPase can
work against)
(Removes 40% of fixed acids – 20 mEq / day)
2) Renal Regulation:
B) Secretion of fixed H+:
Buffers are a short-term fix to the problem; in the long term,
H+ must be removed from the system…
Fluid / Electrolyte / Acid-Base Balance
Maintenance of Acid-Base Balance:
Kidneys are ultimate acid-base
regulatory organs
• Reabsorb HCO3-
• Secrete fixed H+
(2)
Excretion of H+
as NH4+
Costanzo (Physiology, 4th ed.) – Figure 7.8
(Removes 60% of fixed acids – 30 mEq / day)
Diffusional trapping:
Lipid soluble NH3 is able to diffuse
into tubule but once combined with
NH4+ it is unable to leave
12
Disturbances of Acid-Base Balance:
1) Respiratory Acid / Base Disorders:
Respiratory Acidosis:
CO2 retained in body
(Most common
acid / base disorder)
B) Respiratory Alkalosis:
CO2 retained in body
Cause:
Hypoventilation (e.g., emphysema)
Cause:
Hyperventilation (e.g., stress)
(Rarely persists long
enough to cause
clinical emergency)
2) Metabolic Acid / Base Disorders:
A) Metabolic Acidosis:
fixed acids generated in body
Causes:
Starvation ( ketone bodies)
Excessive HCO3- loss
(e.g., chronic diarrhea)
Alcohol consumption ( acetic acid)
A) Metabolic alkalosis:
HCO3- generated in body
Causes:
Repeated vomiting (alkaline tide ‘amped’)
Antacid overdose
(Rare in body)
In the short term, respiratory / urinary system
will compensate for disorders…
Fluid / Electrolyte / Acid-Base Balance
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