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13. Electrolytes and Fluid Balance


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What is the normal body water distribution?
Intracellular = 66%

Extracellular = 33%
What primarily determines the body H2O distribution?
Osmotic pressure
What is Osmotic pressure?
The force that moves water between body compartments.
What type of ions contribute to the osmotic pressure?
Colloids and nondiffusible ions.
What is the normal Osmolality of the body?
285-298 mOsm/kg
What instrument is used to measure body osmolality?
a Thermistor - to measure the vapor point of blood.
How can the body's osmolality be estimated instead of measuring with thermistor?
Calculate the Osmolal Gap
What's the osmolality gap calculation?
1.86Na + Glu/18 + BUN/2.8 +9
What 5 things stimulate Aldosterone secretion?
-Decreased Blood volume
-Decreased SODIUM
-Increased POTASSIUM
What determines the H2O distribution between the vasculature and interstitium?
Starling's forces
What is the Starling equation in a nutshell?
NFP=K x (Forces Out - In)
What are forces out in a capillary?
What are forces in for a capillary?
Colloid osmotic
What force is higher in:
Arterioles: Hydrost. > osmotic
Venules: Osmotic > Hydrost.
When all is said and done, what force is greater for water movement?
Filtration > reabsorption; so water has to return via lymphatic system.
What's the anion gap?
The difference between the measured cations and anions.
What 4 conditions can cause an increased anion gap?
1. Diabetic ketoacidosis
2. Decreased Calcium/Mg
3. Starvation
4. Toxins
What 3 things can cause a decreased anion gap?
What's most common?
1. Lab error - most common
2. Multiple myeloma
3. Increased Ca/Mg
List 5 roles of electrolytes:
1. Maintain osmotic pressure
2. Maintain pH
3. Regulate muscle function
4. Participate in redox
5. Participate in enzyme catalysis
What is the major extracellular cation?
Where is most Na reabsorbed?
in the proximal tubule
How would the following hormonal conditions affect Na?
Addisons = Hyponatremia

Cushing's = Hypernatremia
What specimens are tested for Sodium levels?
-GI fluids
Are sodium levels affected by hemolysis?
What is the major intracellular cation?
What is the major controller of Na after reabsorption in the prox tubules?
How does Aldosterone function?
To increase Na reabsorption and H2O also
What hormone counteracts Aldosterone?
Atrial Natiuretic Factor ANF
Where does ADH act on the kidney?
Collecting ducts
What 2 important Receptors affect sodium levels?
What do osmoreceptors detect?
Where are they located?
-increased osmolarity
-in hypothalamus
What happens when increased osmolarity is detected?
Stimulates the release of ADH to increase H2O reabsorption and thus decrease osmolarity
What do Baroreceptors detect?
Where are they located?
Detect changes in blood pressure
-located in smooth muscle of vasculature
What results when baroreceptors detect increased blood pressure?
-Stimulates ADH release from brain
-ADH stimulates RAS to start
How does the RAS result in increased Na/H2o reabsorption?
Because renin is a stimlator of aldosterone release
What are 3 types of hyponatremia?
What type of pH is generally associated with hyponatremia?
Metabolic acidosis
Why does acidosis cause low Na?
Because of sodium wasting - have to excrete Na+ to reabsorb HCO3- and have the charges balance.
What causes a dilutional hyponatremia?
-Excessive water reabsorption
What is hormonal hyponatremia?
Addison's disease
How would increased ADH affect body sodium levels?
Increased ADH = too much H2O reabsorption, so dilutional hyponatremia.
How would decreased ADH affect body sodium levels?
Not enough H2O so hypernatremia
What type of Ca and K levels are usually seen with hypernatremia?
What hormonal problem usually causes hypernatremia?
What is the general reason for most hypernatremia conditinos?
Disproportionate loss of water vs. sodium.
How do urine sodium levels change in:
Hypernatremia = hyponatruria

Hyponatremia = hypernatruria
So back to this.. what is the major INTRACELLUAR cation?
How is K kept inside cells?
By the Na/K pump - ATPase
What affects body potassium levels (hormone)?
How doe aldosterone regulate K?
Increased Aldosterone causes increased K excretion
How does the body respond to hypokalemia?
-Tachycardia (slowed heartrate)
-EKG changes
How does the body respond to hyperkalemia?
-Weak, numb, tingly
-Respiratory weakness
What hormonal disorder causes hypokalemia?
What hormonal disorder causes hyperkalemia?
What are 3 causes of hypokalemia?
1. Decreased dietary intake
2. Increased loss of K-rich fluids
3. Redistribution from extra to intracellular fluid
What are 2 things that make K go from extracellular fluid to intracellular?
1. Insulin therapy
2. Alkalosis (decreased H+ buffering by Hgb so more room for K+)
What are 3 causes of hyperkalemia?
1. IV infusion
2. Redistribute from intra-> extracellular fluid
3. Decreased excretion
What would make K go from intracellular to ECF?
Acidosis - need space inside the cells to buffer extra H+
What hormonal disorder would cause decreased K excretion?
What is the body potassium in:
Hyperkaluria = hypokalemia

Hypokaluria = hyperkalemia
What 4 preanalytical variables falsely increase potassium measurement?
1. Clenching in phlebotomy
2. Clotting (don't use serum)
3. Hemolysis
4. Delay in centrifugn
What are 2 methods used to measure sodium and potassium?
1. Flame emission spec
2. ISE
What membrane is used for Na?
What membrane is used for K?
Valinomycin - ion-exchange
What is the principle of ISE measurement?
Potentiometric - a change in potential as the ions interact with the membrane
What error can occur in ISE?
What causes it?
Electrolyte exclusion effect - caused by other electrolytes in the fluid being measured.
How can you avoid EEE?
By using the direct ISE method where there is no dilution. (Indirect uses high ionic strength diluent)
What is the major extracellular anion?
How is chloride reabsorbed in the kidneys?
Passively AND via active pumps
What are 3 causes of hypochloremia?
1. Salt losing nephritis
2. Prolonged vomiting
3. Alkalosis
Why does alkalosis cause hypochloremia?
Because there is base excess, so to counter the negative charge more chloride ions are lost.
What 3 conditions are associated with hyperchloremia?
1. Dehydration
2. Acute renal failure
3. Metabolic acidosis
What 3 methods are used for Cl measurement?
1. Mercurimetric titration
2. Spectrophotometric
3. Coulometric - Amperometric
What type of electrode is used in coulometric measurement of Cl?
Silver chloride.
What is the principle of amperometric measurement for Cl?
-The electrode generates Cl ions which bind with pt. Cl -> Cl2
-Free Ag ions shut down the system.
What is the other major anion measured in electrolyte measurements?
Bicarbonate - HCO3-
What specimens are analyzed for HCO3 measurement?
Serum or plasma
What special care needs to be taken for CO2 measurement?
Anaerobic conditions
What are 3 methods for tCO2 measurement?
1. Continuous flow
2. pCO2 electrode
3. Enzymatic
What is the principle of the continuous flow method?
1. Acid buffer converts all CO2 to gas
2. Gas diffuses across membrane
3. pH buffer changes and detected with an indicator
How does the pCO2 electrode work?
As CO2 gas is detected in WB, the pH changes; rate of change is measured.
What enzymatic method is used for CO2 measurement?
2 step method; phosphoenolpyruvate produces oxaloacetate; then measure
oxaloacetate + NADH by detecting NADH decrease
What wavelength is NADH detected at?

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