Water, Sodium, and Potassium

urea

freely diffuses across membranes, so it affects osmolality but not water distribution

colligative properties

- boiling point
- freezing point
- osmotic pressure
- vapor pressure

physiological responses to maintain blood pressure

- ADH stimulation
- constricted renal arterioles
- systemic vasoconstriction
- activation of renin-angiotensin-aldosterone system

example of hyponatremia

Addison's disease
- low aldosterone

Little Syndrome clinical features

family history of premature stroke

percentage of intracellular water

66%

osmolality equation

1.86(Na) + (glucose/18) + (BUN/2.8) + 9

sodium and chloride account for _________ of the osmolality value in serum

over 80%

normal sodium levels

135 - 145 mmol/L

sodium renal threshold

110 - 130 mmol/L

normal potassium levels

3.5 - 5.0 mmol/L

normal osmololality in serum

275 - 295 mOsm/kg

normal osmolality in 24-hr urine

301 - 1090 mOsm/kg

even in hypokalemic states

body will continue to excrete potassium in urine

water accounts for ____ of body weight in males and ____ of body weight in females

66%, 55%

water is freely permeable from

extracellular fluid to intracellular fluid, determined by osmotic pressure

percentage of extracellular water

33%

percentage of plasma water

8%

major contributors to osmolality in the ECF

- sodium
- anions (chloride, bicarbonate)
- glucose
- urea

bicarbonate

measure of total carbon dioxide in the blood

chloride

follows sodium

protein contributes to

colloid osmotic pressure in plasma

colloid osmotic pressure

force needed to resist movement of solvent across semi-permeable membrane due to large molecular weight of the protein itself

main cation in extracellular fluid

sodium

what happens when sodium reaches renal threshold

kidneys begin excreting sodium in urine

sodium retention is controlled by

aldosterone

sodium excretion is controlled by

natriuretic peptide hormones (ANP)

ANP

inhibits Na reabsorption, inhibits release of renin, and suppresses effects of norepinephrine and angiotensin II

main cation of intracellular fluid

potassium

during clotting, platelets

release potassium

serum levels are _________ than plasma potassium

higher

osmolality

measure of the number of dissolved particles in a solution, expressed as milliosmoles/kg of water

osmolality in serum is indication for

hydration status

osmolality in urine is used to

- assess electrolyte and fluid balance
- assess the kidneys' ability to concentrate urine

colligative properties are affected by

number of particles in a solution

osmometry

- method used to measure osmolality
- used on serum or urine

freezing point depression osmometry

- higher number of particles in solution will results in decreased freezing point
- determined as observed freezing point compared to freezing point of pure water

advantages of freezing point depression osmometry

- performs rapid and inexpensive measurements
- simple and reliable performance
- industry preferred
- small sample sizes
- ideal for dilute biological and aqueous solutions

disadvantages of freezing point depression osmometry

- samples must be of low viscosity
- may not be suitable for high molality or colloidal solutions

vapor pressure osmometry

vapor pressure/water evaporation will decrease with the higher number of particles present in solution

advantages of vapor pressure osmometry

- performs rapid and inexpensive measurements
- small sample sizes
- ideal for dilute biological and aqueous solutions

disadvantages of vapor pressure osmometry

- less accurate than freezing point depression
- cannot be used for volatile solutes (alcohols)
- may not be suitable for high molality or colloidal solutions

osmolal gap (OG)

difference between measure osmolality and calculated osmolality

normal osmolal gap

< 10 - 15 mOsm/kg

osmolal gap equation

OG = MO - CO

- OG = osmolal gap
- MO = measured osmolality
- CO = calculated osmolality

high osmolal gap is indicative of

presence of unmeasured anions

in event of water loss, osmolality of ECF will

increase

increased osmolality of ECF causes

- passive movement of water from ICF to ECF
- hypothalamus to stimulate thirst center
- hypothalamus releases ADH

ADH

- increases reabsorption of water
- increases urine concentration
- decreases serum osmolality
- increases blood pressure

if osmolality decreases

- hypothalamus does not stimulate thirst
- ADH secretion is inhibited
- dilute urine is produced

aldosterone

- controls Na+, Cl-, and H2O retention
- controls K+ and H+ excretion

aldosterone secretion is activated by

renin-angiotensin system to increase blood pressure

water loss

can occur on its own, but is usually accompanied by sodium loss

sodium loss

always accompanied by loss of water

isotonic loss (water and Na)

- causes no change in osmolality, no movement from ICF to ECF
- affects plasma volume

water excess is typically due to

impaired water excretion

excessive water intake can result in

- water intoxication
- cerebral overhydration

sodium excess can be due to

increased intake or decreased excretion (more common)

sodium excess can cause

cerebral dehydration

secondary aldosteronism (hyperaldosteronism)

renin-angiotensin disorder

depletional hyponatremia

- causes true loss of total body Na+
- caused by overuse of diuretics

dilutional hyponatremia

- a serum sodium that is low not because of an absolute lack of sodium but because of an excess of water
- overhydrating

example of dilutional hyponatremia

Syndrome of Inappropriate ADH (SIADH)

falsely decreased sodium

sample with high protein/lipids if using indirect ion selective electrode method for measurement

diabetes insipidus

- hypernatremia
- deficiency in ADH release

examples of hyperaldosteronism

- hypernatremia
- Conn syndrome (primary)
- Renin-angiotensin disorder (secondary)

Cushing's syndrome

- hypernatremia
- excessive production of ACTH

hypokalemia

- potassium depletion
- usually caused by loss from gut/kidneys or excess diuretic/laxative use

examples of hypokalemia

- Conn syndrome (high aldosterone)
- Cushing's syndrome

Little Syndrome renin and aldosterone

decreased

Little Syndrome blood pressure

increased

Little Syndrome deficiency

distal tubule sodium channel

Gitelman Syndrome renin and aldosterone

increased

Gitelman Syndrome blood pressure

decreased or normal

Gitelman Syndrome clinical features

salt wasting, low magnesium, low calcium

Gitelman Syndrome deficiency

defect in thiazide-sensitive sodium transporter

Bartter Syndrome renin and aldosterone

increased

Bartter Syndrome blood pressure

decreased or normal

Bartter Syndrome clinical features

salt wasting, short stature

Bartter Syndrome deficiency

sodium reabsorption in loop of Henle

hyperkalemia

less common than hypokalemia, but more deadly

risk of cardiac arrest at K+ concentrations of

>6.5 mmol/L

falsely increased potassium levels can be caused by

- hemolysis
- delayed centrifugation
- EDTA contamination
- abnormal blood cells

why does hemolysis increase potassium levels

breakdown of RBC releases intracellular potassium

why does delayed centrifugation increase potassium levels

platelet clotting releases potassium

electrochemical analysis by potentiometry

- voltage change in electrical potential between a detecting electrode and a reference electrode in which the current is kept constant
- measures difference in potential

reference electrode for ion selective electrode (ISE)

silver-silver chloride (Ag/AgCl)

direct ISE

- requires no sample dilution
- lipids and proteins have no effect
- whole blood can be used
- not suitable for urine

indirect ISE

- requires sample dilution
- cannot use whole blood
- suitable for urine samples
- elevated lipids/proteins may falsely decrease electrolyte result

Selectivity of electrodes

- Na+ use glass ion exchange membrane
- K+ use valinomycin membrane