Pathophysiology: Fluids and Electrolytes (Marwaha)

At steady state, what do fluids and electrolytes look like?

fluid intake=fluid output

electrolyte intake=electrolyte output

How is fluid + electrolyte output regulated?

primarily regulated by the kidneys

Kidneys adjust the ________________ to match changes in fluid intake

urine output

Significant blood loss occurs. Which fluid compartment decreases first?

plasma

Describe how total body water is distributed throughout the body.

total body water is about 42L

-intracellular fluid has about 2/3

-extracellular fluid has 1/3

--ECF is further broken down into interstitial fluid, 3/4 of ECF and then plasma, 1/4 of ECF

What is the difference in TBW (% body weight) between men and women

-males is about 60%

-females is about 50%

What affects TBW%?

-more lean tissue increases TBW%

-more adipose tissue decreases TBW%

-increasing age decreases TBW%

What is the equation to figure out TBW

TBW (L) ~ body weight in kg X estimated TBW fraction

What are the major cations and anions of the ECF

Na+ = major cation

Cl- + HCO3- = major anions

What are the major cations and anions of the ICF?

K+ and Mg2+ = major cations

PO4- = major anion

Which has more proteins, ICF or ECF?

ICF

Do ICF and ECF have the same osmlarity?

YES

What determines fluid shifts between plasma and interstitium?

hydrostatic and oncotic forces

What determines fluid shift between the ECF and ICF

osmolality

What are the major contributors to plasma osmolality?

Na+, glucose, and urea (BUN)

What is plasma osmolality

-number of dissolved particles per kg of water

-normal plasma osmolality ~275-295 mOsm/kg

What is the equation for calculating plasma osmolality

2[Na+] + (glucose/18) + (BUN/2.8)

Describe what hypotonic means relative to the cell

lower solute concentration than the cell

Describe what isotonic means relative to the cell

same solute concentration as the cell

Describe what hypertonic means relative to the cell

higher solute concentration than the cell

Describe how osmolality works in terms of reaching equilibrium

-water moves freely across the cell membranes and many solutes d not

-water will move towards the compartment with higher osmolarity

-at equilibrium, ICF osmolality=ECF osmolality

What happens when fluid balance is disrupted?

-changes in fluid or solute input/output can alter ECF volume or ECF osmolality (like when someone is dehydration or there is blood loss)

-changes in ECF osmolality drive water shifts between ECF and ICF

-water shifts until osmotic equilibrium is restored

Is the fluid hypertonic, isotonic, or hypotonic?

dextrose 5% in water (D5W)

hypotonic (going into the body)

Is the fluid hypertonic, isotonic, or hypotonic?

0.9% NaCl

isotonic

Is the fluid hypertonic, isotonic, or hypotonic?

3% NaCl

hypertonic

Is the fluid hypertonic, isotonic, or hypotonic?

lactated ringers

isotonic

Is the fluid hypertonic, isotonic, or hypotonic?

0.45% NaCl

hypotonic

What is the effect of adding 1L of 0.9% NaCl?

-ECF volume would increase

-ECF osmolality would stay the same

-ICF volume would stay the same

-ICF osmolality would stay the same

Where does isotonic fluid go when it goes into the body

it expands ECF without causing a net water shift across cell membranes

What is the effect of adding 1L of D5W?

-ECF volume goes up

-ECF osmolality would go down

-ICF volume would go up

-ICF osmolality would go down

Where does 1L of D5W go when its put into the body?

D5W distributes throughout the TBW, relatively little remains intravascular

(splits up like TBW, 2/3 goes to ICF, 1/3 went ECF)

Which of the following should be considered in a patient with blood loss, resulting in low blood pressure?

a) 1l of normal saline

b) 1 L of D5W

c) both are Ok since they are both 1L

A)

because it is isotonic, most of it will stay in the ECF where it is needed

Is the fluid hypertonic, isotonic, or hypotonic?

5% albumin

isotonic

Where does 1L of 5% albumin go when it is put into the body?

albumin is largely retained within the vascular compartment because proteins can't cross barrier

-it increases plasma oncotic pressure and expands the intravascular volume

What is the effect of adding 2L of 3% NaCl?

-ECF volume increases

-ECF osmolality increases

-ICF volume decreases

-ICF osmolality increases

What are the two distinct but interconnected aspects of fluid balance?

sodium balance and water balance

Sodium balance primarily determines ______________

ECF volume

How is sodium balance regulated?

regulated largely by RAAS/aldosterone and natriuretic mechanisms

What happens to volume if there is a Na+ gain and what happens to volume if there is a Na+ loss?

-Na+ gain means that water is retained and there is an ECF expansion, or increase in volume

-Na+ loss means that water is lost and there is ECF contraction, or decrease in volume

If there is an imbalance in sodium balance, what kind of disorders can arise?

-volume disorders!

--a decrease in volume would be hypovolemia

--an increase in volume would be hypervolemia

Water balance primarily determines __________________

serum [Na+] and osmolality

What happens when there is water gain/excess relative to Na+ and what happens when there is water loss/deficit relative to Na+ ?

-water gain would decrease osmolality, which would decrease serum [Na+] (or hyponatremia)

-water loss would increase osmolality, which would increase serum [Na+] (or hypernatremia)

How is water balance regulated?

regulated largely by ADH and thirst

An imbalance in the water balance would cause what kind of disorders?

sodium disorders!

-water excess=hyponatremia

-water deficit= hypernatremia

What is hypervolemic hyponatremia?

when you have a little bit of sodium increase but ALLOOTT of water increase

What is hypovolemic hyponatremia

decrease in water but moreee decrease in sodium

what is considered hyponatremia and what is considered hypernatremia according to lab values

hyponatremia= Serum [Na+] < 135 mEq/L

hypernatremia= serum [Na+] > 145 mEq/L

How does hyponatremia affect the brain?

a decrease in ECF osmolality causes water to move into cells, which results in cell swelling

How does hypernatremia affect the brain?

an increase in ECF osmolality causes water to move out of cells, which results in cell shrinking

Describe hypovolemic hyponatremia and some causes

-Na+ loss > water loss (but there is still water loss)

-some causes would be thiazide diuretics and adrenal insufficiency

Describe euvolemic hyponatremia and some causes.

-water gain with relatively unchanged Na+

-no clinically apparent volume expansion or contraction

-some causes are SIADH or primary polydipsia

Describe hypervolemic hyponatremia and some causes

-water gain > Na+ gain

-there's an increase in ECF volume

-some causes would be heart failure, cirrhosis and advanced CKD

Describe hypovolemic hypernatremia and some casues

-water loss > Na+ loss

-there is a decrease in ECF volume

-some causes can be sweating, Gi losses/dehydration or osmotic diuresis

Describe euvolemic hypernatremia and some causes.

-no change in total body Na+, decrease in total body H2O, but it is predominantly free water loss

-no clinically apparent volume expansion or contraction

-some causes include diabetes insipidus, inadequate water intake, insensible water loss

Describe hypervolemic hypernatremia and some causes.

-Na+ gain > water gain

-theres an increase in ECF volume

-some causes include hypertonic NaCl administration, sodium bicarbonate load, mineralcorticoid excess