Fluid Regulation

What is osmolarity

the number of solute particles per liter of solvent (mOsmoles/L)

What is osmolality?

the number of solutes per kilogram solvent (mOsmoles/kg_

What is tonicity?

the effective osmoles in solution, i.e. solutes that cannot freely cross membranes such as sodium and potassium

what is the determination of osmotic pressure gradients between two solutions separated by a semi-permeable membrane

Tonicity

what is plasma

the aqueous potion of whole blood (95% water) that contains everything except cells (RBC and WBC) - includes clotting factors

what is serum

the portion of whole blood that is neither cellular (RBC, WBC) or clotting factors - includes proteins, electrolytes, antibodies, hormones, drugs, etc (plasma - clotting factors and blood cells)

How to find total body water (L)?

60% of total body weight in kg

EX: 0.6 × 70 kg = 42 L

what separates the extracellular space from the intravascular fluid?

capillary beds

What makes up the intravascular volume (blood)?

RBCs and plasma (~5 L)

T/F: electrolyte lab values are based on ICF values

FALSEEEEEEEeeeee

which fluid space has higher amounts of sodium? (ICF vs ECF)

ECF

which fluid space has higher amounts of potassium? (ICF vs ECF)

ICF

how does water move across membranes from ICF to ECF?

via concentration gradients

What separates the ICF from the intravascular fluid?

cell membranes

what percentage of body fluid is made up of extracellular fluid?

40%

how to find intracellular fluid (ICF) amount?

0.6 x total body water

how to find extracellular fluid (ECF) amount?

0.4 x total body water

Are red blood cells in ICF or ECF?

ICF (~1.8 L)

Is plasma in ICF or ECF?

ECF (~3.2 L)

Electrolyte distribution in the intracellular fluid

Na 25mEq/L

K 150 mEq/L

Cl 5 mEq/L

Ca 0.04 mg/dL

HCO3 6 mEq/L

electrolyte distribution in the extracellular fluid

Na 140 mEq/L

K 4 mEq/L

Cl 100 mEq/L

Ca 9 mEq/L

HCO3 24 mEq/L

Where is Ca stored?

bones and endoplasmic reticulum

T/F: Osmolality and tonicity are similar between ICF and ECF spaces

True

what is the ICF osmolality distribution?

280 mOsm/L

K+

What is the ECF osmolality distribution?

280 mOsm/L

Na+

what is serum osmolality

reflects the tonicity of the serum

Normal serum osmolality

275-290 mOsm/kg

how to calculate serum osmolality

2Na + Glucose/18 + BUN/2.8

How is serum osmolality tightly regulated?

through hypothalamus-pituitary through release of arginine vasopressin (AVP) also known as antidiuretic hormone (ADH)

What happens when there is an increased plasma osmolality or decreased arterial circulating volume?

Brain signals thirst and AVP which leads to increased fluid intake and antidiuresis. Resulting in decreased plasma osmolality or increased arterial circulating volume

What does release of ADH lead to?

signals thirst and tells kidneys to reabsorb more water and increase the expression of aquaporin channels to promote water going back into the blood stream.

What causes diminished glomerular filtration?

age, renal disease, congestive heart failure, cirrhosis, nephrotic syndromes, and volume depletion

which portion of the nephron has the highest solute concentration?

The descending loop of Henle

what does it mean if something if reabsorbed?

it is reabsorbed out of the tubule and back into the blood stream

what does it mean if something is secreted?

it flows through the efferent artery that wraps around the tubules and is secreted to the nephron with no filtration.

How are we able to reabsorb sodium and therefore water in the thick ascending limb?

Due to solute gradient (highly concentrated descending loop ——> low concentration ascending loop)

what occurs in the collecting duct?

Not a lot of Na+ reabsorption. ADH/hormones regulate presence of aquaporin channels in this area

What reduces Na+/Cl- reabsorption in the distal convoluted tubule?

thiazide diuretics

What determines the delivery of H20 to the loop of Henle?

glomerular filtration rate, proximal tubule H2O and Na+/Cl- rebsorption

What reduces Na+/Cl- reabsorption in the thick ascending limb

loop diuretics, osmotic diuretics, and interstitial disease

What increases permeability of the collecting duct?

vasopressin, drugs

What causes hypovolemia

poor PO intake, infection/fever, hypermetabolic state (burn, trauma), blood loss (hemorrhage)

Clinical Presentation of hypovolemia

hypotension, tachycardia, orthostasis, dry mucus membranes, poor skin turgor, slow capillary refill, cool extremities, low jugular venous pressure

Types of fluid loss:

insensible, sweat, urine, loop diuretic urine, blood, vomiting, diarrhea

Which types of fluid loss are hypotonic?

Insensible, sweat, urine, and loop diuretic urine

what types of fluids do we lose everyday?

Hypotonic

Which types of fluid loss are isotonic?

loop diuretic urine, blood, vomiting, diarrhea

What is the obligate water loss in a day?

1600 mL/day

What do the kidneys do in hypovolemia

preserve as much salt and water as possible (decrease urine volume, decrease urine sodium, and increase urine specific gravity (concentrated))

How is the fraction excretion of sodium (FeNa) calculated

[ (urine Na [mEq/L] x serum creatinine [mg/dL]) / (serum Na [mEq/L] x urine creatinine [mg/dL]) ] X 100%

When is FeNa not as accurate?

if patient is on a diuretic as they pee more Na+

What is the normal range of FeNa?

1-2%

what is the FeNa in hypovolemia?

< 1%

When is FeUrea used?

If patient is on a loop diuretic

FeUrea in hypovolemia

< 35%

what happens in hypotonic fluid loss?

Free water will shift from the intracellular fluid to the extracellular fluid to maintain tonicity (ECF and ICF both shrink a little)

what happens in isotonic fluid loss?

Free water will NOT shift from the intracellular fluid to the extracellular to maintain tonicity

where does water flow when there is isotonic fluid loss?

Flows from the interstitial space to the intravascular because there is no osmotic gradient to move from ICF to ECF

what is edema?

fluid in the interstitial space

what leads to increased fluid in the interstitial space?

too much hydrostatic pressure or too little plasma colloid osmotic pressure

what does hydrostatic pressure do?

sends water out of the capillary bed

what does plasma colloid osmotic pressure do?

Holds/keeps water in the capillary bed

What are the major determinants of fluid movement between intravascular and intersitium?

hydrostatic and oncotic pressures

Which fluid space is expanded in edema (ICF or ECF)?

ECF

What is accumulated in the ECF due to lack of renal excretion in edema?

Na and water

Presentations of pulmonary edema:

auscultation of breath sounds, crackles/rales, chest X-ray, SOB, dyspnea at rest, and hypoxia

What happens to alveoli in pulmonary edema?

fluid builds up and leaks into the lungs

T/F: acute changes in weight can reflect alterations in kidney system (among others)

true

What weight gain is likely due to fluid accumulation?

if > 1.1 pounds in 24 hours

What weight loss is likely due to fluid loss?

> 0.5 pounds in 48 hours

Clinical presentations of volume overload?

peripheral and/or pulmonary edema, SOB, weight gain, HTN, jugular vein distension

Clinical presentation of volume depletion?

dry mucus membranes, hypotensive/tachycardia, dizziness/orthostasis, poor capillary refill response, low jugular venous pressure

Which electrolyte are CKD patients highly sensitive to?

Sodium! Small changes in sodium balance can lead to big changes in volume status

What does hypovolemic mean?

low volume; total body fluid deficit

What does euvolemic mean?

Normal volume <3

What does hypervolemic mean?

high volume; fluid overload :(

Physical characteristics of hypovolemia

hypotension, tachycardia, poor skin turgor, slow capillary refill time

Euvolemia physical characteristics

normal BP/HR, normal skill turgor, normal capillary refill time

physical characteristics of hypervolemia

HTN, peripheral/pulmonary edema, weight gain, jugular venous distention.

how does hypervolemia cause hypertension

nephron loss of CKD —→ impaired sodium excretion ——> positive sodium balance and volume expansion ——> HTN and release of natriuretic forces (ANP,BNP) ——> Natriuretic forces lead to inhibition of sodium transport —→ decreased renal sodium reabsorption (natriuresis) and altered cellular electrolyte content and membrane potentials ——> uremic manifestations and increased peripheral resistance (and HTN) 😭

How does a low sodium diet effect hypervolemic hypertension?

helps with the impaired sodium excretion

how do diuretics effect hypervolemic HTN

works on the positive sodium balance and volume expansion

how do vasodilators effect hypervolemic HTN

helps decrease the increased peripheral vascular resistance caused by altered cellular electrolyte content and membrane potentials

What are the other names for kidney stones?

nephrolithiasis, kidney calculi

what are kidney stones?

supersaturated salts in the urine

When do kidney stones occur?

crystallization occurs when concentrations exceed solubility limit, and the aggregation of these crystals form large stones; changes to pH

What are kidney stones made of?

calcium, uric acid, struvite (magnesium ammonium phosphate)

What percentage of kidney stones are made of calcium?

~80% - due to idiopathic hypercalciuria

Types of calcium stones?

oxalate in acidic pH or phosphate in alkaline pH salts

What percentage of kidney stones are made up of uric acid?

~10%

Cause of uric acid stones?

gout, hypovolemia, hyperuriccosuria

Which kidney stone composition is associated with UTIs from bacteria that cause urea breakdown?

Struvite

What percentage of kidney stones are made up of struvite?

< 5%

Kidney Stone presentations:

radiating flank pain not relieved by lying down, blood in urine, painful/difficult urination, WNL, hypercalcemia, hyperuricemia, hematuria, hypercalciuria, hyperuricosuria, N/V, sharp sudden severe pain that may be intermittent depending on stone movement.

What drugs cause drug crystallization in the urine

acyclovir, allopurinol, aminopenicillins-amoxicillin, sulfonamides-sulfadiazine-sulfamethoxazole, protease inhibitors-atazanavir, darunavir, indinavir, triamterene

which drugs cause increased urine pH

topiramate and acetazolamide

which drugs decrease urine pH

ascorbic acid

which drugs alter urine electrolyte composition?

calcium containing drugs, loop diuretics, magnesium containing drugs

risk factors for kidney stone formation:

dehydration, high protein diet, high sodium diet, hypercalcemia, obesity

Complications of kidney stones include:

pain, hydronephrosis, post-renal acute kidney injury (bilateral stones), infection surrounding the stone, hypertension (RAAS activation), chronic kidney disease (recurrent obstructive stones)