Fluid Regulation

osmolarity

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

osmolality

the number of solutes per kilogram solvent (mOsmoles/kg)

tonicity

the effective osmoles in solution, i.e. solutes that cannot freely cross membranes such as sodium and potassium; determines osmotic pressure gradient between two solutions separated by a semi-permeable membrane

plasma

aqueous portion of whole blood (95% water); contains everything except cells (WBC and RBC); includes clotting factors

serum

portion of whole blood that is neither cellular (RBC,WBC) or clotting factor; includes proteins, electrolytes, antibodies, hormones, drugs, etc.

total body water (L)=

60% total body weight (kg)

intracellular fluid =

0.6 x total body water

extracellular fluid =

0.4 x total body water

Electrolyte Distribution in ICF

Na 25mEq/L
K 150mEq/L
Cl 5mEq/L
Ca 0.04mg/dL
HCO3 6mEq/L

Electrolyte Distribution in ECF

Na 140mEq/L
K 4mEq/L
Cl 100mEq/L
Ca 9mEq/L
HCO3 24mEq/L

Osmolality Distribution in ICF

280mOsm/L
K+

Osmolality Distribution in ECF

280mOsm/L
Na+

Calculated Serum Osmolality (mOsm/kg)

2Na+glucose/18 +BUN/2.8

Na in mEq/L; Glucose in mg/dL; BUN in mg/dL

How is serum osmolality regulated?

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

Serum Osmolality

reflects tonicity of serum

normal range for measured serum osmolality

275-290mOsm/kg

Causes of 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

Hypotonic and Isotonic

Hypotonic Fluid Loss

insensible, sweat, urine, loop diuretic urine

Isotonic Fluid Loss

loop diuretic urine, blood, vomiting, diarrhea

Obligate Water Loss

800mL insensible + 100mL sweat + 200mL feces + 500mL urine = 1600mL/day

Kidney’s Role in Hypovolemia

Kidney will preserve as much salt and water as possible

Fractional Excretion of Sodium (FeNa)

FeUrea: used if patient on loop diuretic

How does the kidney preserve salt and water in hypovolemia?

↓↓Urine volume, ↓ Urine sodium, ↑urine specific gravity (concentrated)

Fractional Excretion of Sodium

{(Urine Na [mEq/L] x Serum Creatinine [mg/dL])} DIVIDED BY
{(Serum Na [mEq/L] x Urine Creatinine [mg/dL])} x 100%
- Normal range: 1-2%
- Hypovolemia → FeNa<1%

FeUrea

hypovolemia → FeUrea < 35%

Hypovolemia— Hypotonic Loss

free water will shift from ICF to ECF to maintain tonicity

Hypovolemia— Isotonic Loss

free water will NOT shift from ICF to ECF to maintain tonicity

Edematous States

Edema= fluid in the interstitial spaces

Hydrostatic and oncotic pressures are major determinants of fluid movement between intravascular and interstitium

Edema

ECF is expanded; Na and water accumulation in ECF due to lack of renal excretion

Pulmonary Edema Presentation

• auscultation of breath sounds

• crackles/rales

• chest x-ray

• SOB

• dyspnea at rest

• hypoxia

What does acute changes in weight reflect?

alterations in kidney system (among others)

weight gain

> 1.1 lbs in 24 hours → likely fluid accumulation

weight loss

> 0.5 pounds in 48 hours → likely fluid loss

Clinical Presentation of Volume Overload

• peripheral and/or pulmonary edema

• SOB

• weight gain

• hypertension

• jugular venous distention

Clinical Presentation of Volume Depletion

• dry mucus membranes

• hypotensive/tachycardia

• dizziness/orthostasis

• poor capillary refill response

• low jugular venous pressure

What is specific about CKD patients?

they are highly sodium sensitive; small changes in sodium balance can lead to big changes in volume status

Kidney Calculi

kidney stones

what are kidney stones?

supersaturated salts in the urine; crystallization occurs when concentrations exceed solubility limit or changes to pH; aggregation of multiple crystals form large stones

what are the types of kidney stones?

calcium stones, uric acid stones, struvite

calcium stones

most common, idiopathic hypercalciuria, oxalate (acidic pH) or phosphate (alkaline pH) salts

uric acid stones

gout, hypovolemia, hyperuricosuria

struvite

composition: magnesium ammonium phosphate; associated with UTIs from bacteria that cause urea breakdown

Kidney Stone Presentation

Patient Interview: radiating flank pain not relieved by laying down; blood in urine, painful/difficult urination

Physical Exam: WNL

Labs: hypercalcemia, hyperuricemia; urinalysis: hematuria, hypercalciuria, hyperuricosuria

Which drugs cause crystallization in urine?

• acyclovir

• allopurinol

• aminopenicillins-amoxicillin

• sulfonamides-sulfadiazine, sulfamethoxazole

• protease inhibitors-atazanavir, darunavir, indinavir

• triamterene

What drugs increase urine pH?

• topiramate

• acetazolamide

What drugs decrease urine pH?

• ascorbic acid

Which drugs alter urine electrolyte composition?

• calcium containing drugs

• loop diuretics

• magnesium containing drugs

What are the risk factors for kidney stone formation?

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

What are the complications of kidney stones?

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