07 - Volume Regulation and Body Fluid Compartments

water balance

fluid intake = fluid output to maintain homeostasis

• intake → drinking, food, metabolic processes

• output → sweat, stool, skin, respiratory tract, urine, saliva

  • skin and respiratory tract water loss are insensible water loss because it constantly occurs but can’t feel it occurring

obligatory urine volume

minimum volume required to clear metabolic waste

• 500 mL/day

sodium-water balance

where sodium goes, water follows

• water moves towards higher solute concentrations

• sodium intake and excretion are balanced under normal conditions

• in high sodium diet → sodium retention for about 3 days

  • water intake increases, causing water retention and increased blood pressure

  • once equilibrium is restored → sodium excretion = sodium intake

body weight in water

60% of total body weight is water

total body water distribution

total body water is 60% of body weight

• 40% → intracellular (cell water)

• 20% → extracellular

  • 5% plasma

  • 15% interstitial fluid

• cell water, plasma, and interstitial fluid are in osmotic equilibrium → fluid moves between these compartments in response to osmotic gradients

• older adults have lower total body water due to decreased muscle mass and more fat

• newborn babies have 70-75% of their body weight in water

effective arterial blood volume (EABV)

portion of blood actively perfusing tissues, critical for blood pressure and kidney perfusion

• 15% of plasma volume in arteries and arterioles

• plasma volume is 5% of total body water

body water formulas

• total body water (TBW) = body weight in kg x 0.6 L/kg

• intracellular volume (ICV) = body weight in kg x 0.4 L/kg

• extracellular volume (ECV) = body weight in kg x 0.2 L/kg

• plasma volume (PV) = body weight in kg x 0.05 L/kg

• total blood volume (TBV) = PV / (1 - hematocrit)

  • 40% usually hematocrit → can change if person is anemic

hypovolemic shock

life-threatening condition that occurs when there is a significant loss of blood or fluids from the body

• >15% body weight loss (from water), 15-25% total body water loss, 30-40% blood loss

• decreased blood volume flow that causes decreased oxygen delivery to tissues

cation with greatest concentration in extracellular fluid

sodium

major electrolyte concentrations in compartments

• extracellular fluid → Na⁺, Cl^-

• intracellular fluid → K⁺, HPO₄^2-

osmotic equilibrium

equal osmolality between compartments

• composition of fluids are different in different fluid compartments, but are kept in osmotic equilibrium

• osmotic equilibrium at 290 mOsm/L

• water flows freely across most cell membranes

body fluid shifts

• hypertonic → high solute concentration, usually high NaCl

• isotonic → solute concentration approximately same as blood (290 mOsm/L)

• hypotonic → low solute concentration, usually low NaCl

hypertonic fluid shift

if hypertonic fluid is added to body:

• increase in extracellular concentration and volume

• water shifts from cell interior to exterior due to osmosis

• intracellular water loss increases intracellular concentration

isotonic fluid shift

if isotonic fluid is added to body:

• increase in extracellular volume

• no fluid shift or change in concentration

hypotonic fluid shift

if hypotonic fluid is added to body:

• extracellular volume decreases, while extracellular concentration decreases

• water shifts from cell exterior to interior

• intracellular volume increases and intracellular concentration decreases

volume contraction

decrease in body fluids, most commonly the extracellular fluid

• weight loss and low urinary output → loss of TBW reduced BW and urinary output

• reduced blood pressure → decreased blood volume causes blood pressure to decrease

• tachycardia and tachypnea → heart pumps faster to compensate for decreased blood pressure, with increased respiration to blow off more CO₂ for correcting acidosis

• lethargic, dry mucous membranes and skin, skin turgor → CNS effects on mental status, dry mouth/lips/tongue, skin elasticity

• elevated RBC concentration → decrease in ECF causes RBC to become more concentrated

severity of volume contraction

• mild → 3-5% body weight loss

• moderate → 6-9% body weight loss

• severe → >10% body weight loss

hyposmotic volume contraction

decreased ECF, decreased ECF Na⁺ concentration

• ICF fluid is lost and replaced by ECF, fluid shift

• caused by hypertonic fluid loss → decrease volume and large decrease in Na⁺ concentration

  • vomiting, kidney/adrenal problems

  • too much hypotonic fluid intake

isosmotic volume contraction

decreased ECF, no change in ECF Na⁺ concentration

• ECF is lost and not replaced by ICF, no fluid shift

• EABV drops considerably, causing decreased BP

• caused by isotonic fluid loss → decreased volume and Na⁺

  • vomiting, burns, diarrhea

hyperosmotic volume contraction

decreased ECF, increased ECF Na⁺ concentration 

• ECF is lost but replaced by ICF, fluid shift

• EABV does not drop, so BP remains stable

• caused by hypotonic fluid loss → large decrease in volume and decrease in Na⁺

  • sweating, diabetes, diarrhea