Fluid and Electrolytes Part 1 Condensed

Fluid & electrolyte homeostasis

Maintains stable fluid volume, osmolarity, and composition through intake/output, hormones, filtration, osmosis, diffusion, and excretion.

Osmolarity

Concentration of dissolved particles (solutes) in fluid.

Electrolytes

Charged ions dissolved in body fluids, e.g., Na+, K+, Ca2+, Mg2+, Cl−.

Total body water

~55-60% of adult body weight; lower in women, obesity, and older adults; higher in children.

Why does more body fat decrease total body water percentage?

Fat contains less water than lean tissue.

Major functions of body water

Transport; blood volume; metabolism; temperature regulation; lubrication; cell structure/function.

Two major body-fluid compartments

Intracellular fluid (ICF) and extracellular fluid (ECF).

Intracellular fluid (ICF)

Fluid inside cells; ~2/3 of total body water; ~42% body weight; contains most body fluid.

Approximate ICF volume

~28 L in males; ~20 L in females.

Major ICF cation

Potassium (K+).

Extracellular fluid (ECF)

Fluid outside cells; ~1/3 of total body water; includes intravascular, interstitial, and transcellular fluid.

Major ECF cation

Sodium (Na+).

Intravascular fluid

ECF inside blood vessels.

Interstitial fluid

ECF between/surrounding tissue cells.

Transcellular fluid

Specialized ECF, e.g., CSF and peritoneal fluid.

Solute vs solvent

Solute = dissolved particle; solvent = liquid dissolving it, mainly water.

Selectively permeable membrane

Allows some substances to cross while restricting others.

Four methods of fluid/substance movement

Osmosis, diffusion, filtration, active transport.

Osmosis

Water moves across a selectively permeable membrane from lower solute → higher solute concentration to equalize osmolarity.

What primarily drives osmotic water movement?

Sodium; water follows sodium.

Osmotic pressure

Force caused by solute differences that draws water across a membrane.

Diffusion

Solute moves passively from higher → lower concentration; no ATP required.

Lung diffusion example

O2 moves alveoli → blood; CO2 moves blood → alveoli.

Filtration

Fluid moves across a membrane from higher → lower pressure.

Hydrostatic pressure

Pressure from fluid/blood volume that PUSHES fluid out of vessels.

Increased hydrostatic pressure

Can push excess fluid into tissues → edema; occurs with venous congestion such as right-sided HF.

Colloid osmotic/oncotic pressure

Plasma proteins PULL water into/keep water within blood vessels.

Third spacing

Fluid leaves intravascular space and accumulates where it is unavailable to effective circulation, often interstitial tissue.

Why can third spacing cause edema + low circulating volume?

Fluid is in tissues instead of blood vessels.

Substances transported between blood & cells

O2, CO2, nutrients, cellular wastes, electrolytes.

Normal plasma/body-fluid osmolarity

270-300 mOsm/L per slideshow.

Active transport

Moves substances against a concentration gradient using ATP.

Sodium-potassium pump

Active transport: Na+ OUT of cells, K+ IN; uses ATP to maintain concentrations.

Na+/K+ pump ratio

3 Na+ out : 2 K+ in per ATP cycle.

Isotonic IV solution

Same approximate tonicity as plasma; expands ECF/intravascular volume with no major cell-size change.

Isotonic IV examples

0.9% NS; Lactated Ringer's (LR).

Hypotonic IV solution

Lower solute concentration than cells; water moves ECF → cells → cells swell.

Hypotonic IV example

0.45% NS (½ NS).

Major hypotonic-fluid risk

Excess cellular swelling, especially cerebral edema.

Hypertonic IV solution

Higher solute concentration than cells; water moves cells → ECF → cells shrink.

Hypertonic examples from slideshow

3% NS; D10%.

Major hypertonic-fluid risk

Intravascular expansion → fluid overload/pulmonary edema.

Isotonic vs hypotonic vs hypertonic

Isotonic = no major cell shift; hypotonic = water into cells/swelling; hypertonic = water out of cells/shrinking.

Three regulators of body-fluid balance

Fluid intake, hormonal control, fluid output.

Fluid intake regulation

Thirst; average intake ~2200-2700 mL/day.

Routes of fluid loss

Kidneys, skin, lungs, GI tract.

Insensible fluid loss

Unmeasurable fluid loss, mainly through skin and lungs.

Minimum daily urine needed to excrete wastes

~400-600 mL/day per slideshow.

Major fluid-balance hormones

ADH, aldosterone/RAAS, natriuretic peptides.

ADH

Retains WATER by increasing renal water reabsorption → ↓ urine output.

ADH production/release

Synthesized in hypothalamus; stored/released by posterior pituitary.

Aldosterone

Adrenal cortex hormone; retains Na+ and water; increases K+ excretion.

Natriuretic peptides

Released by heart with ↑ volume/pressure/cardiac stretch → ↑ Na+ and water excretion/urine output.

ADH vs aldosterone vs natriuretic peptides

ADH retains water; aldosterone retains Na+/water and loses K+; natriuretic peptides lose Na+/water.

RAAS purpose

Restores circulating volume/BP when BP or renal perfusion falls.

RAAS sequence

↓ BP/renal perfusion → renin → angiotensin I → angiotensin II → vasoconstriction + aldosterone → Na+/water retention → ↑ BP/volume.

Angiotensin II

Vasoconstricts and stimulates aldosterone, increasing BP and fluid volume.

Isotonic fluid imbalance

Water and electrolytes gained/lost equally; osmolarity stays essentially unchanged.

Osmolar fluid imbalance

Water gain/loss disproportionate to solute → altered serum concentration/osmolarity.

Baroreceptors

Pressure receptors that detect BP/circulating-volume changes.

Hypovolemia

Loss of water AND electrolytes from ECF.

Dehydration

Loss of water disproportionately to electrolytes; slideshow describes water loss without electrolyte loss.

Hypovolemia vs dehydration

Hypovolemia = water + electrolyte loss; dehydration = proportionally greater water loss.

Hemoconcentration

Loss of plasma water concentrates blood components → ↑ Hct and often ↑ serum concentration.

Dehydration lab pattern

↑ Hct, ↑ serum osmolarity, ↑ urine specific gravity; serum Na+ depends on relative water/Na+ loss.

Fluid-volume deficit compensation

↑ SNS, thirst, ADH, RAAS/aldosterone → conserve Na+/water and support BP/perfusion.

Fluid-volume deficit cardiovascular findings

Tachycardia, weak/thready pulse, hypotension/orthostasis, possible delayed cap refill.

Fluid-volume deficit skin/neurologic findings

Dry mucosa/skin, poor turgor, weakness, dizziness, fatigue; severe cases may cause confusion.

Fluid-volume deficit renal findings

↓ urine output + concentrated urine.

Fluid-volume deficit urine-output red flag

Labs for fluid-volume deficit

Hct, serum osmolarity, urine specific gravity, serum Na+.

Fluid-deficit prevention

Increase fluids with vigorous exercise, high altitude, and dry climates; alcohol/caffeine may increase fluid excretion.

Why are older adults prone to dehydration?

Lower body-water reserve and less-sensitive thirst mechanism.

Hypovolemic shock

Severe loss of circulating volume → inadequate tissue perfusion.

Hypovolemic shock sequence

Fluid/blood loss → ↓ volume → ↓ venous return/preload → ↓ cardiac output → ↓ tissue perfusion.

Hypovolemic shock findings

Tachycardia, hypotension, weak pulse, cool/pale skin, altered LOC, ↓ urine output.

Dehydrated patient not drinking: what increases?

Blood osmolarity, thirst, ADH.

Dehydrated patient not drinking: what decreases?

BP may decrease with significant volume loss; urine output decreases.

Hypervolemia

Abnormally increased Na+ and water retention.

Overhydration

More water gained relative to electrolytes.

Severe hypervolemia complications

Pulmonary edema and heart failure.

Fluid-volume excess compensation

↑ natriuretic peptides + ↓ aldosterone → ↑ Na+/water excretion → ↓ volume.

Major causes of fluid-volume excess

Excess IV fluids, excess salt, renal failure.

Fluid-volume excess cardiovascular findings

Bounding pulse, ↑ BP, JVD, possible S3, tachycardia.

Fluid-volume excess respiratory findings

SOB, pulmonary congestion, crackles.

Fluid-volume excess neurologic/GI findings

Confusion/LOC changes, headache, possible seizures; anorexia, nausea.

Fluid-volume excess peripheral findings

Dependent pitting edema and weight gain.

Tests for fluid-volume excess

Hct, serum osmolarity, urine specific gravity, Na+, electrolytes, BUN, creatinine, chest x-ray.

Hemodilution

Excess plasma water dilutes blood components → Hct may decrease.

Hemoconcentration vs hemodilution

Fluid deficit → hemoconcentration/↑ Hct; fluid excess → hemodilution/↓ Hct.

Nursing care for fluid-volume excess

Daily weights; strict I&O; assess edema, lungs, LOC; monitor labs; fluid/sodium restriction as ordered.

Daily weights in fluid balance

One of the best measures of acute fluid change; use same scale/time/conditions daily.

Weight-to-fluid conversion

~1 kg acute weight change ≈ 1 L fluid.

Pulmonary edema

Excess fluid in lung tissue/alveoli that impairs gas exchange.

Pulmonary edema findings

Bounding pulses, SOB, edema, crackles, low urine output.

Why do crackles occur in pulmonary edema?

Air moves through fluid-containing small airways/alveoli.

Why is pulmonary edema dangerous?

Impaired gas exchange can cause hypoxemia and respiratory distress.

Normal Na+ range

136-145 mEq/L

Sodium (Na+)

Major ECF cation; controls fluid movement and supports nerve impulses, skeletal muscle contraction, and cardiac contraction.

Why does sodium control water movement?

As the major ECF cation, Na+ strongly determines extracellular osmolarity; water follows Na+.