fluid and electrolytes

what does fluid balance affect

- Fluid balance affects blood pressure, oxygen delivery, and organ perfusion, and cellular fx

o The body is mostly water, but fluid also contains electrolytes (solutes)

what fx do electrolytes allow?

• Nerve signals

• Muscle movement

• Heart rhythm

fluid balance depends on

- Osmosis

- diffusion

- Filtration

- Pressure

- Kidney regulation

Fluid needs vary by:

- Age → older adults have less total body water than younger adults

- Muscle vs. fat → higher muscle mass=more water and more fat=less water

key fx intracellular fluid

- inside cells (~⅔ of body water)

-Supports cell metabolism, electrical activity, energy

· Imbalance → cells swell or shrink

· Can cause confusion, muscle weakness, dysrhythmias

- Ex: hypo/hypernatremia, severe dehydration → intracellular shifts

key fx extracellular fluid

- Extracellular fluid (ECF) - outside cells (~⅓ of body water)

· Intravascular (plasma)

· Interstitial (between cells)

· Transcellular (CSF, pleural, peritoneal, synovial)

Ex: sepsis, trauma, burns, HF, renal failure causes fluid to move from intravascular into the tissues → seem like they have fluid but are intravascularly depleted → explains why swelling and weight gain can exist along side hypotension and low urine output → why you should never rely on a single assessment finding

dx for ecf

- i and o, daily weights, lung sounds, urine output, hemodynamics, mental status, and lab trends → must all be interpreted together to give the story of where fluid is located and if its supporting circulation

ecf supports

· Blood pressure

· Oxygen delivery

- Nutrient transport

· Medication distribution

what is filtration

• Filtration = water movement caused by hydrostatic pressure

  • Hydrostatic pressure = force fluid exerts against the walls of a space (water-pushing pressure)

    • If fluid is confined in a space, the pressure increases → The more fluid present = the higher the pressure becomes

    • Blood pressure is a key example

      • Water moves: from higher pressure to lower pressure → Continues until pressure equalizes

    • Occurs across capillary walls

      • Capillaries are thin and porous

      • Allow fluid to move easily between the bloodstream and surrounding tissue

      • High capillary pressure → fluid moves into tissues from vascular space

      • High tissue pressure → fluid moves back into bloodstream

    • In critical care: ↑pressure → edema, crackles, ↓ oxygen exchange

      • Seen in heart failure, sepsis, trauma, fluid overload

what is diffusion

• Movement of particles (not water)

• Particles (electrolytes, oxygen, glucose) move from high → low concentration until balance is reached

• Driven by a concentration gradient

  • Bigger difference = faster diffusion

• Requires a permeable or selective membrane

  • Capillaries: allow many particles through

  • Cell membranes: tightly controlled → maintains normal electrical and metabolic function

ex of diffusion

• Sodium (Na⁺): higher outside the cell → tightly regulated for nerve & heart function

• Glucose: requires insulin to enter cells (facilitated diffusion) → when insulin is absent, the glucose remains in bloodstream

what is osmosis

• Osmosis = movement of water only across a selectively permeable membrane in response to a particle concentration

• Water moves: Toward higher particle concentration because that side has less water available

• Key rule: Water follows solute

• Osmotic shifts are a major reason PTs deteriorate quickly

causes of osmosis?

• Cell swelling or shrinking depending on the direction of water movement

• Changes in blood volume

ex of osmosis

Changes in sodium, glucose, or administered IV fluids can alter osmolarity and trigger rapid movement

what is fluid spacing

Where Fluid Ends up

1st spacing

• Normal fluid balance

• Fluid evenly distributed between:

  • ICF

  • ECF (intravascular + interstitial)

• Normal perfusion and circulation

• No abnormal fluid accumulation and organ function is normal

2nd spacing

• Normal fluid movement into interstitial space

• Part of normal capillary exchange → Hydrostatic pressure pushes fluid out of the capillaries, while oncotic pressure pulls it back in

• Small amounts of interstitial fluid act as a buffer to help maintain capillary and lymphatic balance

• Becomes edema when pressures are disrupted → Hydrostatic pressure increases or oncotic pressure decreases

• Fluid is part of normal function and can return to normal circulation and perfusion once the underlying imbalance is corrected

3rd spacing

• Fluid shifts out of the bloodstream into areas where it is no longer available to support circulation

• Fluid becomes trapped (extensive interstitial edema, peritoneal cavity, pleural space) and unavailable for circulation

• Total body fluid may be normal or high

• Circulating volume is low

• PTs may appear fluid overloaded with obvious edema → BUT intravascularly depleted

s/s of third spacing

• HOTN, tachycardia, decreased urine output, s/s of shock → circulation is failing

• Edema does not = adequate perfusion

causes of third spacing

• Sepsis, severe inflammation, trauma, burns, surgical stress, low plasma protein levels (inflammatory mediators increase capillary permeability → allowing both fluid and protein to leak into tissues → oncotic pressure drops and fluid follows → becomes trapped outside of circulation)

management of third spacing

• Restoring intravascular volume

• Supporting perfusion

• Treating underlying cause of capillary leak while carefully avoiding excessive fluid administration

what is osmolarity

• Osmolarity = particle concentration dissolved in body fluids

• Normal osmolarity ≈ 300 mOsm/L

• When fluids are balanced:

  • Isotonic

  • No net water movement

  • Cells maintain normal size

• Key rule: Water follows solute → water moves toward the area with higher particle concentration 

key fx isotonic

- No net water movement, fluids are balanced

key fx hypertonic fluid

• High particle concentration

• Pulls water in

• If used inappropriately → dehydrate cells and worsen instability

• Ex: When a hypertonic IV fluid is administered, the blood becomes more concentrated → water shifts from cells and interstitial space into the bloodstream to dilute plasma → expands circulating volume → causes swelled cells to shrink 

key fx hypotonic fluid

• Low particle concentration

• Pushes water out

• Can lower BP and worsen tissue/cerebral edema

• Ex: When a hypotonic IV fluid is administered, the blood becomes more dilute → water shifts out of bloodstream into cells and interstitial tissues → plasma volume decreases and cells swell

what does osmolarity affect

• Blood volume

• Cell size

• Organ function

serum osmolality key fx

- The Big Picture of water balance

- Reflects overall water balance and predicts where water will shift

o Normal: 270-300 mOsm/L → balanced compartments, minimal water movement

o Changes often explain acute mental status changes and perfusion instability

high osmolality

- concentrated blood (fluid deficit) → water moves out of cells into plasma→ cellular dehydration, neurologic risk

low osmolality

o Low osmolality: diluted blood ( excess free water) → water moves into cells → cellular swelling, seizure risk

cations and anions

• electrolytes carry these charges → Cations (+) and anions (-)

• Balance = electrical stability

• body fluids are electrically neutral

electrolytes and fluid compartments

• Electrolytes are unevenly distributed between fluids compartments 

  • ECF vs ICF gradients

  • Gradients allow nerve to fire, muscles to contract, and heart to conduct electrical impulses in a normal function

  • Required for normal cell function

• Small changes = big effects

  • Confusion, weakness, seizures, dysrhythmias

high risk electrolyte pts

- Older adults

- Kidney/endocrine disease (k increases when kidney fx decreased)

- Diuretics

Key electrolytes to monitor:

• Sodium (Na⁺)

• Potassium (K⁺)

• Calcium (Ca²⁺)

• Magnesium (Mg²⁺)

• Chloride (Cl⁻)

• Phosphorus (PO43-)

key fx na

• Major cation in extracellular fluid (ECF)

• Primary determinant of intravascular volume

• Water follows sodium → Abnormal sodium levels almost always reflect a water imbalance

• Normal range: 135 – 145 mEq/L

• Key electrolyte for maintaining blood volume, blood pressure, osmolarity and perfusion

low na

• (Excess Water → shifts into cells)

  • Cell swelling →↑Neurologic Risk (Confusion, altered LOC, seizures, cerebral edema)

high na

• (water loss → shifts out of cells)

  • Cell shrinkage → ↑Dehydration

cues for sodium imbalance

- new confusion, restlessness, decreased loc, seizure activity or h/a w/o clear cause, rapid sodium changes over hours not days, sodium abnormalities paired with fluid shifts, diuretics, or iv fluid changes → early warning signs

chloride key fx

• Major extracellular anion

• Normal: 95-105 mEq/L 

• Moves with sodium→ reflects fluid status

• Supports osmotic balance and acid-base stability (acts as a major counter-ion to bicarbonate)

• Helps maintain resting membrane potential of cells

• Abnormal levels often signal fluid or acid-base disturbance, not isolated chloride problems

what is elevated chloride associated w?

metabolic acidosis

what is low chloride associated w?

metabolic alkalosis

key fx k

• Major intracellular cation → critical for cardiac conduction & neuromuscular function

• Normal range: 3.5-5.0 mEq/L

• Small changes = big risk → life-threatening dysrhythmias

• Kidney dependent electrolyte → always assess renal function and urine output

ecg changes k

- low : flat t waves, u waves

- High: tall, peaked t waves, wide QRS → can progress to heart block or cardiac arrest

clinical cues of potassium imbalance

• Rapid potassium shifts over hours (not days), new or unexplained ECG changes, potassium abnormalities in PTs with AKI, CKD, diuretics, ACEI, ARBS, or acidosis should be carefully monitored, rising potassium with decreasing urine output

calcium key fx

• Essential for neuromuscular stability, muscle contraction, and blood coagulation

• Supports cardiac conduction and nerve transmission

• Two types: Bound and Unbound

• Regulated by PTH, vitamin D, and calcitonin → control absorption, storage, and release of calcium

low ca

→ tetany, muscle spasms, seizures →

high ca

· High Ca2+ → weakness, dysrhythmias, altered mental status

ionized vs total calcium

• Total calcium (includes calcium bound to albumin + free calcium) is affected by albumin → low albumin can falsely lower total Ca2+

• Ionized calcium = free active form → most accurate in critically ill patients

clinical cues of calcium imbalances

Neuromuscular irritability, tetany, seizures, cardiac dysrhythmias, low total calcium with low albumin, calcium abnormalities in patients with massive transfusions, sepsis, pancreatitis, or renal failure

mg key fx

• Primarily intracellular electrolyte Essential for neuromuscular stability and cardiac rhythm

• Essential for neuromuscular stability and cardiac rhythm

• Normal range: 1.3-2.1 mg/dL

• Works closely with potassium and calcium

cues mg

persistent hypokalemia despite K replacement, unexplained or refractory rhythms, neuromuscular irritability (tremors, increase reflexes), diuretics, alcoholics, malnutrition, sepsis, renal dysfx

low mg

· Low Mg²⁺ → dysrhythmias, muscle twitching, neuromuscular irritability, tremors, hyperrelfexia,

· Required to retain potassium inside the cell

· Low Mg²⁺ can cause refractory hypokalemia

· Often mg must be corrected before K+ or Ca2+ will stabilize

phosphorus key fx

- Energy & Respiratory Strength

· Primary role: ATP production → cellular energy

· Normal: 3.0-4.5 mg/dL

· Regulated by kidneys + bone storage

Increaed risk for resp failure

· Contributes to difficulty weaning from ventilation

· Commonly abnormal in critical illness, renal dysfunction, malnutrition

low phosphorus

• Low phosphorus → cells can’t generate energy for normal function → muscle weakness

  • ↓ diaphragmatic strength

  • ↓ respiratory reserve

  • ICU PTS: Shallow breathing, difficulty weaning from ventilation, increased risk for

clinical cues of phosphorus imbalances

Unexplained muscle weakness or fatigue, difficulty weaning from ventilation despite improving lung status, shallow respirations, phosphorus abnormalities in PTS with malnutrition, refeeding syndrome, sepsis, or prolonged ICU stay, normal potassium and calcium with persistent weakness

what hormones help regulate fluid balance

• aldosterone

• ADH

• ANP/BNP

• RAAS

aldosterone key fx

- trigger: low na, low blood volume

- saves na and water, loses k, increases bp

- decreases urine vol

- meaning: increases bp and vol, risk-> fluid retention, hypokalemia

ADH key fx

• trigger: high osmolarity, concentrated blood rather than vol

• saves water only

• urine effect:

  • High ADH: ↓ Urine, more concentrated

  • Low ADH: High output, reduced water retention

• meaning: dilutes blood, affects na

• clinical cues: Low urine output with rising sodium or serum osmolality, sudden changes in mental status, dilutional hyponatremia

ANP/BNP key fx

- trigger: high blood vol, high bp

- dumps na and water

- increases urine vol

- meaning: lowers bp and vol, prevents fluid overload

- clinical cues: Elevated BNP in PTS w/ s/s of overload, persistent edema or crackles despite diuresis, HOTN in PTS aggressively diuresed, worsening renal function during volume removal

RAAS key fx

- trigger: low bp, vol, na, o2 delivery

- saves na and water, vasoconstriction-> triggers aldosterone release

- decreases urine output

- meaning: protects perfusion in shock states

what signals the kidneys to regulate fluids? how do the kidneys adjust?

• Hormones (ADH, aldosterone, natriuretic peptides, RAAS) signal the kidneys to decide what to retain or excrete

• Kidneys continuously adjust:

  • Water retention or loss

  • Sodium and potassium reabsorption

  • Urine concentration (concentrated vs dilute)

• These adjustments maintain blood volume, blood pressure, osmolarity, and perfusion

what is the clearest indicator of the kidney’s response to hormones?

• Urine output is the clearest indicator of the kidney’s response

• Concentrated urine → conserving volume (↓ perfusion, ADH/aldosterone active)

• Dilute urine → excreting excess volume

• Urine output must always be interpreted with trends, vitals, labs, and mental status

clinical cues that kidneys are unable to regulate

• Decreased urine output despite stable BP, sudden oliguria, anuria, rising creatinine with declining urine output, concentrated urine with signs of hypoperfusion, adequate urine output but worsening mental status or labs, urine output below 400-600 mL/day 

Minimum (obligatory) urine output:

~400-600 mL/day to clear waste

BUN

o BUN increases early with low volume or decreased renal blood flow

- ability to clear nitrogen waste, highly sensitive to hydration status and renal blood flow

- High BUN w little or delayed creatinine change: decreased perfusion or volume depletion rather than intrinsic kidney injury

- When rise together: concern shifts to impaired kidney function

creatinine

• Creatinine is produced at a relatively constant rate → less affected by fluid shifts

• Creatinine = reflects true kidney function

BUN: creatinine ratio

o High ratio = Low circulating volume, renal hypoperfusion

Normal ratio w rising values → intrinsic injury rather than fluid distribution problem

o Both ↑ = Kidney dysfunction

albumin key fx

o Maintains oncotic pressure → keeps fluid in intravascular space

o Low albumin → edema and third spacing

o Patient may look "wet" but be intravascularly depleted

creatinine clearance

• Based on 24-hour urine creatinine + serum creatinine

• Determines how much blood the kidneys are actually filtering over time

• More sensitive than serum creatinine alone for early renal decline

• Decreasing CrCl may signal early kidney dysfunction before labs spike

• Best interpreted with urine volume, concentration, and trends

what is creatinine clearance equation?

(Urine creatinine x Urine volume) / Serum creatinine

what does creatinine clearance help anticipate?

• Medication dosing concerns

• Fluid and electrolyte instability

buffers for maintaining ph

- Buffers in the blood immediate response → act immediately to bind or release H ions to limit sudden ph changes → fast but limited capacity, stabilize temporarily

lungs for maintaining ph

· Lungs regulate carbon dioxide (acid) → within minutes, earliest sign of compensation increased ventilation removes co2 to lower acidity, decreased ventilated retains co2 to lower ph

kidneys for maintaining ph

· Kidneys provide long-term control. Manage Acid (H) and Bicarbonate → hours to days, most powerful and sustained correction → accompanied with changes in urine output and electrolytes

-diagnostics: abgs, urine output, lab trends, respiratory patterns

clinical cues of acid-base imbalances

Rapid respiratory changes without an obvious pulmonary cause, acid-base abnormalities paired with electrolyte instability, worsening mental status with subtle ABG changes, rising potassium or falling calcium with acidosis, urine output or kidney markers changing alongside pH shifts

bicarb as neutralizer

• Primary base and key buffer in metabolic acid–base balance

• Neutralizes acid to keep pH within survivable range

• Key principle: ↑ Acid → ↓ Bicarbonate (it is consumed) → metabolic acidosis

• Low HCO₃⁻ = metabolic problem

what can low bicarbonate reflect?

• Excess acid production

• Loss of base

• Impaired kidney compensation

what is the clinical significance of bicarbonate?

• Falling HCO₃⁻ = ongoing acid production or loss of buffering capacity

• Often precedes pH collapse

anion gap key fx

• Signals ongoing acid production and limited buffering reserve

• Tool used to determine the cause of metabolic acidosis

• Compares measured ions to expected balance

what are the ions that are measured?

• Sodium is primary measured positive ion

• Chloride and bicarbonate are the primary measured negative ions

what is the calculation for anion gap?

• Na+ - (Cl- + HCO3-)

normal anion gap

no excess unmeasured acids → suggests bicarbonate loss w chloride replacement

elevated anion gap

• Indicates acid accumulation in the blood

• Acids consume bicarbonate → bicarbonate level falls and gap widens

• Helps answer if metabolic acidosis is d/t acid accumulation or simple bicarbonate loss → elevated anion gap indicates acid accumulation