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Fluid & Electrolyte
Balance

Explore how the body maintains water distribution, regulates ion concentrations, and preserves homeostasis through dynamic physiological mechanisms.

Anatomy & Physiology

What Are Body Fluids?

Body fluids are water-based solutions that dissolve and transport electrolytes, nutrients, gases, and wastes. In an average adult, water accounts for approximately 60% of total body mass, making it the single most abundant chemical substance in the body.

The Two Major Fluid Compartments

ICF Intracellular Fluid ~28 L (2/3 of total)
\( \text{K}^+ \; \text{Mg}^{2+} \; \text{HPO}_4^{2-} \)
 Proteins Cell Membrane ECF Extracellular Fluid ~14 L (1/3 of total)
\( \text{Na}^+ \; \text{Cl}^- \; \text{HCO}_3^- \)
\( \text{Ca}^{2+} \) Glucose
 Plasma (3 L) + Interstitial (11 L) Total Body Water: ~42 L (60% body mass)

What Are Electrolytes?

Electrolytes are ions that dissociate in water and carry electrical charges. They regulate nerve impulses, muscle contractions, pH balance, and fluid distribution between compartments.

Cations (+)
\( \text{Na}^+, \text{K}^+, \text{Ca}^{2+}, \text{Mg}^{2+} \): positively charged ions vital for signaling and contraction
Anions (−)
\( \text{Cl}^-, \text{HCO}_3^-, \text{HPO}_4^{2-} \): negatively charged ions essential for buffering and osmotic balance

The Osmosis Principle

Water moves across semipermeable membranes from areas of lower solute concentration to areas of higher solute concentration until equilibrium is reached. This passive process is called osmosis.

Low Solute
\( \text{H}_2\text{O} \) +++
 Semipermeable Membrane High Solute
\( \text{H}_2\text{O} \) +

Osmolarity & Tonicity

Osmolarity measures the total concentration of solute particles per liter of solution. Normal plasma osmolarity is approximately \( 275\text{--}295 \; \text{mOsm/L} \). Tonicity describes how a solution affects cell volume: it considers only non-penetrating solutes.

Isotonic
Same effective osmolarity as cell interior: no net water movement, cell stays normal
Hypotonic
Lower solute outside: water enters cell, cell swells (may lyse)
Hypertonic
Higher solute outside: water leaves cell, cell shrinks (crenation)

How Tonicity Affects Cells

Isotonic Normal
No net \( \text{H}_2\text{O} \)
movement
 Hypotonic Swollen (lysis risk)
\( \text{H}_2\text{O} \) enters cell
 Hypertonic Shrunk (crenation)
\( \text{H}_2\text{O} \) leaves cell

ICF vs ECF: Key Distinctions

ICF (Intracellular) ECF (Extracellular)
Volume ~28 L (2/3) ~14 L (1/3)
Major Cation \( \text{K}^+ \) \( \text{Na}^+ \)
Major Anion \( \text{HPO}_4^{2-} \) \( \text{Cl}^- \)
Protein Content High Low (except plasma)
Separated By Cell (plasma) membrane

How the Body Regulates Balance

Fluid and electrolyte balance is maintained by hormonal feedback loops and organ systems working together. The kidneys are the primary regulators, adjusting water and ion excretion based on hormonal signals.

Kidneys ADH
Retain \( \text{H}_2\text{O} \)
 Aldosterone
Retain \( \text{Na}^+ \)
 ANP
Excrete \( \text{Na}^+/\text{H}_2\text{O} \)
 RAAS ↑ BP & Volume

ADH & Aldosterone in Detail

ADH (Vasopressin)
Released by the posterior pituitary when osmolarity rises. Inserts aquaporins into collecting ducts, increasing water reabsorption. Result: concentrated urine, diluted blood.
Aldosterone
Released by the adrenal cortex via RAAS when blood pressure drops. Increases \( \text{Na}^+ \) reabsorption (and \( \text{K}^+ \) secretion) in the distal tubule. Water follows \( \text{Na}^+ \), raising blood volume.

How to Analyze a Fluid Imbalance

1
Identify the input/output change. Is the patient losing fluid (vomiting, diarrhea, sweating) or gaining excess (IV overload, kidney failure)?
2
Determine the osmolarity shift. Is the fluid lost/gained isotonic, hypotonic, or hypertonic relative to plasma?
3
Predict the compartment effect. Which compartment changes volume? Does water shift between ICF and ECF?
4
Identify the hormonal response. Will ADH, aldosterone, or ANP be triggered to restore balance?
5
Predict the clinical signs. Thirst, low BP, edema, concentrated/dilute urine, altered heart rate?

Example 1: Dehydration

Scenario: A marathon runner sweats heavily for 3 hours without drinking water. Sweat is hypotonic (mostly water with some \( \text{Na}^+ \)).

Before ICF ECF → Sweat loss After ICF ↓ ECF ↓↓ ECF osmolarity ↑ → water leaves ICF

Example 2: Hyponatremia

Scenario: A patient drinks excessive water (water intoxication). Plasma \( \text{Na}^+ \) drops below \( 135 \; \text{mEq/L} \).

1
Input change: Massive hypotonic fluid (pure water) ingested.
2
Osmolarity shift: ECF osmolarity drops sharply (diluted).
3
Compartment effect: ECF is now hypotonic relative to ICF → water rushes into cells → cells swell.
4
Hormonal response: ADH secretion is inhibited → kidneys excrete dilute urine.
5
Clinical signs: Headache, confusion, nausea, seizures (cerebral edema from brain cell swelling).

Example 3: Hyperkalemia

Scenario: A patient with kidney failure cannot excrete \( \text{K}^+ \). Serum \( \text{K}^+ \) rises above \( 5.0 \; \text{mEq/L} \).

Cause
Renal failure, acidosis (\( \text{H}^+/\text{K}^+ \) exchange), cell lysis, excessive \( \text{K}^+ \) intake
Danger
Cardiac arrhythmias, peaked T-waves on ECG, muscle weakness, potential cardiac arrest
Treatment
IV calcium (cardioprotective), insulin + glucose (shifts \( \text{K}^+ \) into cells), kayexalate, dialysis

Common Mistakes

✕ Confusing Osmolarity with Tonicity
Osmolarity counts all solutes; tonicity counts only non-penetrating solutes. Urea raises osmolarity but not tonicity because it crosses membranes freely.
✕ Swapping \( \text{Na}^+ \) and \( \text{K}^+ \) Locations
\( \text{Na}^+ \) is the dominant ECF cation; \( \text{K}^+ \) dominates ICF. Mixing these up reverses every prediction about fluid shifts.
✕ Thinking ADH = Aldosterone
ADH acts on water channels (aquaporins) in response to osmolarity. Aldosterone acts on \( \text{Na}^+ \) channels in response to blood volume/pressure.

Summary of Key Electrolytes

Electrolyte Normal Range Primary Location Key Function
\( \text{Na}^+ \) \( 135\text{--}145 \; \text{mEq/L} \) ECF Fluid balance, nerve impulses
\( \text{K}^+ \) \( 3.5\text{--}5.0 \; \text{mEq/L} \) ICF Resting membrane potential
\( \text{Ca}^{2+} \) \( 8.5\text{--}10.5 \; \text{mg/dL} \) ECF / Bone Muscle contraction, clotting
\( \text{Cl}^- \) \( 96\text{--}106 \; \text{mEq/L} \) ECF Osmotic balance, HCl in stomach
\( \text{HCO}_3^- \) \( 22\text{--}26 \; \text{mEq/L} \) ECF pH buffering (bicarbonate buffer)

Question 1 of 5

Which ion is the most abundant cation in the intracellular fluid (ICF)?

Question 2 of 5

A red blood cell is placed in a hypotonic solution. What will happen to the cell?

Question 3 of 5

ADH (antidiuretic hormone) is released in response to increased plasma osmolarity. What is its primary effect on the kidneys?

Question 4 of 5

A patient presents with serum \( \text{Na}^+ \) of \( 125 \; \text{mEq/L} \) (normal: \( 135\text{--}145 \)), confusion, and headache after drinking excessive water. Which condition does this represent?

Question 5 of 5

Aldosterone primarily acts on the distal convoluted tubule of the nephron. Which of the following best describes its action?