Fluid Compartments, Movement, and Electrolyte Balance

Intracellular Fluid (ICF)

Inside the cells, making up 2/3 of Total Body Water (TBW).

Extracellular Fluid (ECF)

Outside the cells, making up 1/3 of TBW.

Plasma (Intravascular fluid)

The fluid portion of blood inside blood vessels.

Interstitial Fluid

The fluid between cells and outside the blood vessels.

Osmosis

Water moves between compartments primarily through osmosis (driven by solute concentration differences).

Starling Forces

Starling forces regulate the movement of fluids across the capillary membrane, determining whether fluid enters or exits the capillaries.

Capillary Hydrostatic Pressure (CHP)

Pushes fluid OUT of capillaries into interstitial spaces (due to blood pressure).

Capillary Oncotic Pressure (COP)

Pulls fluid INTO capillaries from interstitial space (due to plasma proteins like albumin).

Interstitial Hydrostatic Pressure (IHP)

Pushes fluid INTO capillaries (but is usually low).

Interstitial Oncotic Pressure (IOP)

Pulls fluid OUT of capillaries into the interstitial space (minimal under normal conditions).

Net Filtration Equation

Net Filtration= NFP = GHP - CsHP - GCOP.

Edema

Fluid shifts from the vascular space into the interstitial space due to an imbalance in Starling forces.

Localized Edema

Limited to a specific area (e.g., DVT, pulmonary edema, cerebral edema).

Generalized Edema

Affects the whole body (e.g., anasarca in kidney or heart failure).

Atrial Natriuretic Peptide (ANP)

Released by the heart when atria are stretched due to high blood volume.

Brain Natriuretic Peptide (BNP)

Released by the heart when ventricles are stretched due to high blood volume.

Antidiuretic Hormone (ADH)

Released by the posterior pituitary in response to high osmolality (dehydration) or low blood pressure.

Renin-Angiotensin-Aldosterone System (RAAS)

Activated by low blood pressure or low sodium levels.

Normal Serum Osmolality

280-294 mOsm/kg.

Isotonic

Equal solute concentration inside & outside cells (280-294 mOsm/L).

Hypertonic

Higher solute concentration outside cells (>294 mOsm/L) → water moves out → cells shrink.

Hypotonic

Lower solute concentration outside cells → water moves in → cells swell.

Example of Hypotonic Solution

0.45% NaCl (½ Normal Saline).

Uses of Hypotonic Solutions

Used for: Dehydration, hypernatremia.

Risks of Hypotonic Solutions

Avoid in patients with increased intracranial pressure (ICP) or third-spacing (edema, burns).

Water Excess (Hypervolemia) Causes

Excess IV fluids, heart or kidney failure, SIADH (Syndrome of Inappropriate ADH secretion).

Water Excess (Hypervolemia) Symptoms

Edema, weight gain, high blood pressure, pulmonary edema (crackles, dyspnea), diluted blood (low hematocrit, sodium).

Water Excess (Hypervolemia) Management

Diuretics, fluid & sodium restriction, treat underlying cause (e.g., heart failure).

Water Deficit (Hypovolemia) Causes

Excess fluid loss (vomiting, diarrhea, burns, sweating), inadequate intake (dehydration, NPO status), third-spacing (fluid shifts into interstitial spaces).

Water Deficit (Hypovolemia) Symptoms

Low BP, tachycardia, poor skin turgor, dry mucous membranes, concentrated urine, low urine output, confusion, dizziness (especially in elderly).

Water Deficit (Hypovolemia) Management

Isotonic fluids (0.9% NS, LR) for volume restoration, hypotonic fluids (0.45% NS) for severe dehydration, monitor vital signs, urine output, electrolytes.

Starling Forces

Regulate fluid movement in capillaries based on pressure gradients.

Edema

Fluid accumulation in tissues due to increased CHP, decreased COP, increased permeability, or lymphatic obstruction.

ANP & BNP

Promote sodium & water excretion, lower BP.

ADH

Increases water retention, reduces urine output.

RAAS

Increases BP & fluid retention by stimulating Aldosterone & vasoconstriction.

Isotonic

No net fluid shift (e.g., 0.9% NaCl).

Hypertonic

Fluid shifts out of cells (cells shrink).

Hypotonic

Fluid shifts into cells (cells swell).

Hypervolemia

Fluid overload, edema, high BP.

Hypovolemia

Dehydration, low BP, dizziness.

Sodium (Na⁺)

Primary Extracellular Cation with a normal serum range of 135-145 mEq/L.

Functions of Sodium

Primary determinant of serum osmolality & fluid balance, regulates extracellular fluid (ECF) volume, works with potassium (Na⁺-K⁺ pump) to generate nerve impulses & muscle contraction.

Hypernatremia

Na⁺ >145 mEq/L caused by water loss (dehydration, sweating, diarrhea), excessive sodium intake (IV fluids, high-sodium diet), diabetes insipidus (low ADH, high urine output).

Symptoms of Hypernatremia

Thirst, dry mucous membranes, confusion, irritability, seizures, cell shrinkage (intracellular dehydration) → brain cells affected.

Treatment of Hypernatremia

Hypotonic IV fluids (0.45% NaCl), oral hydration (if mild).

Hyponatremia

Na⁺ <135 mEq/L caused by excessive water intake (dilutional hyponatremia), loss of sodium (vomiting, diarrhea, diuretics), SIADH (excess ADH → water retention, low Na⁺).

Symptoms of Hyponatremia

Confusion, seizures, coma (brain cell swelling), muscle weakness, nausea, headache.

Treatment of Hyponatremia

Hypertonic IV fluids (3% NaCl) (severe cases), fluid restriction (SIADH), oral sodium replacement (if mild).

Chloride (Cl⁻)

Follows Sodium with a normal serum range of 96-106 mEq/L.

Functions of Chloride

Maintains acid-base balance (binds with H⁺ to form HCl in stomach), follows sodium (Na⁺) in the ECF to maintain osmotic balance, involved in CO₂ transport in blood.

Hyperchloremia

Cl⁻ >106 mEq/L caused by dehydration, kidney disease, metabolic acidosis.

Symptoms of Hyperchloremia

Similar to hypernatremia, thirst, hypertension, lethargy.

Treatment of Hyperchloremia

Hydration, treat underlying cause.

Hypochloremia

Cl⁻ <96 mEq/L caused by vomiting (loss of HCl), metabolic alkalosis.

Symptoms of Hypochloremia

Muscle cramps, confusion, weakness.

Treatment of Hypochloremia

Replace Cl⁻ via saline IV.

Potassium (K⁺)

Primary Intracellular Cation with a normal serum range of 3.5-5.0 mEq/L.

Functions of Potassium

Regulates intracellular fluid (ICF) osmolality, essential for cardiac, skeletal, and smooth muscle contraction, maintains resting membrane potential, helps regulate acid-base balance (exchange with H⁺).

Hyperkalemia

K⁺ >5.0 mEq/L

Causes of Hyperkalemia

Renal failure (↓K⁺ excretion), Acidosis (K⁺ shifts out of cells as H⁺ enters), Tissue breakdown (burns, trauma, rhabdomyolysis).

Symptoms of Hyperkalemia

EKG changes (peaked T waves, wide QRS, arrhythmias), Muscle weakness, paralysis, Nausea, diarrhea.

Treatment for Hyperkalemia

IV calcium gluconate (stabilizes heart), Insulin + glucose (shifts K⁺ into cells), Diuretics, dialysis (remove excess K⁺).

Hypokalemia

K⁺ <3.5 mEq/L

Causes of Hypokalemia

Vomiting, diarrhea, diuretics, Alkalosis (K⁺ shifts into cells as H⁺ exits).

Symptoms of Hypokalemia

Muscle weakness, leg cramps, respiratory depression, EKG changes (flattened T waves, U waves, arrhythmias).

Treatment for Hypokalemia

Oral/IV potassium replacement (never IV push K⁺!), Correct underlying cause.

Acid-Base Balance

The body maintains pH balance via three buffer systems.

Bicarbonate Buffer System

Most important buffer in the blood, maintains pH via the CO₂-HCO₃⁻ system.

Protein Buffer System

Binds or releases H⁺ to regulate pH.

Phosphate Buffer System

Works inside cells & kidneys to maintain pH.

Acidosis

pH < 7.35, ↑ H⁺ (Respiratory = CO₂ retention, Metabolic = HCO₃⁻ loss).

Alkalosis

pH > 7.45, ↓ H⁺ (Respiratory = CO₂ loss, Metabolic = HCO₃⁻ excess).

Respiratory Acidosis

Hypoventilation (COPD, opioids, head trauma) → ↑ CO₂, kidneys retain HCO₃⁻.

Respiratory Alkalosis

Hyperventilation (Anxiety, fever, mechanical ventilation) → ↓ CO₂, kidneys excrete HCO₃⁻.

Metabolic Acidosis

DKA, diarrhea, renal failure → ↓ HCO₃⁻, lungs increase breathing (↓ CO₂).

Metabolic Alkalosis

Vomiting, diuretics, antacid overdose → ↑ HCO₃⁻, lungs decrease breathing (↑ CO₂).

Stomach Acid

The stomach is acidic (HCl production, pH ~1-2).

Intestinal Alkalinity

The intestines are alkaline (bicarbonate secretion, pH ~7-8).

Vomiting/NG Suctioning

Causes Metabolic Alkalosis due to loss of HCl (acid).

Diarrhea

Causes Metabolic Acidosis due to loss of HCO₃⁻ (base).

Sodium (Na⁺) Normal Range

135-145 mEq/L, fluid balance, osmolality, symptoms include seizures, confusion, weakness.

Chloride (Cl⁻) Normal Range

96-106 mEq/L, follows Na⁺, acid-base balance, symptoms include dehydration, alkalosis symptoms.

Potassium (K⁺) Normal Range

3.5-5.0 mEq/L, muscle contraction, nerve function, symptoms include arrhythmias, weakness, paralysis.

pH Normal Range

7.35-7.45, acid-base balance, acidosis (<7.35) or alkalosis (>7.45).

PaCO₂ Normal Range

35-45 mmHg, respiratory acid, high = acidosis, low = alkalosis.

HCO₃⁻ Normal Range

22-26 mEq/L, metabolic base, low = acidosis, high = alkalosis.

Intracellular Fluid (ICF)

Located inside cells, makes up ~2/3 of Total Body Water (TBW), contains potassium (K⁺) as the main cation and phosphate (PO₄³⁻) as the main anion.

Extracellular Fluid (ECF)

Located outside cells, makes up ~1/3 of TBW, subdivided into Plasma, Interstitial Fluid, and Transcellular Fluid.

Capillary Hydrostatic Pressure (CHP)

Pushes fluid out of capillaries into interstitial spaces (increased in hypertension).

Capillary Oncotic Pressure (COP)

Pulls fluid into capillaries (due to plasma proteins like albumin).

Interstitial Hydrostatic Pressure (IHP)

Pushes fluid into capillaries.

Interstitial Oncotic Pressure (IOP)

Pulls fluid out of capillaries into interstitial space.

Net Filtration Equation

(CHP + IOP) - (COP + IHP)

Causation of Edema

1. ↑ Capillary Hydrostatic Pressure (e.g., hypertension, heart failure). 2. ↓ Capillary Oncotic Pressure (e.g., low albumin in liver disease, malnutrition). 3. ↑ Capillary Permeability (e.g., inflammation, sepsis). 4. Lymphatic Obstruction (e.g., tumor, surgery).

Pathophysiology of Edema

Imbalance in Starling forces → Fluid shifts from blood vessels into tissues.

Clinical Manifestations of Edema

Localized edema (e.g., DVT, pulmonary edema). Generalized edema (e.g., anasarca in kidney/heart failure).

Antidiuretic Hormone (ADH)

Released by the posterior pituitary in response to high osmolality or low BP; increases water reabsorption in the kidneys.

Renin-Angiotensin-Aldosterone System (RAAS)

Renin → Angiotensin I → Angiotensin II (vasoconstriction); Aldosterone (from adrenal glands) → Sodium and water retention.

Natriuretic Peptides (ANP & BNP)

ANP (Atria) & BNP (Ventricles) are released when the heart is stretched; promote sodium & water excretion → Lower BP.