Fluid Shifts Between Compartments

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Body Fluid Compartments - Our Inner Oceans

  • Total Body Water (TBW): Approx. 60% of body weight in adult males (lower in females, obese individuals).
  • Intracellular Fluid (ICF): Constitutes 2/3 of TBW.
    • Primary cation: $K^+$.
    • Primary anions: $PO_4^{3-}$, proteins.
  • Extracellular Fluid (ECF): Constitutes 1/3 of TBW.
    • Primary cation: $Na^+$.
    • Primary anion: $Cl^-$.
    • Subdivisions:
      • Interstitial Fluid (ISF): ~75% of ECF.
      • Plasma: ~25% of ECF.
      • Transcellular fluid (e.g., CSF, synovial fluid; small volume). Total Body Water Distribution Pie Chart

⭐ The 60-40-20 rule is a key clinical approximation: 60% of total body weight is water, 40% is ICF, and 20% is ECF.

Starling's Forces - Capillary Pressure Play

  • Fluid exchange between plasma & interstitium is governed by Starling's forces.

  • Net Filtration Pressure (NFP) dictates movement: $NFP = (P_c - P_{if}) - (\mathbf{\pi}c - \mathbf{\pi}{if})$.

    • $P_c$: Capillary Hydrostatic Pressure; pushes fluid OUT (main outward force).
    • $\mathbf{\pi}_c$: Capillary Oncotic Pressure (albumin); pulls fluid IN (main inward force).
    • $P_{if}$: Interstitial Fluid Hydrostatic Pressure; pushes fluid IN.
    • $\mathbf{\pi}_{if}$: Interstitial Fluid Oncotic Pressure; pulls fluid OUT.
  • Arteriolar end: Filtration favored ($P_c$ > $\mathbf{\pi}_c$).

  • Venular end: Reabsorption favored ($\mathbf{\pi}_c$ > $P_c$).

    ![Capillary Fluid Exchange](starling forces diagram)

⭐ Increased capillary permeability (e.g., inflammation) allows protein leakage into interstitium (↑$\mathbf{\pi}_{if}$), causing edema.

Osmolality & Tonicity - Solute's Mighty Pull

  • Osmolality: Measure of total solute concentration per kg solvent.
    • Normal plasma: 280-295 mOsm/kg $H_2O$.
    • Calculated: $2 \times [\text{Na}^+] + \frac{[\text{Glucose}]}{18} + \frac{[\text{BUN}]}{2.8}$ (units: Na mEq/L, others mg/dL).
    • Mainly regulated by ADH.
  • Tonicity: Effective osmolality; drives H₂O movement across membranes. Depends on impermeable solutes.
    • Isotonic: No net H₂O movement (e.g., 0.9% NaCl).
    • Hypotonic: H₂O enters cells → cells swell (e.g., 0.45% NaCl).
    • Hypertonic: H₂O leaves cells → cells shrink (e.g., 3% NaCl, Mannitol).
  • 📌 Osmolality: all solutes. Tonicity: only non-penetrating solutes. Urea: ineffective osmole.

RBCs in Isotonic, Hypotonic, and Hypertonic Solutions

⭐ Mannitol (osmotic diuretic) ↑ ECF osmolality, draws H₂O from ICF. Used to ↓ intracranial & intraocular pressure.

Fluid Shift Scenarios - Clinical Case Clues

Ascites: Fluid Buildup in the Abdomen

  • Heart Failure: ↑$P_c$ (systemic/pulmonary) → Peripheral/pulmonary edema. Clues: Dyspnea, JVD, crackles.
  • Liver Cirrhosis: ↓$\pi_p$ (↓albumin synthesis) + ↑$P_c$ (portal HTN) → Ascites, peripheral edema. Clues: Jaundice, spider nevi.
  • Nephrotic Syndrome: ↓$\pi_p$ (proteinuria) → Generalized edema (anasarca), periorbital puffiness. Clues: Frothy urine, >3.5 g/day proteinuria.
  • Sepsis/Burns: ↑Capillary permeability → Generalized edema, intravascular volume depletion. Clues: Fever, hypotension, inflammation.
  • Lymphatic Obstruction: Impaired interstitial fluid drainage → Lymphedema. Clues: Non-pitting edema, history of surgery/radiation.

⭐ In nephrotic syndrome, massive proteinuria (typically >3.5 g/day) leads to decreased plasma oncotic pressure, a primary driver of generalized edema (anasarca).

Regulation of Balance - Hormonal Harmony

  • ADH (Vasopressin): From posterior pituitary. Trigger: ↑plasma osmolality, ↓volume. Action: ↑$H_2O$ reabsorption (collecting ducts).
  • Aldosterone (RAAS): From adrenal cortex. Trigger: ↓renal perfusion (Angiotensin II), ↑$K^+$. Action: ↑$Na^+$ & $H_2O$ reabsorption, ↑$K^+$ excretion (DCT/CD).
  • Natriuretic Peptides (ANP/BNP): From heart. Trigger: Atrial/ventricular stretch. Action: Natriuresis, diuresis, vasodilation; oppose RAAS.

Hormonal Regulation of Water and Sodium Balance

⭐ SIADH (Syndrome of Inappropriate ADH) leads to euvolemic hyponatremia due to excessive water retention and resultant sodium dilution.

High‑Yield Points - ⚡ Biggest Takeaways

  • Osmolality differences drive water movement between ICF and ECF.
  • Sodium (Na+) is the main determinant of ECF osmolality and volume.
  • Water moves passively from lower to higher osmolality areas.
  • Isotonic solutions (0.9% Saline) expand ECF volume without major ICF shifts.
  • Hypotonic solutions (0.45% Saline) shift water into cells, causing ICF expansion.
  • Hypertonic solutions (3% Saline) draw water out of cells, causing ICF contraction.
  • Starling forces govern fluid exchange between plasma and interstitial fluid.

Practice Questions: Fluid Shifts Between Compartments

Test your understanding with these related questions

A decrease in which of the following is the most likely cause of peripheral edema in a patient with long-term alcoholism and liver disease?

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Flashcards: Fluid Shifts Between Compartments

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On average, _____ fluid is 20% of body weight

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On average, _____ fluid is 20% of body weight

extracellular

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