Lecture: Body Fluid Compartments

Introduction

• Human body is 50-70% water.
  • Varies with factors: age, body fat percentage.
• Total body water divided into:
  • Intracellular Fluid (ICF)
  • Extracellular Fluid (ECF)

Fluid Compartments

Intracellular Fluid (ICF)

• Inside cells.
• Constitutes two-thirds of total body water.
• Major cations: Potassium, Magnesium.
• Anions: Organic phosphates (ATP, ADP), Proteins.

Extracellular Fluid (ECF)

• Outside cells.
• Constitutes one-third of total body water.
  • Approximately 20% of body weight.
• Divided into:
  • Plasma (25% of ECF)
  • Interstitial Fluid (75% of ECF)

Plasma

• Inside blood vessels.
• Has plasma proteins.

Interstitial Fluid

• Outside vessels.
• Ultrafiltrate of plasma.
• Very little protein due to capillary membrane.

Other Fluids

• Blood: Contains both ECF and ICF.
  • RBCs have ICF; Plasma has ECF.
  • Hematocrit: Fraction of blood that's RBCs.
• Transcellular Fluid
  • Includes pericardial, peritoneal, cerebrospinal fluid.

Membranes and Permeability

Cell Membrane

• Separates ICF and ECF.
• Selectively permeable.
  • Freely permeable to water.
  • Impermeable to most solutes.

Capillary Membrane

• Separates plasma and interstitial fluid.
• Allows solutes through but not proteins.

Solute Composition

• ECF: Predominant cation - Sodium; Anions - Chloride, Bicarbonate.
• ICF: Predominant cations - Potassium, Magnesium; Anions - Phosphates, Proteins.
• Overall electrochemical neutrality in compartments.

Volume Measurement

• Total Body Water = ICF + ECF.
• ECF = Interstitial Fluid + Plasma Volume.

Indicator-Dilution Principle

• Indicators measure compartment volume.
  • Total Body Water: Indicator crosses cell membrane.
  • ECF: Indicator does not cross cell membrane.
  • Plasma: Indicator does not cross capillary membrane nor enters RBCs.
• Indirect Calculations:
  • ICF = Total Body Water - ECF.
  • Interstitial Fluid = ECF - Plasma Volume.
  • Blood Volume: Plasma Volume / (1 - Hematocrit).

Osmolarity and Water Movement

• ECF and ICF osmolarity must be equal.
• Water moves across membranes to balance osmolarity (Osmosis).

Conclusion

• Summary of body fluid compartments and their significance in maintaining homeostasis.