Lecture: Body Fluid Compartments

Introduction

• Human body: 50-70% water.
  • Varies by age, body fat, etc.
• Total body water distributed into:
  • Intracellular Fluid (ICF)
  • Extracellular Fluid (ECF)

Intracellular Fluid (ICF)

• Water inside cells.
• Comprises 40% of body weight if total body water is 60%.

Extracellular Fluid (ECF)

• Water outside cells.
• Comprises 20% of body weight if total body water is 60%.
• ECF further divided into:
  • Plasma (inside vessels)
  • Interstitial fluid (outside vessels)

Plasma and Interstitial Fluid

• Plasma: 1/4 of ECF
• Interstitial Fluid: 3/4 of ECF
• Separated by capillary membrane

Blood as a Compartment

• Contains both ECF and ICF:
  • Plasma (ECF)
  • Red Blood Cells (ICF)
• Hematocrit: fraction of blood that is RBCs

Transcellular Compartment

• Includes fluids in:
  • Pericardial cavity
  • Peritoneal cavity
  • Cerebrospinal fluid
• Small part of ECF

Solutes in Fluid Compartments

• ECF and ICF separated by cell membrane
• Plasma and Interstitial Fluid separated by capillary membrane
• Interstitial fluid is an ultrafiltrate of plasma
• Capillary membrane allows solute pass except proteins

Differences in Composition

• ECF:
  • Main cation: Sodium
  • Main anions: Chloride and Bicarbonate
• ICF:
  • Main cations: Potassium and Magnesium
  • Main anions: Organic phosphates (ATP, ADP) and proteins

Fluid Volume Measurement

• Total body water = ICF + ECF
• ECF = Interstitial fluid + Plasma volume
• Indicator-dilution principle used for measurement
  • Total body water indicator crosses cell membrane
  • ECF indicator does not cross cell membrane
  • Plasma volume indicator does not cross capillary membrane or enter RBCs
• Indirect calculations:
  • ICF = Total body water - ECF
  • Interstitial fluid = ECF - Plasma volume

Blood Volume Calculation

• Formula: Plasma volume / (1 - Hematocrit)

Osmolarity and Equilibrium

• ECF and ICF osmolarity must be equal
• Cell membrane is selectively permeable
• Water moves by osmosis to maintain equilibrium