MD137 Body Fluid Compartments Quiz

Module Information

• Module: MD137
• Year: 2024-2025
• Topic: Body Fluid Compartments
• Lecturer: Dr. Karl McCullagh
• Suggested Reading: Vander 15th Edition, Chapter 4

Learning Outcomes

• Students will learn about the fluid compartments of the human body.
• Students will learn about the composition of body fluids, including osmolarity and pH.
• Students will understand the clinical significance of body fluid properties.

Lecture Outline

• Fluid intake and output are balanced in steady-state conditions
• Body fluid compartments, including extracellular and intracellular fluids.
• Measurement of fluid volumes (principles and specific compartments).
• Regulation of fluid exchange and osmotic equilibrium between intracellular and extracellular fluids.
• Hyponatremia and hypernatremia

Body Fluid Distribution

• Total Body Water (TBW): Approximately 45L in a 70 kg adult.
• Extracellular Fluid (ECF): Approximately 14L in a 70kg adult male
  • Plasma: ~3L
  • Interstitial Fluid: ~11L
• Intracellular Fluid (ICF): Approximately 28L in a 70 kg adult male. This accounts for 40% of the body weight.

Fluid Balance

• Intake: Fluids ingested (~2100ml) and from metabolism (~200ml) = ~2300ml/day in normal conditions
• Output: Insensible losses (skin/lungs), sweat, feces, urine = ~2300ml/day in normal conditions

Effect of Sodium Intake

• A 10-fold increase in sodium intake leads to sodium retention and an increase in extracellular fluid volume.
• The kidneys will excrete more sodium in response after ~1-2 days to maintain homeostasis.

Fluid Balance during Exercise

• Exercise substantially increases fluid needs.
• Fluids ingested = ~6400ml/day during exercise
• Fluid output increase to maintain the fluid balance
• Sweat, urine, and other losses increase significantly during exercise (~6600ml/day in active conditions)

Body Fluid Compartments (Numerical Data)

• Plasma: 3.5L
• Interstitial Fluid: 8.5L
• Lymph: 1.5L
• Transcellular Fluid: 1.5L

Ionic Composition

• Extracellular fluid electrolytes: high Na+, Cl-, HCO3-.
• Intracellular fluid electrolytes: high K+, Mg++, PO4-.

Measuring Body Fluid Volumes (Indicator Dilution Principle)

• Indicator Mass = Volume x Concentration (useful to find total body water).
• Volume = Indicator Mass / Concentration.
• Indicator must only dissolve in the measured compartment and disperse evenly to avoid errors.
• The indicator should stay in the fluid, not be metabolized or excreted by the body.

Example Question (Total Body Water)

• 10 ml of antipyrine (150 mg/ml) injected.
• Equilibrium concentration in blood = 0.03 mg/ml.
• Total body water = 1500 mg / 0.03 mg/ml = 50,000 ml, or 50 liters.

Types of solutions (mOsm/L)

• 3% normal saline: 1026 mOsm/L (hypertonic)
• 5% glucose: 278 mOsm/L (isotonic)

Controlling fluid distribution

• Osmotic forces play a vital role in fluid distribution across cell membranes.
• Electrolyte concentrations drive the movement of water.
• Hydrostatic and colloid osmotic forces influence fluid movement across capillaries.
• Many solutes (e.g. sodium chloride) are relatively impenetrable to cell membranes.
• Water easily crosses membranes.

Other important concepts

• Water, as well as electrolytes, are critically important in maintaining homeostasis in the body; balance between their intake and output is essential.
• The volumes and composition of body fluids must be carefully regulated by cellular and bodily mechanisms to maintain health and function; it is important to understand what may alter fluid homeostasis.

Test your understanding of body fluid compartments as covered in Vander's 15th Edition, Chapter 4. This quiz will assess your knowledge on fluid distribution, osmolarity, pH, and their clinical significance in health and disease. Prepare to explore the dynamics of extracellular and intracellular fluids.