Fluid and Electrolyte Balance: Body Fluid Compartments and Regulation

Fluid and Electrolyte Balance

Percentage of Body Fluid

The human body is composed of a significant proportion of water, which is distributed across various tissues and compartments. The percentage of body fluid varies based on several factors, including age, sex, and the relative amounts of adipose (fat) and muscle tissue.

• Body Fluid Percentage: Ranges between 45% and 75% of total body mass.

• Influencing Factors: Age and the ratio of adipose tissue to muscle tissue.

• Muscle vs. Adipose Tissue:

  • Muscle contains about 75% water (muscle stores water).

  • Adipose tissue contains about 20% water.

• Sex Differences: On average, males have about 60% water, while females have about 50% due to a higher proportion of adipose tissue.

Example: A young, muscular male will have a higher percentage of body water than an older female with more adipose tissue.

Fluid Compartments

Main Fluid Compartments

Body fluids are distributed in two main compartments: intracellular fluid (ICF) and extracellular fluid (ECF). Each compartment has distinct characteristics and functions.

• Intracellular Fluid (ICF):

  • Fluid within cells.

  • Accounts for about two-thirds of total body fluid.

  • Separated from ECF by the plasma membrane, which is selectively permeable.

• Extracellular Fluid (ECF):

  • Fluid outside cells.

  • Divided into two subcompartments:

    • Interstitial Fluid (IF): Fluid surrounding cells; makes up about two-thirds of ECF.

    • Plasma: Fluid within blood vessels; separated from IF by the capillary vessel wall, which is more permeable than the plasma membrane.

  • Interstitial fluid and plasma are similar in composition, except plasma contains more proteins.

Additional info: The movement of water and solutes between these compartments is essential for maintaining homeostasis and is regulated by osmotic and hydrostatic pressures.

Summary Table: Body Fluid Compartments

Compartment	Location	Percentage of Total Body Fluid	Main Features
Intracellular Fluid (ICF)	Inside cells	~66%	High in K+, Mg2+, PO43−; separated by plasma membrane
Extracellular Fluid (ECF)	Outside cells	~33%	Divided into interstitial fluid and plasma; high in Na+, Cl−, HCO3−
Interstitial Fluid (IF)	Between cells	~22%	Surrounds cells; low protein content
Plasma	Within blood vessels	~11%	Contains proteins; separated from IF by capillary wall

Key Points

• Selective Permeability: The plasma membrane allows passage of some substances but not others, maintaining distinct compositions in ICF and ECF.

• Fluid Movement: Water and solutes move between compartments to maintain equilibrium, influenced by osmotic gradients.

Example: During dehydration, water moves from the ICF to the ECF to balance osmolarity, potentially affecting cell function.