Body Fluid Compartments, Osmosis, and Membrane Transport in Anatomy & Physiology

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Body Fluid Compartments

Overview of Body Fluid Compartments

The human body is composed primarily of water, distributed between distinct fluid compartments. Understanding these compartments is essential for studying physiological processes such as transport, osmosis, and homeostasis.

Intracellular Fluid (ICF): Fluid within cells; constitutes about two-thirds of total body water.
Extracellular Fluid (ECF): Fluid outside cells; makes up about one-third of total body water. Subdivided into:
- Interstitial Fluid: Surrounds tissue cells.
- Plasma: The liquid component of blood.
Selective Permeability: The cell membrane separates ICF and ECF, allowing selective movement of substances.
Chemical Disequilibrium: The compartments are in osmotic equilibrium but have different chemical compositions.

Example: Sodium ions (Na+) are more concentrated in the ECF, while potassium ions (K+) are more concentrated in the ICF.

Diagrammatic Representation

Box diagrams and bar graphs are commonly used to illustrate the relative volumes and compositions of fluid compartments.

ICF: ~2/3 of total body water
ECF: ~1/3 of total body water (further divided into plasma and interstitial fluid)

Body Water Content Variation

The percentage of body weight that is water varies with age and sex, but the relative proportions of ICF and ECF remain fairly constant.

Age	Male	Female
Infant	65%	65%
1–9	62%	62%
10–16	59%	57%
17–39	61%	51%
40–59	55%	47%
60+	52%	46%

Key Point: Males generally have a higher percentage of body water than females, and total body water decreases with age.

Osmosis and Tonicity

Osmosis

Osmosis is the movement of water across a selectively permeable membrane in response to a solute concentration gradient.

Direction: Water moves from an area of lower solute concentration to an area of higher solute concentration.
Equilibrium: Osmosis continues until the concentrations on both sides of the membrane are equal or until opposed by another force.

Example: If a cell is placed in a solution with higher solute concentration than its cytoplasm, water will move out of the cell, causing it to shrink.

Osmotic Pressure

Osmotic pressure is the pressure required to prevent the movement of water by osmosis.

Definition: The minimum pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
Formula:

Where = osmotic pressure, = van 't Hoff factor (number of particles per formula unit), = molarity, = gas constant, = temperature in Kelvin.

Example: In a U-tube with pure water on one side and a glucose solution on the other, water moves into the glucose solution, and osmotic pressure is the force needed to stop this movement.

Tonicity

Tonicity describes how a solution affects cell volume, based on the concentration of non-penetrating solutes.

Isotonic Solution: No net movement of water; cell volume remains unchanged.
Hypertonic Solution: Higher solute concentration outside the cell; water moves out, and the cell shrinks.
Hypotonic Solution: Lower solute concentration outside the cell; water moves in, and the cell swells.

Membrane Transport Processes

Diffusion

Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration.

Simple Diffusion: Movement directly through the lipid bilayer (e.g., O2, CO2).
Facilitated Diffusion: Movement via membrane proteins (channels or carriers).

Protein-Mediated Transport

Some substances require specific membrane proteins to cross the cell membrane.

Channel Proteins: Form water-filled passages for ions and small molecules.
Carrier Proteins: Bind to substrates and undergo conformational changes to transport them across the membrane.

Vesicular Transport

Large molecules or particles are transported via vesicles in processes such as endocytosis and exocytosis.

Endocytosis: Uptake of materials into the cell by engulfing them in vesicles.
Exocytosis: Release of materials from the cell by fusion of vesicles with the plasma membrane.

Epithelial Transport

Transport of substances across epithelial layers involves movement through or between cells, often requiring both active and passive mechanisms.

Transcellular Transport: Through the cell, involving membrane transporters.
Paracellular Transport: Between adjacent cells, through tight junctions.

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell at rest.

Generated by: Differences in ion concentrations and selective permeability of the membrane to ions.
Typical Value: For most animal cells, the resting membrane potential is between -60 mV and -90 mV.
Key Ions: Na+, K+, Cl-

Formula (Nernst Equation):

Where = equilibrium potential for the ion, = gas constant, = temperature, = charge of the ion, = Faraday's constant.

Summary Table: Body Fluid Compartments

Compartment	Location	Proportion of Total Body Water	Main Ions
Intracellular Fluid (ICF)	Inside cells	~2/3	K+, Mg2+, PO43-
Interstitial Fluid	Between cells	~3/4 of ECF	Na+, Cl-
Plasma	Blood vessels	~1/4 of ECF	Na+, Cl-, proteins

Additional info: Some explanations and formulas have been expanded for academic completeness and clarity.