Learning Objective: By the end of this topic, the learner will be able to describe the major body fluid compartments, their approximate proportions, and the role of membrane barriers in maintaining fluid distribution.

Total Body Water (TBW)

• Approximately 60% of body mass

Major Compartments

Intracellular Fluid (ICF)

• Makes up ~2/3 of total body water

Extracellular Fluid (ECF)

• Makes up ~1/3 of total body water

Subdivisions of the ECF

Interstitial Fluid (ISF)

• ~3/4 of ECF

Plasma Volume (PV)

• ~1/4 of ECF

Vascular Compartment

• Contains blood volume
• Blood volume = plasma + cellular elements (mainly RBCs)

Activity

Membrane Barriers and Their Importance

1. Cell Membrane

• Selectively permeable
• Relative barrier to Na+
• Freely permeable to water
• Maintains distinct ICF and ECF composition

2. Capillary Membrane

• Freely permeable to ions and small solutes
• Barrier to plasma proteins
• Maintains oncotic pressure
• Helps keep fluid in the vascular space

Activity