M04.03.01 Total Body Water

Total Body Water (TBW) refers to the sum of all fluids within the human body. It is divided into two main compartments: the Intracellular Fluid (ICF) and the Extracellular Fluid (ECF), with each having distinct components and functions.

Major Fluid Compartments

1. Intracellular Fluid (ICF)
   • Accounts for approximately 2/3 of the total body water.
   • This fluid is located inside the cells, serving vital cellular functions.
2. Extracellular Fluid (ECF)
   • Makes up around 1/3 of the total body water.
   • This compartment is further divided into:
     • Interstitial Fluid (ISF): Constitutes about 3/4 of the ECF.
     • Plasma Volume (PV): Comprises 1/4 of the ECF, found within blood vessels.
3. Vascular Compartment
   • Contains the blood volume, which includes both:
     • Plasma: The liquid component of the blood.
     • Cellular elements of blood: Mainly red blood cells.

Membrane Barriers and Functions

Membranes act as selective barriers between different compartments, regulating the movement of substances across them.

Important Membranes:

1. Cell Membrane:
   • Acts as a barrier for sodium ions (Na⁺), controlling their movement in and out of the cells.
2. Capillary Membrane:
   • Functions as a barrier for plasma proteins, restricting their passage from the blood into the interstitial space.

Table: Distribution of Total Body Water

Points to Remember

• ICF is the major fluid compartment within cells, while ECF is outside the cells.
• ISF makes up the majority of the ECF, with PV being a smaller portion.
• The cell membrane primarily restricts sodium movement, while the capillary membrane acts as a barrier for plasma proteins.

Bibliography Reference Links

• Guyton and Hall Textbook of Medical Physiology
• Boron and Boulpaep Medical Physiology