Body Fluid Compartments and Osmosis Notes

Body Fluid Compartments and Osmosis

Body Fluid Compartments

• The average adult body is 60-70% water. This fluid is stored in two main compartments:
  • Intracellular Compartment: Fluid within the cell.
    • "Intra" means within.
    • Accounts for two-thirds of the body's water.
  • Extracellular Compartment: Fluid outside the cell.
    • "Extra" means beyond or outside.
    • Made up of:
      • Intravascular Fluid (Plasma): Inside blood vessels.
      • Interstitial Fluid: Surrounds the cells.
      • Transcellular Fluid.
    • Accounts for one-third of the body's water.

Closer Look at Fluid Compartments

• Intracellular Space:
  • Fluid found inside the cell.
  • Accounts for $2/3$ of body water.
• Extracellular Space:
  • Fluid outside the cell.
  • Accounts for $1/3$ of body water.
  • Includes:
    • Interstitial Fluid Compartment:
      • Fluid surrounds the outside of cells.
      • A medium for electrolytes and other substances to move to and from the cell to the plasma, with the help of capillaries.
    • Intravascular Fluid Compartment (Plasma):
      • Fluid inside blood vessels.
      • Contains electrolytes, blood cells, etc.
    • Transcellular Fluid Compartment:
      • Smallest compartment.
      • Fluid within certain body cavities (spinal fluid, fluid around the heart/lungs, joints).

These compartments are interconnected and maintain homeostasis by shifting water, electrolytes, and nutrients. This shifting occurs through processes like osmosis. IV fluids can be administered to the intravascular compartment to expand it or shift fluids via osmosis to correct imbalances.

Osmosis

• Process where water moves from a higher water concentration to a lower water concentration (or from a low solute concentration to a high solute concentration).
• Passive process (no energy required).
• Occurs through a semi-permeable membrane (permeable to water molecules only).

*Water moves from an area with more water molecules (less solutes) to an area with fewer water molecules (more solutes).

• Key Takeaway: Osmosis is influenced by fluid solute concentration. The solute concentration determines how water shifts between extracellular and intracellular spaces.

Solutes

• A solid that has been dissolved in a liquid (e.g., sodium and chloride).
• When dissolved, they become electrolytes but remain solutes.
• The amount of solute in an IV fluid affects osmosis in the extracellular and intracellular compartments.

Osmolarity

• The amount of solutes within a specific fluid volume.
• Total solute concentration per liter of solution.
• An IV fluid's osmolarity determines how osmosis works to shift fluid.
• Fluids can have high or low osmolarity.
  • High Osmolarity: Many solutes, less water.
  • Low Osmolarity: Few solutes, more water.

In healthcare, osmolarity is used to treat patients needing fluid replacement by administering fluids with different osmolarities to shift water in or out of compartments.