The body’s water is organized into two main compartments separated by the cell membrane: Intracellular Fluid (ICF), found within the cells, and Extracellular Fluid (ECF), which bathes the outside of the cells. The ECF includes both the fluid in the spaces between cells (interstitial fluid) and the plasma portion of the blood. The chemical makeup of the ICF and ECF are dramatically distinct, a separation necessary for virtually all biological processes.
Cation Concentration Differences
The primary difference between ICF and ECF is the concentration of major positive ions (cations). Sodium (Na+) is the primary cation in the extracellular fluid (ECF), typically 142 millimoles per liter (mM). Inside the cell, sodium concentration is kept low, often less than 15 mM.
Potassium (K+) is the most abundant cation within the intracellular fluid (ICF), maintained near 149 mM. In the ECF, potassium is kept at a very low concentration, usually around 4 to 5 mM.
This inverted distribution is fundamental to life processes, especially excitability. The concentration difference creates an osmotic driving force that regulates water movement, preventing cells from swelling or shrinking. These opposing gradients establish the resting membrane potential, the necessary electrical charge difference across the cell membrane. This potential is crucial for the function of excitable tissues like nerve and muscle cells.
Anion and Macromolecule Differences
Anions (negative ions) also differ significantly between the compartments. Chloride (Cl-) is the major anion in the extracellular fluid (ECF), balancing the high concentration of sodium ions. Its concentration in the ECF commonly ranges between 103 to 110 mM.
The concentration of chloride inside the cell is considerably lower, generally kept below 10 mM. This contrasts sharply with the high concentration of large, negatively charged molecules found within the ICF. The ICF is rich in proteins and organic phosphate groups (like ATP intermediates). These large, fixed anions contribute significantly to the overall negative charge inside the cell.
The Role of the Cell Membrane in Maintaining Balance
The chemical differences between ICF and ECF depend entirely on the plasma membrane. This highly selective semipermeable structure tightly controls substance movement. It allows water to pass relatively freely but strictly limits the passage of most ions and large molecules.
Maintaining the steep sodium and potassium gradients requires specific transport proteins embedded in the membrane. The primary mechanism is the sodium-potassium pump (Na+/K+-ATPase). This pump actively transports three sodium ions out of the cell for every two potassium ions moved in, utilizing ATP for energy.
This active transport works against the natural tendency for ions to diffuse down their concentration gradients, which would otherwise quickly equalize the concentrations. The membrane’s selective permeability also explains the concentration differences for large macromolecules. Negatively charged proteins and phosphate compounds are too large to pass through channels, ensuring they remain confined within the ICF.