Full Answer Section

1. Extracellular Fluid (ECF):

   • Location: Fluid found outside the cells. It constitutes about one-third (35-40%) of the total body water. The ECF is further divided into:
     • Interstitial Fluid (ISF): The fluid that bathes the cells, found in the spaces between cells and outside of blood vessels. It makes up about 75% of the ECF.
     • Intravascular Fluid (Plasma): The fluid component of blood, found within blood vessels. It makes up about 20-25% of the ECF.
     • Transcellular Fluid: A small, specialized component of ECF found in specific body cavities (e.g., cerebrospinal fluid, pleural fluid, peritoneal fluid, synovial fluid).
   • Composition: The primary cation in the ECF (both ISF and plasma) is Sodium (Na+), and the primary anion is Chloride (Cl−). Bicarbonate (HCO3−​) is also present in significant amounts. Potassium (K+), Magnesium (Mg2+), and Phosphate (PO43−​) concentrations are relatively low compared to ICF. Plasma also contains a significant amount of protein (albumin) which is largely absent in the ISF.
   • Movement:
     • Between ISF and Plasma: Water and small solutes move freely between the plasma and ISF through capillary walls, primarily driven by hydrostatic pressure (pushing fluid out of capillaries) and oncotic pressure (pulling fluid into capillaries due to proteins, especially albumin, in the plasma). This is known as Starling forces.
     • Between ECF and ICF: As mentioned, water moves between the ECF and ICF via osmosis across the cell membrane, striving for osmotic equilibrium. Solutes move via diffusion (passive) or active transport.

Overall Principle: The body maintains an osmotic equilibrium between the ICF and ECF. While their electrolyte compositions differ dramatically, their total solute concentration (osmolality) is normally very similar (around 280-295 mOsm/kg H2O). Changes in ECF osmolality will cause water to shift between the compartments, impacting cell volume.

Regulation of Water Balance within the Body

Water balance is tightly regulated by a complex interplay of hormonal and neural mechanisms to ensure that water intake matches water output, maintaining overall body fluid volume and osmolality.

1. Antidiuretic Hormone (ADH) / Vasopressin:
   • Mechanism: ADH is produced by the hypothalamus and released by the posterior pituitary gland. Its secretion is primarily stimulated by an increase in ECF osmolality (detected by osmoreceptors in the hypothalamus) or a decrease in blood volume/pressure (detected by baroreceptors in the cardiovascular system).
   • Role: ADH acts on the collecting ducts and distal tubules of the kidneys, increasing their permeability to water. This leads to increased water reabsorption back into the bloodstream, producing a smaller volume of more concentrated urine and conserving body water.
2. Thirst Mechanism:
   • Mechanism: The thirst center in the hypothalamus is stimulated by similar factors as ADH secretion: increased ECF osmolality, decreased blood volume/pressure, and even dry mouth.
   • Role: Thirst is the conscious desire to drink water, representing the primary means of regulating water intake.
3. Renin-Angiotensin-Aldosterone System (RAAS):
   • Mechanism: While primarily regulating blood pressure and sodium balance, RAAS also influences water balance. A decrease in renal perfusion pressure (low blood volume/BP) triggers renin release from the kidneys, initiating the cascade that leads to angiotensin II formation. Angiotensin II then stimulates aldosterone release from the adrenal cortex.