QS Study

All the fluids outside the cells are collectively called extracellular fluid. Extracellular fluid (ECF) usually denotes all body fluid outside the cells. All body cells survive in an environment of fluid cooperatively designated extracellular fluid. This includes the blood plasma, interstitial fluid, and lymph. Na+ (and its associated anions) is the predominant ions in the extracellular fluid (ECF). Because Na+ and its associated anions are non-penetrating solutes, they produce an osmotic pressure that holds water in the ECF. Because the osmolarity of the ECF is kept constant, the amount of Na+ in the body determines the volume of the ECF.

Total concentration of Sodium (Na+) in ECF = 142 mEq/ L this conc. is the 90% of total Na+ cone. of body fluid.

Fig: Extracellular Fluid (ECF) Sodium Concentration

Na+ cone is regulated by –

(1) Osmoreceptor — ADH mechanism

  • ↑ ECF osmolarity
  • stimulate osmoreceptor
  • Supraoptic nuclei of Hypothalamus.
  • Stimulate the post-pituitary gland.
  • ↑ ADH secretion
  • Increased ADH increased the permeability of DT and CD.
  • ↑ H2O reabsorption
  • ↑ ECF Volume.
  • ↑ Loss of Na” and come back to normal.

(2) Thirst mechanism

  • Dehydration
  • Hypovolemia and Na+ and other osmolar elements rise
  • ↓ Blood volume
  • ↓ Blood to kidney
  • Release of rennin
  • Angiotensinogen → Angiotensin-1
  • Angiotensin-11 (Plasma protein and ACE in lung )
  • (+) thirst centre
  • ↓ Intake
  • Na+ goes to normal.