Discuss in brief about the regulation of ECF volume.

The extracellular volume is controlled by the total body content of sodium because Na⁺ is by far the most abundant active solutes in ECF. Control of body’s Na⁺ is excreted by tight control over renal excretion, that is achieved by activation of receptors which responds to extracellular volume rather than the change in sodium concentration. Thus the defenses of ECF volume are

·        Extrarenal volume receptors mechanism

o   Vasopressin (ADH) mechanism

o   Atrial natriuretic peptide mechanism

·        Intrarenal mechanism

o   Rennin-angiotensin-aldosterone mechanism

Volume receptor mechanism:

            A slight reduction in effective circulatory volume inhibits volume receptors in the atria, aortic arch sinus body and major thoracic veins. This results in..

·        Increased sympathetic nerve activity and a rise in catecholamines.

·        Increased vasopressin secretion (ADH)

Both this response cause antidiuresis and thus increase circulatory volume. Conversely an increase in ECF volume causes opposite effect.

·        Decrease in ADH secretion – diuresis

·        Increase in ANP secretion – natriuresis, diuresis.

Intrarenal mechanism:

Receptors in the walls of the glomerular afferent arterioles respond via the juxtaglomerular apparatus to changes in renal perfusion and control the activity of the renin-angiotesin-aldosterone system. A decrease in effective ECF volume leads to decrease in renal perfusion and stimulates renin release that in turns increase circulatory angiotensin-II. Angiotensin-II causes

·        Vasoconstriction

·        Aldosterone secretion: Na⁺ and H₂O retention.

·        Thirst mechanism: drinking

·        Vasopressin secretion: antidiuresis

The final common pathway for all the regulatory systems discussed is to increase or decrease the renal excretion of Na⁺ in the event of an increase or decrease in effective ECF volume. An increase in the reabsorbtion of Na⁺ with concomitant retention of H₂O expands the vascular and interstitial compartments while increase Na⁺ excretion has the opposite effect.