question_answer 51)

  Comment upon the hormonal regulation of selective reabsorption.

Answer:

                The functioning of the kidneys is efficiently monitored and regulated by Antidiuretic Hormone (ADH), Juxtaglomerular Apparatus (JGA) and Atrial Natriuretic Factor (ANF). (a) Antidiuretic Hormone (ADH) or vasopressin from the neurohypophysis, facilitates water reabsorption from latter parts of tubule, i.e., distal convoluted tubule and collecting duct by increasing the permeability to water and salt and by accelerating water and ion transfer in a direction determined by the osmotic gradient. (b) Juxta Glomerular Apparatus (JGA) operates a multihormonal. Renin Angiotensin Aldosterone System (RAAS). JG cells secrete an enzyme, renin, which, changes plasma protein called angiotensinogen to a peptide called angiotensin I and further to angiotensin II, which works as a hormone. Angiotensin II, being a powerful vasoconstrictor increases the glomerular blood pressure and thereby GFR. It also stimulate's soduim absorption by proximal tubules. (c) Angiotensin II also activates the adrenal cortex to release aldosterone. Aldosterone induces the distal convoluted tubule to absorb more Na4" and water. (d) Atrial, Natriuretic Factor (ANF) is produced by the atria of heart. It regulates blood flow by causing vasodilation and increasing sodium excretion. It is also involved in checking , of renin-angiotensin mechanism.