19. EXCRETORY PRODUCTS & THEIR ELIMINATION
It is done by hormonal feedback mechanisms involving the hypothalamus, JGA and the heart.Changes in blood volume, body fluid volume and ionic concentration activate Osmoreceptors in the body.
When body fluid level decreases, the osmoreceptors stimulate hypothalamus to release antidiuretic hormone (ADH). It stimulates water reabsorption from DCT & collecting duct. Thus, ADH prevents diuresis and increases body fluid volume.
Increase in fluid volume switches off the osmoreceptors and suppresses ADH release to complete the feedback.
ADH constricts blood vessels resulting in an increase of BP. This increases the glomerular blood flow and GFR.
JGA (Juxta glomerular apparatus) is a sensitive region formed by cellular modification of DCT and the afferent arteriole at the location of their contact.
JGA regulates the GFR.
A fall in glomerular blood flow/glomerular blood pressure/GFR activates the JG cells to release renin.
Renin converts angiotensinogen in blood to angiotensin I and further to angiotensin II (a vasoconstrictor).
Angiotensin II performs the following functions:
- Increases glomerular blood pressure and thereby GFR.
- Activates adrenal cortex to release Aldosterone.
ANF check on the renin- angiotensin mechanism.
An increase in blood flow to the atria of the heart causes the release of Atrial Natriuretic Factor (ANF).
ANF causes vasodilation (dilation of blood vessels) and thereby decreases the blood pressure.
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