Question

Determine whether the afferent arteriole will vasoconstrict or vasodilate to return to normal

infreased filtrate osmolarity-

decreased filtrate osmolarity-

increased BP-

decreased BP-

Answer 1

In renal system, the osmolarity and blood pressure are maintained by the afferent arteriole constriction and vasodilation and it is regulated by a mechanism called tubuloglomerular mechanism that involves the Juxtaglomerular apparatus and a paracrine signaling mechanism involving adenosine, ATP and nitric oxide.

1) Increased osmolarity of GFR attracts more amount of water which increases the flow rate within the distal convulated tubule. This increased flow rate activates the macula densa cells present in the DCT which in turn release ATP and adenosine. ATP and adenosine act on the myogenic juxtaglomerular cells and stimulate these cells by acting as a paracrine factors. Thus, finally constrict the afferent arteriole and ultimately decreases the osmolarity.

2) When there is decreased filterate osmolarity, there will be less sodium and chloride ions in the filterate. This low concentration then activates the macula densa present in DCT and decreases the release of ATP and adenosine from these cells. Now, these paracrine factors will not stimulate the myogenic juxtaglomerular cells and thus facilitate the dilation of afferent arteriole.

3) When there is increased blood pressure, the Glomerular filteration rate will also increases. This increased GFR in turn activates the macula densa cells present in the DCT. This activation decrease the release of ATP and adenosine and increase the release of nitric oxide (powerful vasodilator). These paracrine factors act by vasodilating the afferent arteriole and thus increases more GFR and more excretion of fluids. This ultimately decreases the blood pressure.

4) The kidneys are regulated by the sympathetic neurons of the autonomic nervous system through splanchnic nerves and the celiac plexus. When there is decreased blood pressure, the sympathetic nervous system gets activated which directly causes vasoconstriction of afferent arterioles and it also facilitates the release of epinephrine from the adrenal medulla that further causes vasoconstriction of afferent arteriole. If the BP still increases, the sympathetic nervous system facilitate the release of renin from the kidney that helps in production of Angiotensin II (vasoconstrictor) and release of Aldosterone from the adrenal cortex. Angiotensin II directly acts on the afferent arteriole and causes vasoconstriction. Aldosterone increases the blood pressure by facilitating the retention or reabsorption of sodium and water. This ultimately control the blood pressure.