JGA (Juxta-glomerular Apparatus): JGA works via Renin-Angiotensin mechanism which works in the following manner:
A. Fall in glomerular blood flow/glomerular blood pressure/GFR $\to$ Activation of the JG cells $\to$ Release of Renin in blood.
B. Angiotensinogen $\stackrel{\hspace{1em}\mathit{R}\mathit{e}\mathit{n}\mathit{i}\mathit{n}\hspace{1em}}{\to }$ Angiotensin I $\stackrel{}{\to }$Angiotensin II.
C. Angiotensin II performs 2 functions:
(i) Angiotensin II is a powerful vasoconstrictor $\to$Vasoconstriction $\to$ Glomerular blood pressure increases $\to$GFR increases.
(ii) Angiotensin II activates the adrenal cortex $\to$ Release of Aldosterone $\to$ Reabsorption of ${\mathrm{N}\mathrm{a}}^{+}$ and water from the distal parts of the tubule $\to$ Increase in blood pressure $\to$ GFR increases.