Juxta  glomerular apparatus:

This is a special region formed in DCT (Distal Convoluted tubule). It controls the regulation of glomerular filtration rate. When Na⁺ level reduces in blood then special cells at Juxta glomerular apparatus releases a protein called renin. This renin converts the angiotensinogen into angiotensin I and angiotensin II.

• Angiotensin I is a powerful vasoconstrictor (the process of contraction of blood vessels due to which blood pressure in them increases is called vasoconstriction).
• Angitensin II releases aldosterome, that increases the rate of re-absorption of Na⁺.
• Re-absorption of water and Na⁺ increases blood volume and blood pressure which further increases the rate of glomerular filtration. This RAAS (Renin – angiotensin – Aldosterone – system) maintains Homeostasis (water and ionic level of blood).