The juxtaglomerular apparatus is a renal structure consisting of the macula densa and juxtaglomerular cells . Juxtaglomerular cells (JG cells) are the site of renin secretion.

The JG cells are found in the afferent arterioles of the glomerulus act as an intra-renal pressure sensor. Lowered pressure leads to secretion of renin which acts to increase systemic blood pressure via the renin-angiotensin system.

The macula densa senses fluid flow rate in the distal tubule of the kidney and secretes a locally active vasopressor which acts on the adjacent afferent arteriole to decrease GFR.