Question

What is a juxtaglomerular apparatus?

Answer 1

A specialized structure formed by distal convoluted tubule and the glomerular arteriole (part of kidney). It has juxtaglomerular cells, derived from smooth muscles, of the afferent arteriole and secrete renin when blood pressure in the arterial falls.

Complete answer:
To answer this question you must know all parts of the kidney.
The initial ,straight part of the distal tubules makes contact with the vascular pole of the renal corpuscle of its present nephron and forms a specialized structure ,called juxtaglomerular apparatus (JGA). It is a functional unit of the kidney or urinary system. It consists of juxtaglomerular cells which secrete renin hormone and macula densa which is sensitive to the concentration of NaCl.
Juxtaglomerular apparatus forms at the point of contact between a nephron’s distal tubule and the vascular pole of its glomerulus. At that point cells of the distal tubules become columnar as a thickened region called the macula densa. There are also lacis cells ,which are extraglomerular mesangial cells adjacent to the macula densa, the afferent arteriole, and the efferent arteriole.
Function of JGA is secretion of hormones like rennin ,prostaglandins and it also regulates blood flow and glomerular filtration rate (GFR). Renin plays an important role in the maintenance of the blood pressure and prostaglandin secrete cytokines like interleukin-2 and tumor necrosis factor.

Note: Juxtaglomerular apparatus is named for its proximity to the glomerulus. Its main function is to regulate blood pressure ,body fluid and electrolytes. juxtaglomerular are the major source of circulating renin but Excessive secretion of renin causes intractable hypertension and hypokalemia . Renin is secreted in response to decrease in blood pressure and decrease in NaCl concentration at the macula densa often due to a decrease in the glomerular filtration rate.