At the glomerulus the nephrons and the afferent and efferent arteriole are organized so that the ascending portion of the Loop of Henle is located between the two arterioles. The cells located in the wall of the Loop of Henle is specialized and is called the macula densa. In the same area, around the afferent arterioles, we find juxtaglomerular cells which are specialized smooth muscle cells. In addition, between the macula densa and the juxtaglomerular cells there are a third group of cells called mesangial cells. Altogether, these specialized cells form the juxtaglomerular apparatus.
The juxtaglomerular cells, which are the smooth muscle cells surrounding the afferent arterioles, are mechanoreceptors which respond to changes in pressure (due to stretching) and detect changes in blood pressure in the arterioles. These cells contain the enzyme renin, which is secreted when blood pressure drops below a certain threshold.
The macula densa cells are chemoreceptors that register the sodium level in the filtrate arriving at the upper portion of the ascending Loop of Henle. Low levels of sodium is an indicator of low blood pressure