Final answer:
The juxtaglomerular apparatus (JGA), specifically the macula densa cells within the distal convoluted tubule (DCT), monitors blood volume and osmolarity by sensing the concentration of sodium and fluid flow.
Step-by-step explanation:
The structure that monitors blood volume and osmolarity of the blood is the juxtaglomerular apparatus (JGA). The JGA, located outside Bowman's capsule and next to the glomerulus, includes a group of specialized cells known as the macula densa, which are part of the distal convoluted tubule (DCT). These cells monitor the sodium (Na) concentration and fluid flow through the DCT and respond by releasing paracrine signals like adenosine triphosphate (ATP) and adenosine, which in turn regulate blood flow and glomerular filtration rate according to the osmolarity and fluid status of the blood.