Question

In the juxtaglomerular apparatus, what is the significance of the macula dense cells being so close to the afferent arteriole?

1) The macula densa cells sense the diameter of the afferent arteriole and then communicate to the juxtaglomerular cells
2) The macula densa cells directly stimulate aldosterone release which will vasoconstrict the afferent arteriole and decrease GFR
3) The macula densa cells receive rich sympathetic innervation and as a result can communicate with the juxtaglomerular cells
4) The macula densa cells can sense sodium content in the distal tubule and then communicate to the juxtaglomerular cells

Answer 1

The macula densa cells are crucial for sensing sodium levels in the distal tubule, which they then communicate to the juxtaglomerular cells to regulate GFR and contribute to the control of renin release, blood pressure, and fluid balance.

Step-by-step explanation:

The significance of macula densa cells being so close to the afferent arteriole within the juxtaglomerular apparatus (JGA) is their role in sensing the sodium content in the distal tubule and communicating these changes to the juxtaglomerular cells. When the macula densa cells detect a high sodium concentration in the filtrate, they release adenosine triphosphate (ATP) and adenosine, which act as paracrine signals to the juxtaglomerular cells. These cells, in response, can contract, reducing the glomerular filtration rate (GFR) to decrease plasma filtration, urine formation, and ultimately lower the osmolarity of blood to physiological levels. Conversely, when the sodium concentration is low, the signals from macula densa cause the juxtaglomerular cells to relax, increasing GFR and water loss to urine, raising blood osmolarity. Additionally, macula densa cells help regulate the renin-angiotensin-aldosterone system by controlling the release of renin from juxtaglomerular cells, which influence blood pressure and fluid balance.