Question

At what point does the kidney detect blood osmolarity? Name the cell and where they are.

Answer 1

Kidneys detect osmolarity at the macula densa portion of juxtaglomerular apparatus. Juxtaglomerular apparatus lies just outside the bowman's capsule and glomerulus. At the entrance of bowman's capsule where the afferent and efferent arterioles enter the leave the capsule, a portion of distal convoluted tubule comes in direct contact with the arterioles. The walls of distal convoluted tubule at that point form a part of juxtaglomerular apparatus known as macula densa. Macula densa consists of juxtaglomerular cell, which is a modified smooth muscle cell lining the afferent arteriole that can contract or relax in response to ATP or adenosine released by the macula densa. This cell is responsible for detecting osmolarity of blood and responding accordingly. If the osmolarity is too high, the juxtaglomerular cells will contract, decreasing the glomerular filtration rate (GFR) so less plasma is filtered, leading to less urine formation and greater retention of fluid, thereby decreasing the blood osmolarity. If the osmolarity of the filtrate is too low, the juxtaglomerular cells will relax, increasing the GFR and enhancing the loss of water to the urine, causing blood osmolarity to rise. Macula densa responds to blood osmolarity through changes in the sodium ion inflow through the nephron, thereby activating the renin -angiotensin system to regulate the osmolarity.