Macula densa cells monitor the ________ concentration of the filtrate.

• A. $H_2O$
• B. $Na^+$
• C. $NaCl$
• D. $Ca^2$$^+$

Answer: (C)

Verified step by step guidance

1. Understand the role of macula densa cells: These cells are part of the juxtaglomerular apparatus in the kidney and play a crucial role in regulating blood pressure and the filtration rate of the glomerulus.
2. Identify the primary function of macula densa cells: They detect changes in the concentration of certain ions in the filtrate, which is the fluid that passes through the nephron in the kidney.
3. Focus on the specific ion concentration monitored by macula densa cells: These cells are sensitive to the concentration of sodium chloride (NaCl) in the filtrate.
4. Consider the physiological response: When the macula densa cells detect low NaCl concentration, they signal for adjustments in the glomerular filtration rate to maintain homeostasis.
5. Conclude that the correct answer is NaCl, as macula densa cells specifically monitor the sodium chloride concentration in the filtrate to regulate kidney function.