Final answer:
Macula densa cells are responsible for sensing the amount of Na+ in the tubular fluid and they are part of the juxtaglomerular apparatus (JGA) in the kidney.
Step-by-step explanation:
The cells that sense the amount of Na+ in the tubular fluid flowing past them are D) Macula densa cells. Found in the juxtaglomerular apparatus (JGA) of the kidney, these cells are a part of the distal convoluted tubule (DCT) and play a crucial role in monitoring the fluid's sodium ion concentration. They respond to changes in Na+ concentration by releasing paracrine signals, which can alter blood flow and filtration rates in the glomerulus by influencing the contraction and relaxation of the juxtaglomerular cells.
In addition, macula densa cells are involved in the tubuloglomerular feedback mechanism, which adjusts glomerular filtration rate (GFR) to maintain a steady filtration process. This regulation of renal function ensures proper balance of electrolytes and blood pressure within the body.