g In order to stimulate the renin-angtiotensin-aldosterone system (RAAS), cells in the kidney detect blood pressure which causes to be released from the juxtaglomerular cells.

Macula densa cells, lower and renin.

Explanation:

Macula densa cells in the kidney detect lowers blood pressure which causes renin to be released from the juxtaglomerular cells which is  an enzyme. The arterial cells observe the drop in blood pressure, and the decrease in Na concentration is transfer to them by the macula densa cells. The juxtaglomerular cells then release an enzyme called renin. Renin converts angiotensinogen which is a peptide, or amino acid derivative into angiotensin-1.

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