Step 1: Understanding the Concept:
Renin is the first enzyme in the renin-angiotensin-aldosterone system, and finding its source means finding the cell type built to detect low blood pressure or low sodium.
Step 2: Key Formula or Approach:
Trace the pathway backward. Renin acts on angiotensinogen (made in the liver) to form angiotensin I, so the source of renin must be a separate structure from the liver, and one placed to sense pressure changes in the kidney's blood supply.
Step 3: Detailed Explanation:
The kidney's afferent arteriole carries blood into each glomerulus, and its wall contains modified smooth muscle cells called juxtaglomerular cells. These cells act like pressure sensors: when blood pressure or sodium delivery drops, they release renin directly into circulation.
Hepatocytes only supply angiotensinogen, the raw material renin needs, so they are ruled out.
Beta-cells of the pancreas handle blood sugar through insulin, and microglial cells handle immune defense in the nervous system, so neither has any link to blood pressure regulation.
Step 4: Final Answer:
Renin is released by the juxtaglomerular cells of the kidney, since they are the structures built to sense pressure changes and trigger the pathway.