A fall in glomerular filtration rate (GFR) activates

Options

A. Adrenal medulla to release adrenaline
B. Posterior pituitary to release vasopressin
C. Juxta glomerular cells to release renin
D. Adrenal cortex to release aldosterone

Answer

To answer the question, "A fall in glomerular filtration rate (GFR) activates," we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Glomerular Filtration Rate (GFR)**:
- GFR is the rate at which blood is filtered in the kidneys. It is an important indicator of kidney function. A decrease in GFR indicates that the kidneys are not filtering blood effectively.

2. **Identifying the Cause of Decreased GFR**:
- A fall in GFR often occurs due to a decrease in blood volume or blood pressure. This can happen due to various reasons such as dehydration, blood loss, or heart failure.

3. **Detection by Juxtaglomerular Cells**:
- The kidneys have specialized cells known as juxtaglomerular (JG) cells. These cells are sensitive to changes in blood pressure and blood volume. When GFR decreases, these cells detect the drop in blood pressure.

4. **Release of Renin**:
- In response to the detected decrease in blood pressure, the juxtaglomerular cells release an enzyme called renin. Renin plays a crucial role in the regulation of blood pressure.

5. **Renin-Angiotensin-Aldosterone System (RAAS)**:
- Renin converts angiotensinogen (produced by the liver) into angiotensin I. Angiotensin I is then converted into angiotensin II, which is a potent vasoconstrictor and stimulates the adrenal cortex to release aldosterone.

6. **Final Conclusion**:
- Therefore, the correct answer to the question is that a fall in glomerular filtration rate activates the juxtaglomerular cells to release renin.

### Final Answer:
The correct option is: **Third - juxtaglomerular cell to release the renin.**

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