A fall in GFR can activate the JG cells to release `"________"` which can stimulate the glomerular blood flow and threby the GFR back to normal

Options

A. Renin
B. Angiotensin-II
C. Rennin
D. Erthropoietin

Answer

To answer the question, "A fall in GFR can activate the JG cells to release '________' which can stimulate the glomerular blood flow and thereby the GFR back to normal," we can follow these steps:

### Step-by-Step Solution:

1. **Understanding GFR**: The Glomerular Filtration Rate (GFR) is the rate at which blood is filtered through the glomeruli of the kidneys. A fall in GFR indicates that the kidneys are not filtering blood effectively.

2. **Role of JG Cells**: The Juxtaglomerular (JG) cells are specialized cells located in the juxtaglomerular apparatus of the nephron. They are sensitive to changes in blood volume and pressure.

3. **Trigger for JG Cell Activation**: When there is a decrease in blood volume, it leads to a drop in blood pressure (BP), which in turn causes a decrease in GFR. The JG cells detect this fall in blood volume and pressure.

4. **Release of Renin**: In response to the detected decrease in blood volume and pressure, the JG cells release an enzyme called **renin**. Renin is crucial in the regulation of blood pressure and fluid balance.

5. **Function of Renin**: Renin acts on a protein called angiotensinogen (produced by the liver) to convert it into angiotensin I. Angiotensin I is then converted to angiotensin II by the action of angiotensin-converting enzyme (ACE) primarily in the lungs.

6. **Effects of Angiotensin II**: Angiotensin II has several effects:
- It constricts blood vessels, which increases blood pressure.
- It stimulates the thirst center in the brain, prompting increased water intake.
- It promotes reabsorption of sodium and water in the kidneys, which helps restore blood volume and pressure.

7. **Restoration of GFR**: As blood volume and pressure are restored, the glomerular blood flow increases, leading to an increase in GFR back to normal levels.

### Final Answer:
The blank in the question should be filled with **"renin."**

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