A fall in glomerular filtration rate (GFR) activates

Options

A. juxtaglomerular cells to release renin
B. adrenal cortex to release aldosterone
C. adrenal medulla to release adrenaline
D. posterior pituitary to release vasopressin.

Explanation

### Step-by-Step Solution:

1. **Understanding Glomerular Filtration Rate (GFR)**:
- The Glomerular Filtration Rate (GFR) is the rate at which blood is filtered in the kidneys, specifically through the glomeruli. It is a crucial indicator of kidney function.

2. **Identifying the Impact of a Fall in GFR**:
- When there is a decrease in GFR, it indicates that the kidneys are not filtering blood effectively. This can trigger compensatory mechanisms to restore normal function.

3. **Role of the Juxtaglomerular Apparatus**:
- The juxtaglomerular apparatus (JGA) is a specialized structure located near the glomerulus, formed by the distal convoluted tubule and the afferent arteriole. It plays a significant role in regulating blood pressure and GFR.

4. **Components of the Juxtaglomerular Apparatus**:
- The JGA consists of three types of cells:
- **Macula Densa Cells**: These cells detect sodium concentration in the filtrate.
- **Juxtaglomerular Cells**: These cells respond to low GFR by releasing renin.
- **Extraglomerular Mesangial Cells**: These cells support the structure of the JGA.

5. **Activation of Juxtaglomerular Cells**:
- When GFR falls, the juxtaglomerular cells are activated. They respond to the decrease in filtration by releasing the enzyme renin into the bloodstream.

6. **Renin-Angiotensin-Aldosterone System (RAAS)**:
- Renin initiates the renin-angiotensin-aldosterone system (RAAS), which leads to an increase in blood pressure and subsequently helps to restore GFR.
- This system causes vasoconstriction and promotes sodium and water retention, which helps to increase blood volume and pressure.

7. **Conclusion**:
- Therefore, a fall in GFR activates the juxtaglomerular cells to release renin, which is the correct answer to the question.