A fall in glomerular filtration rate (GFR) activates

Options

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

Answer

### Step-by-Step Solution:

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

**Hint**: Remember that GFR is a crucial indicator of kidney function and health.

2. **Activation of Juxtaglomerular Cells**:
- When GFR falls, the juxtaglomerular cells, which are specialized cells located in the walls of the afferent arterioles of the nephron, detect this change.

**Hint**: Think about where juxtaglomerular cells are located and their role in monitoring blood flow.

3. **Release of Renin**:
- In response to the detected decrease in GFR, the juxtaglomerular cells release the enzyme renin into the bloodstream. Renin is an important enzyme in the regulation of blood pressure and fluid balance.

**Hint**: Consider the relationship between renin and blood pressure regulation.

4. **Renin-Angiotensin-Aldosterone System (RAAS)**:
- Renin initiates a cascade of reactions that lead to the production of angiotensin II, which causes vasoconstriction and stimulates the adrenal cortex to release aldosterone.

**Hint**: Recall the role of angiotensin II in the body and how it affects blood vessels and hormone release.

5. **Effects of Aldosterone**:
- Aldosterone promotes sodium reabsorption in the kidneys, which leads to increased water retention. This helps to increase blood volume and, consequently, blood pressure, which can help restore GFR to normal levels.

**Hint**: Think about how sodium and water balance affects blood volume and pressure.

6. **Overall Outcome**:
- The overall effect of this process is to increase blood flow to the glomeruli, thereby increasing GFR and maintaining kidney function.

**Hint**: Consider how the body maintains homeostasis in response to changes in kidney function.

### Final Answer:
A fall in glomerular filtration rate (GFR) activates the juxtaglomerular cells to release renin.