A fall in glomerular filtration rate (GFR) activates

Options

A. juxtaglomerular cells to release renin
B. atria of the heart to release ANF
C. adrenal medulla to release adrenanline
D. posterior pituitary to release vasopressin

Answer

**Step-by-Step Solution:**

1. **Understanding Glomerular Filtration Rate (GFR):**
- GFR is a measure of how well the kidneys are filtering blood. A fall in GFR indicates that the kidneys are not filtering blood effectively.

2. **Activation of Juxtaglomerular Cells:**
- When there is a decrease in GFR, the juxtaglomerular cells in the kidneys are activated. These cells are located in the walls of the afferent arterioles of the glomeruli.

3. **Release of Renin:**
- The activation of juxtaglomerular cells leads to the release of an enzyme called renin. Renin plays a crucial role in the body’s blood pressure regulation.

4. **Renin-Angiotensin-Aldosterone System (RAAS):**
- 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 enzyme ACE (Angiotensin-Converting Enzyme) primarily in the lungs.

5. **Effects of Angiotensin II:**
- Angiotensin II has several effects, including vasoconstriction (narrowing of blood vessels), which increases blood pressure, and stimulating the adrenal cortex to release aldosterone, which promotes sodium and water reabsorption in the kidneys.

6. **Conclusion:**
- Therefore, a fall in GFR activates the juxtaglomerular cells to release renin, which is part of the RAAS, ultimately leading to increased blood pressure.

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

---