ANF is secreted in response to decreased blood pressure/increased blood pressure.

Explanation

**Step-by-Step Solution:**

1. **Understanding ANF**: ANF stands for Atrial Natriuretic Factor, which is a hormone produced by the heart's atrial walls.

2. **Stimulus for Secretion**: ANF is secreted in response to increased blood pressure. When blood pressure rises, the heart detects this change.

3. **Function of ANF**: The primary function of ANF is to lower blood pressure. It does this by promoting the excretion of sodium (natriuresis) and water through the kidneys.

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