In tubular secretion, are substances entering or leaving the bloodstream?

Explanation

**Step-by-Step Solution:**

1. **Understanding Tubular Secretion**: Tubular secretion is a process that occurs in the nephrons of the kidneys, specifically in the renal tubules. It involves the movement of substances from the blood into the tubular fluid.

2. **Identifying the Direction of Movement**: In tubular secretion, substances are leaving the bloodstream. This means that they are being transported from the blood into the renal tubules.

3. **Types of Substances Involved**: The substances that are secreted can be categorized into two main types:
- **Substances Too Large to be Filtered**: These are molecules that cannot pass through the glomerular filtration barrier due to their size. Examples include certain antibiotics and toxins.
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