What do you understand by polycythaemia?

Explanation

**Step-by-Step Solution:**

1. **Definition of Polycythemia**: Polycythemia, also known as erythrocytosis, refers to a condition characterized by an increased concentration of red blood cells (RBCs) in the blood.

2. **Causes of Polycythemia**: This increase in red blood cells can occur due to various factors, including living at high altitudes, chronic hypoxia, or certain diseases that stimulate red blood cell production.

3. **Effects on Blood Viscosity**: The elevated levels of red blood cells lead to thickening of the blood, which can impede its flow through blood vessels.

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