In polycythemia the number of red cells increases greatly.This causes adverse effects due to

Options

A. Decreased blood volume
B. Increased circulation time
C. Increased viscosity of blood
D. Increased availability of oxygen

Answer

### Step-by-Step Solution:

1. **Understanding Polycythemia**: Polycythemia is a condition characterized by an increased number of red blood cells (RBCs) in the bloodstream. This can lead to various physiological changes.

2. **Analyzing the Effects of Increased RBCs**: When the number of red blood cells increases, it can lead to several adverse effects on the body. We need to identify which of these effects is primarily responsible for the complications associated with polycythemia.

3. **Evaluating the Options**:
- **Option 1: Decreased Blood Volume**: This option is incorrect. In polycythemia, the blood volume does not decrease; rather, it may remain stable or increase due to the increased number of cells.
- **Option 2: Increased Circulation Time**: This option is also incorrect. While the circulation time may be affected, it is not the primary adverse effect associated with the increase in red blood cells.
- **Option 3: Increased Viscosity of Blood**: This option is correct. An increase in red blood cells leads to increased blood viscosity (thickness), which can impair blood flow and increase the risk of clotting and other cardiovascular issues.
- **Option 4: Increased Availability of Oxygen**: This option is incorrect. While more red blood cells can carry more oxygen, the adverse effects of increased viscosity outweigh this benefit.

4. **Conclusion**: The primary adverse effect of polycythemia due to the increased number of red blood cells is the increased viscosity of blood, which can lead to complications such as hypertension and thrombosis.

### Final Answer:
The correct answer is **increased viscosity of blood**.