Average Blood Volume in Adults
The average blood volume in an adult human is approximately 5 liters. This is a crucial physiological parameter and varies based on several factors.

Factors Influencing Blood Volume
- **Body Size**: Larger individuals tend to have a greater blood volume compared to smaller individuals.
- **Gender**: Males usually have a higher blood volume than females, typically due to differences in body composition.
- **Health Conditions**: Certain medical conditions, such as dehydration or heart failure, can significantly alter blood volume.

Composition of Blood
Blood is composed of various components, including:
- **Plasma**: Makes up about 55% of blood volume, consisting primarily of water, electrolytes, and proteins.
- **Red Blood Cells**: Approximately 45% of blood volume, crucial for oxygen transport.
- **White Blood Cells and Platelets**: Make up less than 1% of blood volume but are essential for immune function and clotting.

Importance of Blood Volume
Maintaining an appropriate blood volume is vital for:
- **Oxygen Transport**: Ensures adequate oxygen supply to tissues.
- **Nutrient Delivery**: Facilitates the transport of nutrients and hormones throughout the body.
- **Thermoregulation**: Helps in regulating body temperature.

Conclusion
Understanding blood volume is essential for healthcare professionals. It aids in assessing various health conditions and determining appropriate medical interventions. For most adults, the average blood volume is around 5 liters, making option 'D' the correct answer.