1. What gas do humans exhale when breathing?

2. What is the hardest substance produced by the human body?

3. Which bone in the human body is unique because it is not connected to any other bone?

4. Which protein in red blood cells uses iron to transport oxygen throughout the body?

5. Besides fingerprints, which of the following is unique to every individual and can be used for biometric identification?

6. If uncoiled, the DNA from all the cells in a single human body would reach approximately how far?

7. Which branch of the nervous system is responsible for the 'fight or flight' response?

8. What is the primary function of platelets in the human blood?

9. Which part of the central nervous system processes a reflex action like the 'knee-jerk' or pulling away from heat?

10. Where is the vestibular system, which controls balance and spatial orientation, located?

11. What is the snail-shaped structure in the inner ear responsible for hearing?

12. Which organ in the lymphatic system acts as a major filter for the blood?

13. What percentage of human blood is typically composed of plasma?

14. Which antibacterial enzyme is found in human tears and helps protect the eyes from infection?

15. Which of the five senses is the only one that does NOT pass through the thalamus before reaching the cerebral cortex?

16. What are the tiny projections in the small intestine that increase surface area for nutrient absorption?

17. What is the name of the fatty insulation that speeds up nerve impulses?

18. What is the term for the body's ability to sense its own position and movement without using sight?

19. What two gases are exchanged across the blood-air barrier in the lungs?

20. Which part of the brain is primarily responsible for fine-tuning motor movements and maintaining balance?