3. Capillaries - Circulatory systems that bathe the cells in blood.

4. Nutrients - What is dropped off at the cells in the body tissue.

5. Hemoglobin - The protein in blood that binds to oxygen.

6. Trachea - A cartilaginous tube that connects the larynx to the bronchi in the lungs.

7. Smoking - A habit that can severely damage your lungs and severely shorten your life.

8. Respiration - Gas exchange in which oxygen is brought in and carbon dioxide is released.

9. Platelets - Blood cells that help with clotting.

10. Valves - Veins have valves to prevent the backflow of blood.

11. Circulatory - Circulatory system provides oxygen, nutrients, and eliminates wastes for larger organisms.

12. Plaques - Build-ups in the walls of the arteries that cause atherosclerosis.

13. Murmur - A defect in a heart valve.

14. Gravity - The biggest problem for the circulatory system of a giraffe.

15. Systolic - The upper pressure in a blood pressure reading.

16. Warm - In cold environments, nostrils also help warm the air before it enters the lungs.

17. Increased - Capillaries have increased surface area compared to arteries.

18. Arteries - Vessels that move blood away from the heart.

19. Stroke - Damage to brain tissue due to blocked or ruptured arteries.

20. Two - Gas exchange occurs in two phases.

21. Larynx - The vocal cords are contained in the larynx.

22. Gills - Gas exchange system used by some aquatic organisms such as fish.

23. Dead air - The air left in the lungs after exhalation is called dead air.

24. Diastolic - Portion of the heart cycle where the heart is relaxed.

25. White - White blood cells help fight infection.

26. Closed - Circulatory system where blood travels in vessels.

27. Iron - The element in hemoglobin that oxygen binds to.

28. Copper - The blood of arthropods has copper instead of iron.

29. Double - Four-chambered hearts use a double circulatory system in which 2 circuits flow.

30. Leukemia - Cancer of the blood.

31. Alveoli - The most important organs of the respiratory system that remove oxygen from air.

Down:

1. Veins - Vessels that move blood to the heart.

2. Left atrium - The chamber in which blood is received from the lungs.

3. Countercurrent - In the gills, a countercurrent exchange system passes blood in one direction next to water flowing in the opposite direction.

4. Coronary - The arteries that feed the heart muscle itself.

5. Gravity - Blood is helped to move back up the legs by gravity.

6. Sanode - The natural built-in pacemaker for the heart.

7. Blood pressure - The force blood exerts on the vessel walls.

8. Cardiac output - Amount of blood pumped by the heart per minute.

9. Air - Which of the following contains more oxygen: air or water?

10. Left ventricle - The ventricle in the human heart that pumps blood to the entire body.

11. Hypertension - High blood pressure is also known as hypertension.

12. Inhalation - The diaphragm moves down during this process, and air comes into the lungs.

13. Ventricle - A heart chamber that pumps blood out.

14. Oxygen - The brain automatically responds to the level of oxygen in the blood.

15. Heart attack - Damage to the heart muscle.

16. Capillaries - Small vessels in which only one red blood cell can move at a time.

17. Red - Red blood cells carry carbon dioxide and wastes away from the cells.

18. Exhalation - The diaphragm moves up during this process, and air leaves the lungs.

19. Open - Very small simple organisms with one or two tissue layers use open circulation instead of circulation.

20. Right atrium - A heart chamber that receives blood from the rest of the body.

21. Lowest - Blood pressure is highest or lowest in the veins?

22. Mucus - Hair and mucus in the nostrils provide protection for the respiratory system.