Circulatory system crossword

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Across
  1. 2. Does not have collagen fibres, smooth muscle and elastic fibres
  2. 7. Waste gas produced in muscles
  3. 9. Oxygen rich blood
  4. 13. Blood vessels that carry blood away from the heart
  5. 14. Bottom chambers of the heart
  6. 16. Organ where gas exchanged with environment
  7. 19. Heart does not require stimulus
  8. 20. Capillaries are blood vessels with lowest blood ?
  9. 21. Another word that means heart
Down
  1. 1. Blood vessels that swap oxygen and carbon dioxide
  2. 3. Top chambers of the heart
  3. 4. Carry oxygenated blood for the heart to use
  4. 5. Carbon dioxide rich blood
  5. 6. Controls contraction of heart
  6. 8. Heart rate is measured in beats per
  7. 10. Prevent backflow of blood through veins
  8. 11. Blood vessel that carries blood towards the heart
  9. 12. Veins have the largest ? diameter
  10. 15. Blood flow around the body
  11. 17. Number of chambers in the heart
  12. 18. Your blood vessels could reach around this 2.5 times