Need for Energy

Click on a number in the grid to see the clue or clues for that number.
Complete the crossword, then click on "Check" to check your answer. If you are stuck, you can click on "Hint" to get a free letter.

1        2      3            
                    
 4                     
                    
                    
   5             6        
                  7    
   8        9       10        
                    
      11                
  12                    
    13                  
                    
  14      15                
                    
   16     17    18          19      
                    
20          21              
                    
                    

Across:

1. Muscle attached to the ribs. (11,6)
4. Vessel carrying blood away from the heart. (6)
5. Tiny tube in the lung terminating in an air sac. (10)
8. Lower chamber of the heart. (9)
10. Main artery carrying blood from the heart to the body. (5)
11. Dome-shaped muscular sheet at the bottom of the chest cavity. (9)
13. Tiny air sac in the lungs. (8)
14. Main vein returning to the heart from the body. (4,4)
17. Voicebox found at the top of the trachea. (6)
19. Vessel that carries blood back towards the heart. (4)
20. Tube connecting the mouth and nose to the bronchi. (7)
21. Branch of the trachea going to each lung. (8)

Down:

2. Term used to describe blood that contains a high oxygen concentration. (10)
3. Organ of gas exchange. (4)
4. Upper chamber of the heart. (6)
6. Yellowish fluid in the blood. (6)
7. Red pigment in blood cells that carries oxygen. (11)
9. Smallest blood vessel. (9)
12. Structure present in the heart and veins which prevents the backflow of blood. (5)
15. Hair-like projections on the inner surface of the trachea. (5)
16. Sticky fluid that traps dirt and bacteria in the trachea. (5)
18. One of the bones surrounding the chest cavity. (3)