Across 1. carry oxygen to all of the body's cells. They resemble disks with pinched in centers. They develop in the marrow or the center portion of large bones. They have no nuclei and live for about 120 days. (3 Words) 3. a long tube in the chest cavity (windpipe). 6. the clear, yellowish fluid portion of blood. More than 50% of blood is this. 8. a muscular organ that is about as large as your fist and is located at the center of your chest. This hollow organ pumps blood throughout the body. 11. an individual air sac. each one has a thin wall, only one cell thick and is surrounded by very thin capillaries. 16. the exchange of gases between the blood and the body's cells. (2 Words) 18. oxygen rich blood or oxygenated blood is carried away from the heart in these large blood vessels. They are strong, thick walled vessels that are elastic and durable. 19. the trachea branches into 2 large tubes which lead to the lungs. 20. larger veins in the body that have flaps of tissue which prevent blood from flowing backward.

Down 2. the exchange of gases between the atmosphere and the blood which occurs in the lungs. (2 Words) 4. blocked arteries, when blood flow is reduced or blocked, the heart must work even harder to pump blood and vessels can burst. 5. cell fragments that are important in forming blood clots. When a blood vessel is cut they collect and stick to the vessel at the site of the wound. 7. a group of cells located in the right atrium called sinoatrial (SA) node, send out signals that tell the heart muscle to contract. 9. the body's disease fighters that are produced in bone marrow, some of these recognize disease causing organisms and alert the body that it has been invaded, others produce chemicals to fight the invaders. (3 Words) 10. the largest organs in the respiratory system and gas exchange takes place here. 12. microscopic blood vessels where the exchange of important substances and wastes occurs. 13. bean shaped organs that filter out wastes, water and salts from the blood. they are divided into 2 distinct regions. 14. carry oxygen poor blood or deoxygenated blood back to the heart. The walls of these are much thinner than the walls of arteries. 15. the mechanical movement of air into and out of your lungs. 17. water and substances dissolved in the water, a nitrogenous waste product.