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Across 6. Vessels which carry blood away from the heart 8. Amount of blood pumped by each ventricle 9. Middle layer of the heart 12. Phase of a heartbeat, when pressure in the ventricles drops until atrioventricular valves open and blood flows into ventricles 14. Receptors which detect CO2 levels 15. Primary type of metabolism used by cardiac cells 17. Direct connection between the pulmonary artery and aorta in fetal circulation 19. Direct connection between right and left atriums in fetal circulation 22. Valve between the left atrium and left ventricle 23. Membrane surrounding the heart 26. Receptors which detect pressure 27. Heart circuit which transports blood to and from the lungs 28. Heart circuit which transports blood to and from the body 29. Connection of cytoplasm between two cells to help synchronize contraction 30. Stage of contraction when calcium begins to enter cell and potassium begins to leave cell 31. Lower chambers of the heart

Down 1. Phase of a heartbeat when contraction occurs 2. Innermost layer of the heart 3. Valve between the right ventricle and pulmonary trunk 4. Cells which fire action potentials to trigger rhythmic heart contraction 5. Mechanical connection between cells to prevent separation during contraction 7. Vessels which carry blood toward the heart 10. Clump of myocardial conducting cells known as the pacemaker of the heart 11. Stage of contraction when rapid Na+ influx causes membrane potential to increase to +30mV 13. Valve between the left ventricle and aorta 15. Upper chambers of the heart 16. Stage of contraction when rapid outflow of potassium re-establishes a resting potential 18. Outer layer of the heart 20. Vessels which carry blood to and from the myocardium 21. How fast the heart beats 24. Valve between the right atrium and right ventricle 25. Stage when a new contraction cannot occur until relaxation has occured 27. Fibers which conduct nerve impulses through the myocardium of the ventricles