Which vein would most likely use gravity for venous return to the heart?

Which of the following structures is referred to as the pacemaker of the heart?

A sudden change in a gradient that brings the thresholds closer together is called:

The SA node requires a stimuli to achieve an action potential.

Which of the following conditions might lead to polycythemia?

At which point in the Cardiac Cycle does isovolumetric contraction occur?  

What causes the semilunar valves to open?

Extra Credit: Choose one of the following questions and respond in three or more sentences for 5 extra credit points.   1. Explain what an EKG is used for and its limitations in diagnosing heart conditions. What can/cannot it diagnose? When would an EKG be appropriate or not? List one condition that the EKG can detect and one that it cannot detect. Be specific! 2. What are patients with A-fib more at risk for and what type of treatment are they typically on?  

What is the study of the heart and its disorders called?

What is happening in Phase 3 of the diagram?