Which statement best describes an organ? A. This is the most complex level of organization of life.
B. Cells of one type work together to carry out a function
C. Different types of tissues work together to carry out a function
D. This is the simplest level of organization of life.

1. beginning with all chambers of the heart in the "relaxed" phase, describe the cardiac cycle in mammals, with systole and diastole of the atria and ventricles. for each time point in the cycle you select, state the following: • whether the atria and ventricles are contracting or relaxing • whether the av valves and semilunar valves are closed or open • whether the pressure in the atria and ventricles is high or low