They prevent the pressure from filling ventricles from causing the valves from collapsing back up into the atria, which would facilitate backflow. At the appropriate time, the papillary muscles contract to pull the guy wires taut. These muscles don't have any effect on the valves except to keep them from collapsing up into the atria. The semilunar valves prevent backflow from the ventricles into the aorta or pulmonary vein. Systolic pressure is measured as arterial pressure generated as blood is forced by left ventricular contraction out through the aorta and into the rest of the body.
Diastolic pressure is the pressure remaining as the left ventricle relaxes.
Blood pressures typically vary all day long depending on things like activity level and psychological stress, so a resting pulse is taken. The conducting system of the heart keeps it beating independent of the nervous system. There are certain cardiac muscle cells that maintain the heartbeat. The signals go quickly from the SA node to the atria to effect simultaneous contractions of both atria. The signals are stopped from moving directly to the ventricles by nonconducting tissue.