 The cardiac cycle consists of the events that occur during one complete heartbeat or during which both the atria and ventricles contract.  The term systole refers to heart contraction and diastole refers to heart relaxation.  Normally these terms are only used to describe the ventricles.

 The first step of the cardiac cycle is mid- to-late diastole. Begins with the heart completely relaxed. Blood flows into and through the atria to the ventricles. SL valves are closed, AV valves are open. Then, atria contract forcing all blood into ventricles.

 The second step is ventricular systole. Ventricular contraction begins and pressure increases causing the AV valves to close. SL valves open when pressure is great enough and blood is forced out. Atria relax and begin filling with blood.

 The third step of the cardiac cycle is called early diastole. The SL valves close and the intraventricular pressure drops. When the pressure is low enough, the AV valves reopen allowing blood to begin flowing into the ventricles.

 The heart produces two distinct sounds described as a “lub-dup” sequence.  The “lub” is the closing of the AV valves.  The “dup” is the closing of the SL valves.  A heart murmur in a healthy adult can indicate a valve problem.

 Cardiac output is the amount of blood pumped out by each side of the heart in 1 minute.  It is the product of the heart rate and the stroke volume.  The stroke volume is the volume of blood pumped out by a ventricle with each heartbeat.

 Stroke volume is regulated by the following factors: The stretch of the muscle cells before contracting Exercise Time between heart beats Blood volume

 Heart rate is affected by stroke volume.  If volume is low the heart compensates by beating faster in an effort to move more blood.  Heart rate can also be modified by chemicals, hormones, and ions.  The autonomic nervous system can also change heart rate temporarily.

 During times of physical or emotional stress, the autonomic nervous system stimulates the SA and AV nodes to increase heart rate.  When the situation changes the heart rate returns to normal.

 Epinephrine and thyroxine both cause an increase in heart rate.  A deficit of calcium or potassium can cause slow or weak heart contractions.  An excess of calcium ions can cause a prolonged contraction which may cause the heart to stop completely.

 Age, gender, exercise and body temperature all effect the resting heart rate.  Heart rate slows with age.  Women have a faster heart rate than men.  Heat will cause a heart to speed up, while cold will cause the heart rate to decrease.  Exercise increases both heart rate and stroke volume.