Cardiac Cycle 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.

First Step of Cardiac Cycle 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.

Second Step 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.

Third Step 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.

Heart Sounds 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 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.

Regulation of Stroke Volume

Stroke volume is regulated by the following factors:• The stretch of the muscle cells before

contracting• Exercise• Time between heart beats• Blood volume

Regulation of Heart Rate 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.

Neural Controls 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.

Effect of Chemicals and Hormones on the Heart

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.

Effect of Physical Factors on the Heart

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.