Heart Actions

The cardiac cycle constitutes a complete heartbeat.
The atria fill with blood while they are relaxed. When they contract (atrial systole), the blood flows into the ventricles and then the atria relax again (atrial diastole ). When the ventricles contract (ventricular systole), the tricuspid and bicuspid valves close and blood flows into the arteries. The valves reopen when the ventricles relax (ventricular diastole).
The sounds of the heart are the results of valves closing and blood moving.
The heart acts as two functional units, or functional syncytia ; one is in the atrial walls and one is in the ventricular walls.
The cardiac conduction system is comprised of specialized clumps and strands of cardiac muscle which initiate and distribute cardiac impulses. The sinoatrial node is a key portion of this system and is found in the posterior wall of the right atrium. It is the pacemaker.

With Every Beat of Your Heart…

The ability of the heart to conduct an impulse is intrinsic – it does not depend on the nervous system to beat. The vagal nerve fibers, however, will alter the basic rhythm.
The cardiac conduction system controls the intrinsic beating of the heart. It is composed of special cardiac cells called autorhythmic cells. These cells are localized in a few areas : the sinoatrial node , the atrioventricular node , the atrioventricular bundle (bundle of His), branches of the AV bundle and in ventricular walls as Purkinje fibers .

The sequence of cardiac excitation is the same as the order of the structures given above

The SA node (located just inferior to the superior vena cava) sends impulses about 75 times per minute (without neural control, however, it would be as high as 100 beats/min). The SA node is the pacemaker because it sends impulses out more frequently than the other localized areas of autorhythmic cells.
The AV node (located in the inferior section of the interatrial septum, just above the tricuspid valve) delays the impulse sent by the SA node about 0.1 seconds before initiating ventricular contraction. This is to give the valves time to close. In the absence of the SA node, the AV node would send impulses about 50 times per minute.
The AV bundle starts in the inferior section of the interatrial septum and carries the impulse from the AV node down into the ventricular septum to the Purkinje fibers, which stimulate the ventricles. In the absence of a "pace" from the SA and AV nodes, these fibers would send impulses at the rate of ~30 per minute.
Ventricular contraction starts at the apex and moves upward in a "wringing" motion.
Fibrillation is an extreme form of arrhythmia.
Ectopic heartbeats arise when an abnormal pacemaker appears and takes over a defective SA node. Alternatively, the AV node may take over – this is called a junctional rhythm.

Ectopic heartbeats can appear even with a normal pacemaker. This is usually due to too much caffeine or nicotine and causes extrasystole until the effect wears off.

Extrinsic innervation is conducted by autonomic system fibers. This can be considered the "accelerator" or "brake" as they affect the rate and force of the heartbeat.
Factors Affecting Heart Rate :

In a healthy system, the volume of blood expelled by each ventricular contraction (the stroke volume ) remains fairly constant. Certain events will change stroke volume – blood loss or a weak heart – and the heart will compensate by increasing the rate and the strength of the beat (contractility ). Temporary stressors may do the same thing.

The autonomic nervous system is probably the most important and most powerful extrinsic control. The sympathetic branch increases heart rate and contractility. The parasympathetic branch does the opposite. The heart exhibits vagal tone, as both autonomic branches continuously innervate it and the overall effect is inhibitory. If the vagal nerves to the heart were cut, the heart rate would increase ~25 beats/minute.

Sensory input to the ANS is sent by pressoreceptors that respond to changes in the blood pressure.

Chemical controls include hormones (epinephrine, thyroxine) and ions

Hypocalcemia will depress the heart rate. Potassium and sodium levels will also affect heart rate.

Other extrinsic factors include age, sex, exercise and body temperature.

The resting heartbeat in a fetus averages 150 beats/minutes. This decreases with age, and it is also lower in women.

Exercise ultimately decreases heart rate since the hypertrophied heart increases stroke volume. It does not have to beat as quickly in order to send the same about of blood through the system in a minute.

Congestion

Congestive heart failure occurs when the pumping efficiency of the heart is too low to meet tissue needs. It can happen due to atherosclerosis in coronary vessels or any other event that weakens or kills myocardium, resulting in myocardial infarctions.

Circulation

Circulation is either the flow of blood or the actual volume of blood flowing through a vessel, organ or the system. Blood pressure is the force the blood exerts on the blood vessel wall. It is normally meant as the systemic arterial blood pressure. Resistance is opposition to flow. The viscosity of the blood, length of a vessel and vessel diameter will affect resistance.

Blood flows from high pressure to low pressure, until it gets back to the heart.

Arterial pressure is the greatest in the aorta, during systole. This is systolic pressure . The lowest pressure is after the aortic valve has closed and blood is moving into the arterioles and capillaries. The absolute lowest point is the diastolic pressure.

Pulse pressureis the difference between systolic and diastolic pressures. It is felt as the throbbing we call the pulse.