1. THE CARDIAC CYCLE What is the cardiac cycle.
The cardiac cycle is the sequence of events that occur when the heart beats. There are two phases of this cycle:
Diastole - Ventricles are relaxed.
Systole - Ventricles contract.

2. The cardiac cycle
Exercise increases blood flow through the heart so that the cardiac cycle accelerates to accommodate the increased demand for oxygen.
The normal cycle is around 0.8 seconds. This accelerates with faster and more powerful atrial and ventricular contraction, which is stimulated by the cardiac centre in the brain.
Heart rate:- is defined as the number of heart contractions in each minute.
# There are two distinct periods in the cardiac cycle- one of the heart muscle relaxation (cardiac diastole), the other of contraction (cardiac systole)

3. Cardiac diastole During cardiac diastole
The bicuspid and tricuspid valves are closed and the atrium is full.
Once full with blood, the atria forces the bicuspid and tricuspid valves to open and fill the ventricles.
This lasts for around 0.5 seconds at rest.

4. Cardiac systole Cardiac systole
The atria contract and send blood via the bicuspid and tricuspid valves into the ventricles.
When full, these contract causing blood to be expelled from the heart via the semi-lunar valves.
(the bicuspid and tricuspid valves are closed at this time)
This lasts around 0.3 seconds at rest.

5. What are heart valves?
Valves are flap-like structures that allow blood to flow in one direction. The heart has two kinds of valves, atrioventricular and semilunar valves.
Heart sounds.
* The audible sounds that can be heard from the heart are made by the closing of the heart valves. These sounds are referred to as the “lub-dupp” sounds. The “lub” sound is made by the contraction of the ventricles and the closing of the atria-ventricular valves. The “dupp” sound is made by the semi-lunar valves closing.

6. Stimulation of the heart originates in the cardiac centre, in the “medulla oblongata.”
The “sympathetic and parasympathetic nervous systems” work antagonistically and provide the stimulation for acceleration and deceleration of the heart rate.
Cardiac systole (contraction) is initiated by the electrical cardiac impulse from the “sinu-atrial node” (the pace-maker found in the right atria wall.) This distributes electrical stimulus through the “myocardial” (heart muscle) wall between the heart chambers, where the “atrio-ventricular node” (between the right atrium and right ventricle) continues distribution of the electrical signal across the ventricles.

7. The SA node
In the upper part of the right atrium of the heart is a specialized bundle of neurons known as the sinoatrial node (SA node). Acting as the heart's natural pacemaker, the SA node "fires" at regular intervals to cause the heart of beat with a rhythm of about 60 to 70 beats per minute for a healthy, resting heart. The electrical impulse from the SA node triggers a sequence of electrical events in the heart to control the orderly sequence of muscle contractions that pump the blood out of the heart.
The AV node
The AV node (AV stands for atrioventricular) is an electrical relay station between the atria (the upper) and the ventricles (the lower chambers of the heart). Electrical signals from the atria must pass through the AV node to reach the ventricles.

8. AV node (bundle of his)
The bundle of His is located in the proximal intraventicular septum. It emerges from the AV node to begin the conduction of the impulse from the AV node to the ventricles.

9. Purkinje fibers
Purkinje fibers are heart muscle tissues that are specialized to conduct electrical impulses to ventricular cells, which induce the lower chambers of the heart to contract.
Impulses from the upper chambers of the heart are relayed by this node to large bundles of Purkinje fibers referred to as the Bundle of His. These bundles branch into smaller elements and eventually form terminal ends that burrow into left and right ventricular chamber muscles. As the impulse is passed to the ventricles, the muscles contract and pump blood. The contraction caused by the specialized fibers begins from the bottom of the ventricles and move upwards so that the blood leaves the lower chambers through the pulmonary arteries and the aorta.