The heart

The right side of the heart pumps deoxygenated blood to the lungs.
The left side of the heart pumps oxygenated blood to the body.

The left ventricle has thicker, more muscular walls then the right ventricle, because it needs to contract powerfully to pump blood all around the body.
The ventricles have thicker walls than the atria as they push blood out of the heart whereas the atria push blood into the ventricles.

The atrioventricular (AV) valves link the atria to the ventricles and stop blood flowing back into the atria when the ventricles contract.
The semi-lunar (SL) valves link the ventricles to the pulmonary artery and aorta, and stop blood flowing back into the heart after the ventricles contract.

The cords attach the atrioventricular valves to the ventricles to stop them being forced up into the atria when the ventricles contract.

The valves only open one way - whether they're open or closed depends on the relative pressure of the heart chambers.

If there's higher pressure behind a valve, it's forced open.
If there's lower pressure behind a valve, it's forced shut.

Cardiac muscle is myogenic - it can contract and relax without receiving signals from nerves. This pattern of contractions controls the regular heartbeat.

It starts in the sino-atrial node (SAN), which is in the wall of the right atrium. It is like a pacemaker - it sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls.
This causes the right and left atria contract at the same time. A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles.
Instead, these waves of of electrical activity are transferred from the SAN to the atrioventricular node (AVN). There's a slight delay before the AVN reacts, to make sure the ventricles contract after the atria have emptied.

The bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity to the finer muscle fibres in the right and left ventricle walls.
These finer muscle fibres are called Purkinje fibres. They carry the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously, from the bottom up.

Cardiac output - the volume of blood pumped by th heart per minute (measured in cm3 per minute).

Cardiac output = stroke volume x heart rate

Stroke volume - the volume of blood pumped during each heartbeat (cm3).
Heart rate - the number of heart beats per minute.

Cardiac cycle - an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continuously circulating round the body.

Pressure changes take place, due to the changes in chamber volume.
Ventricles relax, atria contract - this decreases the volume of the atria and increases the pressure within the atria, pushing the blood into the ventricles. (Atrial systole)

Ventricles contract, atria relax - the ventricles contract, decreasing their volume and increasing their pressure to above the pressure in the atria, forcing the AV valves shut. The pressure is higher in the ventricles than in the aorta and pulmonary artery, forcing the SL valves open and blood is pushed out into these arteries. (Ventricular systole)

Ventricles relax, atria relax - as they both relax, the higher pressure in the aorta and pulmonary artery closes the SL valves to prevent back-flow into the ventricles. Blood returns to the heart and the atria fill again due to higher pressure in the vena cava and pulmonary vein. As the atria fill, the pressure increases and so the AV valves open. This allows the blood to flow passively into the ventricles, then the atria contract and the process starts again. (Atrial and ventricular diastole)