Cardiac Contractions

Heart Beat

• 
• 4 chambers of heart:
• Right atria
• Left atria
• Right ventricle
• Left ventricle

Diastole & Systole

• 
• Stage A: Atrial Systole - Ventricular Diastole
• Pressure of blood flow in atria increases
• Force open atrioventricular valves
• Blood pass through valves to ventricles
• Stage B: Ventricular Systole - Atrial Diastole
• Pressure of blood in ventricles increases
• Closes atrioventricular valves
• Blood stays in ventricles (pressure not higher than in main arteries so won't open aortic or pulmonary valves)
• Pressure increases as ventricles continue to contract
• Pressure in ventricles exceeds pressure in main arteries
• Pulmonary & aortic valves open
• Blood to pulmonary & aorta
• Stage B: Ventricular Diastole - Atria diastole
• Pressure quickly drops below that of the main arteries
• Pulmonary & aortic valves close
• Pressure drops below atria value
• The atrioventricular valves open
• Blood flows into atria due to open valves even though it isn't contracting

Controlling Cardiac Cycle

• Sinoatrial Node (SA)
• Consists of a cluster of cells in upper wall of the right atrium
• It's a natural pacemaker
• Generates electrical impulses causing the heart to contract
• This electrical impulse is passed cell to cell down the heart until it reaches the atrioventricular node.
• Atrioventricular node (AV)
• Cluster of cells in the centre of the heart between atria and ventricles
• The AV slows down the electrical impulses before they are permitted to pass down through the ventricles. This delay allows chance to ensure the atria has fully contracted before ventricles are stimulated.
• After passing AV node the electrical impulse passes down through the ventricles along purkinje fibres embedded in the walls of the lower part of the heart.

Electrocardiogram