The heart and cardiac cycle

• Due to the heart’s ability to initiate its own contraction, it is referred to as myogenic
• In the wall of the right atrium there is a region of specialised fibres called the sinoatrial node which is the pacemaker of the heart, as it initiates a wave of electrical stimulation which causes the atria to contract at roughly the same time
• The ventricles do not start contracting until the atria have finished due to the presence of tissue at the base of the atria which is unable to conduct the wave of excitation
• The electrical wave eventually reaches the atrioventricular node located between the two atria which passes on the excitation to ventricles, down the bundle of His to the apex of the heart
• The bundle of His branches into Purkyne fibres which carry the wave upwards
• This causes the ventricles to contract, thus emptying them
• Stages of the cardiac cycle:

  1. Atrial systole – during atrial systole the atria contract and this forces the atrio-ventricular valves open and blood flows into the ventricles.
• 2) Ventricular systole – contraction of the ventricles causes the atrio-ventricular valves to close and semi-lunar valves to open thus allowing blood to leave the left ventricle through the aorta and right ventricle through the pulmonary artery
• 3) Cardiac diastole – atria and ventricles relax, elastic recoil of the heart lowers the pressure inside the heart chambers and blood is drawn from the arteries and veins thus causing semilunar valves in the aorta and pulmonary arteries to close, preventing backflow of blood.