Describe the physiology of the cardiac cycle and how it is regulated.

The heart is a myogenic muscle, meaning the cells contract with no stimulus. However, this is not sufficient to keep a person alive so the cellular contractions are co-ordinated within the heart and the rate of those contractions are controlled by the autonomic nervous system. A normal heart contraction begins in the sinoatrial node, or the ‘pacemaker’. This sends an electrical impulse which initiates the contract through the left and right atria forcing blood to move through the tricuspid and mitral valves into the ventricles. The walls separating the atria and ventricles are unable to conduct the electrical stimulus, so it must pass through the atrioventricular node located between both atria and ventricles. The AV node also helps in regulating heart rate by slowing down conduction through the heart. The stimulus then pass down the septum between the two ventricles via the bundle of His and then the Purkynjie fibres towards the apex of the heart. It then spreads back up the walls of the ventricles causing them to contract from the apex upwards and forcing blood through the pulmonary and aortic valves into the pulmonary artery and the aorta where it travels to the lungs and body respectively. The rate of these contractions is controlled by the autonomic nervous system and depends on what the person is doing. Sympathetic fibres, those used in the fight or flight response, cause the heart rate to increase when stimulated, while the parasympathetic fibres, used in the rest and digest response, cause the heart rate to slow.