The cardiac cycle

The heart consists of four chambers that are connected in such a way that contraction of them causes the heart to act as a pump, the right half of the heart passing blood from the venae cavae to the pulmonary circulation for oxygenation and the left half of the heart pumping oxygenated blood from the pulmonary veins into the aorta and systemic circulation. The heart beats rhythmically due to spontaneous firing of cells in the sino-atrial node and passage of the electrical activity throughout the heart via cell-to-cell contacts and specialized conducting tissue. Systole causes the pressure inside the four chambers of the heart to rise which, coupled with the activity of valves, forces blood to move through the heart in one direction. Even though the heart is spontaneously active, its rate of beating can be altered by outputs from the autonomic nervous system. There is a relationship between the length of a cardiac muscle fibre at the end of diastole and its force of contraction (the ‘Law of the Heart’), but this can be changed by altered inotropism of the cardiomyocytes due to their sympathetic innervation and circulating catecholamines. Whilst physical exercise promotes an individual's health, there is evidence that extended prolonged exercise can cause transient ‘cardiac fatigue’. Recent work has begun to investigate this phenomenon using the technique of echocardiography.