Nervous control of heart beat - Page 4
             Broker, (2011) states that 'the cardiac cycle is the cyclical contraction (systole) and the relaxation (diastole) of the two atria and the ventricles.' Each cycle is initiated by the spontaneous generation of an action potential in the sinoatrial node. Diastole lasts around 0.4 seconds. Increasing pressure in the ventricles closes the triaspid and mitral valves; this then means that all four valves are closed. Ventricular pressure continues to rise until eventually the pulmonary and aortic valves are forced open and blood is ejected into the pulmonary artery and aorta (Crowley, 1996). When the heart rate is increased the amount of time the diastole and systole take place is shortened, so that more blood is pumped out in one minute.
             During aerobic exercise cellular respiration in muscle cells need to increase significantly to maintain its performance. When exercising, adenosine triphosphate (ATP) is needed in order for muscles to contact (Brown, 2006). This provides energy and allows movement on demand. When muscles contract hard and as fast, we use ATP far faster than it can be possibly made. Whilst exercising, ATP is made using a chemical reaction that requires oxygen. The oxygen comes from the air breathed into the lungs and is then carried to the muscles in the hemoglobin of the red blood cell. This way of making ATP is called aerobic respiration. This is the breakdown of glucose with oxygen to provide energy for the muscle cells. It produces heat which increases body temperature and carbon dioxide which we breath out. To maintain a steady supply of glucose and oxygen to working muscles, and the removal of carbon dioxide will allow an increase in stroke volume (Sherwood, 2012). During exercise, the cardiovascular system works harder. It needs to deliver oxygen to muscles. The volume delivered by each beat of the heart (stroke volume) increases. Whilst exercising, the initial increase i...