Heart rate node, where the faster pacemaker cells driving the self-generated rhythmic firing and responsible for the heart's autorhythmicity are located. For endurance athletes at the elite level, it is not unusual to have a resting heart rate between 33 and 50 bpm. The "maximum heart rate" (HR) is the highest heart rate an individual can achieve without severe problems through exercise stress, and generally decreases with age. Since HR varies by individual, the most accurate way of measuring any single person's HR is via a cardiac stress test. In this test, a person is subjected to controlled physiologic stress (generally by
Cardiac physiology large numbers of gap junctions that allow the passage of ions between the cells and help to synchronize the contraction. Intercellular connective tissue also helps to strongly bind the cells together, in order to withstand the forces of contraction. Cardiac muscle undergoes aerobic respiration patterns, primarily metabolizing lipids and carbohydrates. Oxygen from the lungs attaches to haemoglobin and is also stored in the myoglobin, so that a plentiful supply of oxygen is available. Lipids, and glycogen are also stored within the sarcoplasm and these are broken down by mitochondria to release ATP. The cells undergo twitch-type contractions with long refractory