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( Cardiac output)
Cardiac output (Q) is the volume of blood being pumped by the heart, in particular by a ventricle in a minute. This is measured in dm3 min-1 (1 dm3 equals 1000cm3 or 1 litre). Cardiac output is equal to the stroke volume (SV) multiplied by the heart rate (HR). SV is the volume of blood pumped per beat and the HR is the number of beats per minute. Therefore, if there are 70 beats per minute, and 70 ml blood is ejected with each beat, (SV), the cardiac output (Q) is 4900 ml/minute. This value is typical for an average adult at rest, although Q may reach up to 30 litres/minute during extreme exercise by elite athletes. The function of the heart is to transport the blood to deliver oxygen, nutrients and chemicals to the cells of the body to ensure their survival and proper function and to remove the cellular wastes. Q indicates how well the heart is performing this function. Q is regulated principally by the demand for oxygen by the cells of the body. If the cells are working hard, with a high metabolic oxygen demand then the Q is raised to increase the supply of oxygen to the cells, while at rest when the cellular demand is low, the Q is said to be baseline. Q is regulated not only by the heart as it pumps, but also by the function of the vessels of the body as they actively relax and contract thereby increasing and decreasing the resistance to flow. When Q increases in a healthy but untrained individual, most of the increase can be attributed to an increase in HR. Change of posture, increased sympathetic nervous system activity, and decreased parasympathetic nervous system activity can also increase cardiac output. HR can vary by a factor of approximately 3, between 60 and 180 beats per minute, whilst SV can vary between 70 and 120 ml, a factor of only 1.7. [1][2][3]
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