ABSTRACT
Cardiac output must increase in response to increased demand of the tissues for oxygen supply and carbon dioxide removal. Since cardiac output is the product of heart rate and stroke volume, increases in either of these two variables can produce increases in cardiac output. Cardiac output can increase by as much as fourfold during exercise, facilitated by an increase in heart rate at low-to-moderate exercise levels and by an increase in both heart rate and stroke volume at heavy exercise levels. During severe exercise, however, stroke volume falls somewhat because of the reduction in diastolic filling time at extremely high heart rates. This, together with a maximal heart rate of 180-200 beats min-1, means that cardiac output is maximal at around 20 1 min-1 Denervation does not markedly affect the ability of the heart to increase output to match demand; hormonal influences and intrinsic heart mechanisms are important.
