Increases in both heart rate and stroke volume are responsible. Maximum heart rate within individuals is relatively fixed and does not alter much with training. Stroke volume however, is extremely responsive to endurance training and can increase significantly. Reductions in exercise heart rate and resting heart rate that typically occur with training are indicators that stroke volume has increased. Blood on average has a haemoglobin level of 15 grams per 100 mls and carries about 200 mls of oxygen per litre.
Capillaries are very small blood vessels that surround muscle fibres.
An increase in the number and density of the capillaries surrounding each muscle fibre will deliver more blood and therefore more oxygen to the muscle. Endurance training increases the total number of functional capillaries surrounding muscle fibres and thereby allows more oxygen to be available to the muscle. Research indicates that training will increase the speed and efficiency of the blood flow to working muscles. These are beyond the scope of this course, but be aware that they do occur. That is, it is of a low enough intensity, that anaerobic sources are not required to supply extra energy.