Exercise
When exercise first starts large, contracting muscles in the limbs squeeze more blood along veins. Stretch receptors in the cardiac muscle recognise more blood entering the atria and send more nerve receptors to the cardiovascular centre of the brain.
The increased stretching also directly increases the strength of contraction, without involving the brain.
Stress, or anticipation of exercise
Hormonal changes can also alter cardiac output; even in the absence of exercise stress can cause release of adrenaline which acts on the SAN, speeding the heart rate up.
Pressure
Baroreceptors in the carotid artery respond to blood pressure changes, and send impulses to the cardiovascular centre to affect heart rate or change width of blood vessels to reverse the change detected
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