The right side of the heart pumps blood through the pulmonary circuit.
The left side pumps blood through the systemic circuit.
-> The main parts of the heart, seen in cross-section from the front 
Effect of exercise and adrenaline
During exercise, the muscle cells need more energy than usual.
They therefore need to respire more
As a consequence, need more oxygen and glucose, and they produce more waste carbon dioxide.
To meet this demand, the heart:
- contracts more often (the heart rate increases)
- contracts more powerfully (the stroke volume increases, pumping a greater volume of blood with each beat)
The changes in heart rate are easily observed by measuring the pulse rate (for example, at the wrist or side of the neck).
The hormone adrenaline is secreted when we are afraid, stressed or angry.
It also causes the heart rate and stroke volume to increase, preparing us for ‘fight or flight’.