passive stretching force exerted on
the
ventricular muscle at the end of diastole. Preload is caused by
the volume of blood in the ventricle at the end of diastole.
Preload is a passive stretching force exerted on
the
ventricular muscle at the end of diastole. Preload is caused by
the volume of blood in the ventricle at the end of diastole.
Afterload is the 
force resisting the contraction of
the
cardiac muscle fibers. Afterload can also be considered as the
end-systolic wall stress in the ventricle.
Contractility refers to the ability of cardiac muscle fibers to shorten when stimulated and is considered to be independant of preload and afterload. Contractility can be affected by a variety of chemicals, both natural as well as externally aquired.
As heart rate increases, the amount of blood that it can pump during a given time also increases until a critical heart rate is reached then cardiac output decreases. In combination these factors all work together to allow the heart to compensate over a wide range of conditions. This allows for the continuous supply of oxygen and nutrients to all parts of the body.