These contractions occur so closely together that one can think of them as one contraction, but that is not actually the case. The myocardium actually causes the heart to contract, as its muscle fibres create a wringing movement that efficiently squeezes blood from the heart with each beat. The thickness of the myocardium varies according to the pressure generated to move blood to its destination. Thus the myocardium of the left ventricle is thicker than the myocardium of the right ventricle. The volume of blood expelled by the heart is referred to as the cardiac output.
Blood pressure is a measure of the force that results from the heart's pumping action.
When the blood pressure is too low, the patient is said to have hypotension. When blood pressure is too high, the patient is said to have hypertension. The heart is suspended in a membranous sac called the pericardium. The heart is covered by the visceral pericardium or epicardium. Between the heart and the epicardium lies the pericardial cavity, which is normally filled with a very small amount of pericardial fluid.