Heart, computer illustration. The heart is a hollow sac (lower frame) of muscular tissue that pumps blood around the body. Small surface coronary blood vessels supply the heart muscle with blood. Four major blood vessels (upper frame) bring blood to and from the heart. From upper left to centre right they are: the vena cava (the vein returning deoxygenated blood from the body); the aorta (artery through which oxygenated blood is pumped to the body); the pulmonary artery (through which deoxygenated blood is pumped to the lungs); and the pulmonary vein (which returns oxygenated blood from the lungs). The heart can pump 5 litres of blood every minute at rest. Heart muscle is composed of spindle-shaped cells grouped in irregular bundles. Boundaries between individual cells are faintly visible here. Each cell contains one nucleus, visible as a dark stained spot. Cardiac muscle is a specialised muscle tissue that can contract regularly and continuously without tiring.