Heart, in anatomy, the hollow muscular organ that receives blood from the veins and propels it into and through the arteries. The heart of a human is about the size of a closed fist. It is situated behind the lower part of the breastbone, extending more to the left of the midline than to the right. It is roughly conical in shape, with the base directed upward and to the right and slightly backward; the apex touches the chest wall between the fifth and sixth ribs. The heart is held in place principally by its attachment to the great arteries and veins, and by its confinement in the pericardium, a double-walled sac with one layer enveloping the heart and the other attached to the breastbone, the diaphragm, and the membranes of the thorax.