A bee's heart is located in the dorsal region of the abdomen. The heart pumps the blood in the aorta through the petiole and into the thorax while the venial blood, pumped by the ventral diaphragm, runs from the thorax through the petiole into the ventral region of the abdomen. The ventral diaphragm is a very thin sheet of muscle that directly touches the aorta in the petiole. When it contracts, it forces the blood from the thorax in the posterior direction towards the abdomen (Heinrich, 1976). 
 

diagram adapted from Heinrich, 1976