People sometimes hear their heartbeat in their stomach because the abdominal aorta descends from the heart and into the abdomen. The abdominal aorta is the largest of the abdomen's arteries.
Sounds from the heart transmit throughout the body. The liver is a firm organ, so it readily transmits sounds from the heart throughout the abdominal cavity.
The abdominal aorta is a direct continuation of the descending aorta, and it is responsible for supplying blood to most of the abdominal organs. Because it is so large and directly attached to the descending aorta, the abdominal aorta transmits sounds from the heart.
The abdominal aorta starts at the diaphragm and runs parallel to the inferior vena cava. It is located just left of the midline of the body and is covered by the stomach as well as the omentum.
Feeling the pulsation, or heartbeat, in the stomach tends to be more common in thin people. The liver sits on the right side of the stomach. It's a meaty organ that weighs roughly 3 pounds. The abdominal aorta runs to the left of the liver. People with narrow frames are more likely to have the liver sit close to the abdominal aorta, thus amplifying the sound of a heartbeat within the stomach.