The organ that is approximately the size of a fist is the heart. The heart is a muscular organ of the circulatory system that pumps blood to different parts of the body. It is located in the chest and is roughly the size of a person's left-hand fist. The liver is a large organ located in the upper right part of the abdomen. The intestine is a long, tube-like organ that is part of the digestive system. The stomach is a muscular sac that is part of the digestive system and is located between the oesophagus and the small intestine.