Structure of the Heart. The human heart is a four-chambered muscular organ, shaped and sized roughly like a man's closed fist with two-thirds of the mass to the left of midline. The heart is enclosed in a pericardial sac that is lined with the parietal layers of a serous membrane.
https://training.seer.cancer.gov/anatomy/cardiovascular/heart/structure.html
A ventricular septal defect (VSD) is a defect in the septum between the right and left ventricle. The septum is a wall that separates the heart's left and right sides. Septal defects are sometimes called a hole in the heart.
http://www.heart.org/HEARTORG/Conditions/CongenitalHeartDefects/AboutCongenitalHeartDefects/Ventricular-Septal-Defect-VSD_UCM_307041_Article.jsp
Overview of the heart valves and related structures. The atria are divided into the left atrium and the right atrium, which correspond to the left and right sides of the heart. They are situated just above the ventricles of the heart and are separated by the cuspid valves. The atria are built and held up by rigid endocardial muscle fibers that completely encompass the blood as it flows through them.
https://www.kenhub.com/en/library/anatomy/the-atria-of-the-heart
The muscle that separates the right and left side of the heart is  called the interventricular septum and is just a continuation of  the cardiac (Heart) muscle of the heart'…s walls.
http://www.answers.com/Q/What_separates_the_left_and_right_side_of_a_heart
Normally, the left side of the heart only pumps blood to the body, and the heart's right side only pumps blood to the lungs. When a large opening exists between the ventricles, a large amount of oxygen-rich (red) blood from the heart's left side is forced through the defect into the right side.
http://www.heart.org/HEARTORG/Conditions/CongenitalHeartDefects/AboutCongenitalHeartDefects/Ventricular-Septal-Defect-VSD_UCM_307041_Article.jsp
It means your heart is in middle of your chest, with a slight tilt towards the left. Located in the front and middle of your chest, your heart is no bigger than the size of your clenched fist. A wall separates the left and right of your heart, and each of these sides has a small chamber called atrium. The chamber then leads into a large pumping chamber known as ventricle. Your heart has four chambers, called right ventricle and right atrium, and then left atrium and left ventricle.
http://www.newhealthadvisor.com/What-Side-Is-Your-Heart-On.html
The left side of the heart is for to collecting oxegynated blood  from the lungs, then pumps blood around your body.   The right side of the heart is for to collecting bl…ood from the  body, then so pumps blood to the lungs.
http://www.answers.com/Q/What_separates_the_left_and_right_side_of_a_heart
Normally, the left side of the heart only pumps blood to the body, and the heart's right side only pumps blood to the lungs. In a child with VSD, blood can travel across the hole from the left pumping chamber (left ventricle) to the right pumping chamber (right ventricle) and out into the lung arteries.
http://www.heart.org/HEARTORG/Conditions/CongenitalHeartDefects/AboutCongenitalHeartDefects/Ventricular-Septal-Defect-VSD_UCM_307041_Article.jsp
While it is convenient to describe the flow of blood through the right side of the heart and then through the left side, it is important to realize that both atria and ventricles contract at the same time. The heart works as two pumps, one on the right and one on the left, working simultaneously.
https://training.seer.cancer.gov/anatomy/cardiovascular/heart/structure.html
because less pressure is needed to pump the blood to the lungs as oppose to the left side which has the duty of pumping the blood to the body. and also due to the fact that th…e blood coming to right side of the heart from the body very low pressure.
http://www.answers.com/Q/What_separates_the_left_and_right_side_of_a_heart
what separates the left and right side of the heart
The Interventricular septum separates the left and right side of the heart. The interventricular septum separates the left and right side of the heart.