The heart is almost the size of a large fist and weighs between about 280 to 340 grams in men and 230 to 280 grams in women.
The heart is situated between the lungs in the thoracic cavity. The name of this area is mediastinum. The cone-shaped heart base is at the top, behind the sternum, and the great vessels are entering or leaving here. The heart's apex (tip) points down and is just above the midline diaphragm to the left. That's why we might think of the heart as being on the left side because here you can hear or feel the strongest beat.
The three layers of heart’s wall are the epicardium (external layer), the myocardium (middle layer), and the endocardium (inner layer)
The walls of the heart's four chambers are made of the myocardium called cardiac muscle. The chambers are lined with the endocardium, a simple squamous epithelium which also covers the heart valves and continues as its lining (endothelium) into the vessels. The endocardium's important physical feature is not its thinness, but its smoothness. This very smooth tissue prevents blood clotting as blood contact with a rough surface would initiate clotting. The heart's upper chambers are the right and left atria, with relatively thin walls separated by a common myocardial wall called the interatrial septum. The lower chambers are the right and left ventricles with thicker walls and the interventricular septum separates them. As you can see, the atria receive blood from either the body or the lungs and the ventricles pump blood into the lungs or the body.