Heart is a hollow muscular organ, which is somewhat pyramidal in shape. It lies within the pericardium in the mediastinum. It lies free within the pericardium except at its base where it is connected to great blood vessels.
Surfaces of heart:

Because of its shape, the heart has three surfaces: anterior, inferior and posterior. Often the surfaces are referred to as: sternocostal (anterior), diaphragmatic (inferior) and base (posterior). The apex of the heart is directed downward, forward and to the left.

* Anterior (Sternocostal) surface: It is formed mainly by the right atrium and right ventricle. They are separated from each other by the vertical atrioventricular groove. The right border of the anterior surface is formed by the right atrium while the left border is formed by left atrium and part of left auricle.
* Inferior (Diaphragmatic) surface: It is formed mainly by the right and left ventricles separated by the posterior interventricular groove. The inferior surface of the right atrium into which the inferior vena cava opens, also forms part of this surface.
* The base of the heart (posterior surface): It is formed mainly by the left atrium, into which the four pulmonary veins drain. It lies opposite to the apex. Often, the beginners think of the diaphragmatic surface of the heart as its base because of the fact that the heart rests on it, however, it should be kept in mind that the heart does not rest on its base. It rests on the diaphragmatic surface which is not the base. The posterior surface is called the base because it lies opposite to the apex of the pyramidal shaped heart.

Apex of the heart: It is formed by the left ventricle and is directed downward, forward and the left. It lies at the level of the fifth intercostals space, about 3.5 inches from the midline. The apex beat can be palpated in the region of apex of the heart.
Borders of the heart:

Because of pyramidal nature of its shape, the heart has three borders: right, left and lower. Right border is formed by the right atrium. The left border is formed by the left auricle and left ventricle. The lower border is formed by right ventricle, however, some part of it is also formed by the right atrium.
Wall of the heart:

As it was stated earlier, the heart is a hollow muscular chamber. It has strong wall that are composed of three main layers. The bulk of the wall of the heart is formed by cardiac muscles called the endocardium. On the outer side, the endocardium is covered with visceral layer of serous pericardium, known as epicardium. On the inner side, the endocardium is line with a layer of endothelium known as endocardium.
Chambers of the heart:

Human heart is not a simple hollow pump. It has been divided by vertical septa into four chambers: two atria (right and left) and two ventricles (right and left). The atria lie superior to the ventricles. In anatomic position, the right atrium lies anterior to the left atrium and the right ventricle lies anterior to the left ventricle.
Right atrium:

It consists of two regions: the main concavity and a small outpouching called auricle. At the region of junction between these two parts, on the outer side, there is a vertical groove called sulcus terminalis, which on the inner side forms a ridge known as crista terminalis. The main part of the atrium lies posterior to crista terminalis and is derived embryologically from sinus venosus. The part of the atrium, which lies in front of crista terminalis, is roughened by bundles of muscle fibers, the musculi pectinati. This anterior part is derived embryologically from primitive atrium. Read the rest of this entry »