Heart

Cardiac orientation

The general shape and orientation of the heart are that of a pyramid that has fallen over and is resting on one of its sides . Placed in the thoracic cavity, the apex of this pyramid projects forward, downward, and to the left, whereas the base is opposite the apex and faces in a posterior direction (Fig. 1) . The sides of the pyramid consist of:

• diaphragmatic (inferior) surface on which the pyramid rests,
• an anterior (sternocostal) surface oriented anteriorly,
• a right pulmonary surface, and
• a left pulmonary surface.

Fig. 1 Base of the Heart

Because the great veins enter the base of the heart, with the pulmonary veins entering the right and left sides of the left atrium and the superior and inferior venae cavae at the upper and lower ends of the right atrium, the base of the heart is fixed posteriorly to the pericardia! wall, opposite the bodies of vertebrae TV to TVIII (TVI to TIX when standing) . The esophagus lies immediately posterior to the base.

From the base the heart proj ects forward, downward, and to the left, ending in the apex. The apex of the heart is formed by the inferolateral part of the left ventricle (Fig. 2) and is positioned deep to the left fifth intercostal space, 8-9 em from the midsternal line.

Fig. 2 Anterior Surface of the Heart

Surfaces of the heart

The anterior surface faces anteriorly and consists mostly of the right ventricle, with some of the right

atrium on the right and some of the left ventricle on the left (Fig. 2 ).

The heart in the anatomical position rests o n the diaphragmatic surface, which consists of the left ventricle

and a small portion of the right ventricle separated by the posterior interventricular groove (Fig. 3) . This surface faces inferiorly, rests on the diaphragm, is separated from the base of the heart by the coronary sinus, and extends from the base to the apex of the heart.

The left pulmonary surface faces the left lung, is broad and convex, and consists of the left ventricle and a

portion of the left atrium (Fig. 3).

The right pulmonary surface faces the right lung, is broad and convex, and consists of the right atrium

(Fig. 3).

Figs. 3 Diaphragmatic surface of the Heart 

Margins and Borders

Some general descriptions of cardiac orientation refer to right, left, inferior (acute), and obtuse margins:

• The right and left margins are the same as the right and left pulmonary surfaces of the heart.
• The inferior margin is defined as the sharp edge between the anterior and diaphragmatic surfaces of the heart (Figs. 2)-it is formed mostly by the right ventricle and a small portion of the left ventricle near the apex.
• The obtuse margin separates the anterior and left pulmonary surfaces (Fig. 2)-it is round and extends from the left auricle to the cardiac apex (Fig. 2) , and is formed mostly by the left ventricle and superiorly by a small portion of the left auricle.

External sulci

Internal partitions divide the heart into four chambers (i.e. , two atria and two ventricles) and produce surface or external grooves referred to as sulci.

• The coronary sulcus circles the heart, separating the atria from the ventricles {Fig. 4) . As it circles the heart, it contains the right coronary artery, the small cardiac vein, the coronary sinus, and the circumflex branch of the left coronary artery.
• The anterior and posterior interventricular sulci separate the two ventricles-the anterior interventricular sulcus is on the anterior surface of the heart and contains the anterior interventricular artery and the great cardiac vein, and the posterior interventricular sulcus is on the diaphragmatic surface of the heart and contains the posterior interventricular artery and the middle cardiac vein.

CARDIAC CHAMBERS

The heart functionally consists of two pumps separated by a partition (Fig.1). The right pump receives deoxygenated blood from the body and sends it to the lungs. The left pump receives oxygenated blood from the lungs and sends it to the body. Each pump consists of an atrium and a ventricle separated by a valve.

The thin-walled atria receive blood coming into the heart, whereas the relatively thick-walled ventricles pump
blood out of the heart.
More force is required to pump blood through the body than through the lungs, so the muscular wall of the left ventricle is thicker than the right.

Fig. 1 The Heart has two pumps

Right Atrium

In the anatomical position, the right border o f the heart is formed by the right atrium. This chamber also contributes to the right portion of the heart's anterior surface.

Blood returning to the right atrium enters through one of three vessels. These are:

• the superior and inferior venae cavae, which together deliver blood to the heart from the body; and
• the coronary sinus, which returns blood from the walls of the heart itself.

The superior vena cava enters the upper posterior portion of the right atrium, and the inferior vena cava and

coronary sinus enter the lower posterior portion of the right atrium.

From the right atrium, blood passes into the right ventricle through the right atrioventricular orifice. This opening faces forward and medially and is closed during ventricular contraction by the tricuspid valve.

The interior of the right atrium is divided into two continuous spaces. Externally, this separation is indicated by a shallow, vertical groove (the sulcus terminalis cordis) , which extends from the right side of the opening of the superior vena cava to the right side of the opening of the inferior vena cava. Internally, this division is indicated by the crista terminalis (Fig. 2) , which is a smooth, muscular ridge that begins on the roof of the atrium just in front of the opening of the superior vena cava and extends down the lateral wall to the anterior lip of the inferior vena cava.

The space posterior to the crista is the sinus of venae cavae and is derived embryologically from the right horn of the sinus venosus. This component of the right atrium has smooth, thin walls, and both venae cavae empty into this space.

The space anterior to the crista, including the right auricle, is sometimes referred to as the atrium proper.

This terminology is based on its origin from the embryonic primitive atrium. Its walls are covered by ridges called the musculi pectinati (pectinate muscles), which fan out from the crista like the "teeth of a comb . " These ridges are also found in the right auricle, which is an ear-like, conical, muscular pouch that externally overlaps the ascending aorta.

An additional structure in the right atrium is the opening of the coronary sinus, which receives blood

from most of the cardiac veins and opens medially to the opening of the inferior vena cava. Associated with these openings are small folds of tissue derived from the valve of the embryonic sinus venosus (the valve of the coronary sinus and the valve of inferior vena cava, respectively) . During development, the valve of the inferior vena cava helps direct incoming oxygenated blood through the foramen ovale and into the left atrium.

Separating the right atrium from the left atrium is the interatrial septum, which faces forward and to the right

because the left atrium lies posteriorly and to the left of the right atrium. A depression is clearly visible in the septum just above the orifice of the inferior vena cava. This is the fossa ovalis (oval fossa) , with its prominent margin, the limbus fossa ovalis (border of the oval fossa).

The fossa ovalis marks the location of the embryonic foramen ovale, which is an important part of fetal circulation. The foramen ovale allows oxygenated blood entering the right atrium through the inferior vena cava to pass directly to the left atrium and so bypass the lungs, which are nonfunctional before birth.

Finally, numerous small openings-the openings of the smallest cardiac veins (the foramina of the venae

cordis minimae)-are scattered along the walls of the right atrium. These are small veins that drain the myocardium directly into the right atrium.