The human heart is located within the thoracic cavity, medially between the lungs in the space known as the mediastinum. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity.
The dorsal surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages. The great veins, the superior and inferior venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the superior surface of the heart, called the base.
The base of the heart is located at the level of the third costal cartilage.The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly.
It is important to remember the position and orientation of the heart when placing a stethoscope on the chest of a patient and listening for heart sounds, and also when looking at images taken from a midsagittal perspective. The slight deviation of the apex to the left is reflected in a depression in the medial surface of the inferior lobe of the left lung, called the cardiac notch.
Chambers and Circulation through the Heart
The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle. Each of the upper chambers, the right atrium (plural = atria) and the left atrium, acts as a receiving chamber and contracts to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart, propelling blood to the lungs or to the rest of the body.
There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. Although both circuits transport blood and everything it carries, we can initially view the circuits from the point of view of gases. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation.
The blood exiting the systemic capillaries is lower in oxygen concentration than when it entered. The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava, which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle.
The right atrium (RA), lies anteriorly and receives blood from:
- The superior vena cava
- Inferior vena cava
- Coronary sinus
The interatrial septum forms the posteromedial wall of the RA. A shallow depression in this septum, the fossa ovalis is an embryonic remnant of the foramen ovale. Blood flows out of the RA during diastole through the tricuspid valve (TV) into the right ventricle.
The right ventricle (RV), on the surface of the heart, is separated from the RA by the right atrioventricular groove in which the right coronary artery lies.
The RV cavity is crescent shaped in cross-section and is lined by rough muscular ridges known as trabeculae. Specialized trabeculae form the three papillary muscles which attach to the tricuspid valve via the chordae tendineae. One of the trabeculae lies free within the RV cavity and carries the part of the cardiac conducting system known as the ‘moderator band’.
The RV receives blood from the right atrium via the tricuspid valve. Blood is then ejected through the pulmonary valve into the pulmonary artery during systole.
The left atrium (LA), lies directly behind the right atrium.
Posteriorly, it receives blood from the four pulmonary veins (namely, left upper and lower, and right upper and lower).
The left atrial cavity is smooth-walled, with the exception of the LA appendage which is trabeculated by pectinate muscles.
The left ventricle (LV), on the surface of the heart, is separated from the left atrium by the left atrioventricular groove in which the branches of the left coronary artery lie.
The LV is thick and muscular with a wall thickness approximately 3-4 times that of the RV wall.
The LV cavity is circular in cross-section, and has prominent trabeculae carnae.