Anatomy of the Heart: Mediastinum

Location of the Heart

• The heart is located in the mediastinum, a broad central partition that separates the two pleural cavities.
• The mediastinum is subdivided into several smaller regions: superior mediastinum, inferior mediastinum (anterior, middle, and posterior).

Pericardium

• The pericardium is a fibroserous sac surrounding the heart and the roots of the great vessels.
• It consists of two components: fibrous pericardium (tough connective tissue outer layer) and serous pericardium (thin, consisting of two parts: parietal layer and visceral layer).
• The fibrous pericardium defines the boundaries of the middle mediastinum.
• The serous pericardium lines the inner surface of the fibrous pericardium (parietal layer) and adheres to the heart, forming its outer covering (visceral layer).

Orientation and Surfaces of the Heart

• The heart is a pyramid-shaped organ with its apex pointing forward, downward, and to the left.
• The base of the heart is opposite the apex and faces posteriorly.
• The heart has four surfaces: anterior (sternocostal), diaphragmatic (inferior), right pulmonary, and left pulmonary.
• The anterior surface consists mostly of the right ventricle with some of the right atrium on the right and some of the left ventricle on the left.
• The diaphragmatic surface consists of the left ventricle and a small portion of the right ventricle.

External Sulci

• Internal partitions divide the heart into four chambers, producing surface or external grooves referred to as sulci.
• The coronary sulcus circles the heart, separating the atria from the ventricles.
• The interventricular sulcus (anterior and posterior) separates the right and left ventricles.

Right Atrium

• The right atrium consists of a main cavity and a small outpouching, the auricle.
• The sulcus terminalis is a vertical groove on the outside of the heart, forming a ridge, the crista terminalis, on the inside.
• The main part of the atrium is smooth walled, while the part in front of the ridge is roughened or trabeculated by bundles of muscle fibers.

Openings into the Right Atrium

• The superior vena cava opens into the upper part of the right atrium without a valve.
• The inferior vena cava (larger than the superior vena cava) opens into the lower part of the right atrium with a rudimentary, nonfunctioning valve.
• The coronary sinus opens into the right atrium between the inferior vena cava and the atrioventricular orifice, guarded by a rudimentary, nonfunctioning valve.

Right Ventricle

• The walls of the right ventricle are thicker than those of the right atrium, showing internal projecting ridges formed of muscle bundles.
• The projecting ridges give the ventricular wall a spongelike appearance and are known as trabeculae carneae.
• The tricuspid valve guards the atrioventricular orifice, consisting of three cusps (anterior, septal, and inferior).
• The pulmonary valve guards the pulmonary orifice, consisting of three semilunar cusps.

Left Atrium and Left Ventricle

• The left atrium is similar to the right atrium, consisting of a main cavity and a left auricle.
• The left atrium is situated behind the right atrium, forming the greater part of the base or posterior surface of the heart.
• The four pulmonary veins open into the posterior wall of the left atrium without valves.
• The left atrioventricular orifice is guarded by the mitral valve, consisting of two cusps (anterior and posterior).
• The left ventricle communicates with the left atrium through the atrioventricular orifice and with the aorta through the aortic orifice.

Blood Supply of the Heart

• The heart is supplied by the coronary arteries, which arise from the aorta.
• The coronary arteries are divided into two main branches: the right coronary artery and the left coronary artery.

Nerve Supply of the Heart

• The heart is innervated by sympathetic and parasympathetic fibers of the autonomic nervous system via the cardiac plexuses.
• The cardiac plexuses are situated below the arch of the aorta.
• The sympathetic supply arises from the cervical and upper thoracic portions of the sympathetic trunks.
• The parasympathetic supply comes from the vagus nerves.

Great Vessels

• The great vessels of the heart function to carry blood to and from the heart.
• The great vessels of the thorax are the pulmonary trunk, aorta, superior vena cava, inferior vena cava, and pulmonary veins.

Surface Anatomy of the Heart

• The heart has an apex and four borders: superior, right, left, and inferior.
• The apex is formed by the left ventricle and corresponds to the apex beat.
• The surface markings of the heart valves are as follows:
  • The tricuspid valve lies behind the right half of the sternum opposite the fourth intercostal space.
  • The mitral valve lies behind the left half of the sternum opposite the fourth costal cartilage.