The Anatomy of the Human Heart
The heart is a muscular four-chambered pump that beats constantly to keep blood flowing to the rest of your body. To make it’s workings easier to understand, clinical anatomy breaks the heart into three sections:
The pericardium is a fluid-filled sac that encloses the heart and the proximal part of the great vessels. It has several parts:
Fibrous pericardium: This outer covering of the pericardium attaches to the central tendon of the diaphragm.
Serous pericardium: This internal part has two layers: the parietal layer and the visceral layer.
The pericardial cavity lies between these two layers and contains a small amount of fluid.
Blood is supplied to the pericardium by the pericardiacophrenic arteries, which are branches of the internal thoracic arteries. Venous blood is drained by tributaries of the internal thoracic and azygos veins. The pericardium is innervated by the phrenic nerve, the vagus nerve, and branches of the sympathetic trunks.
Examining the surfaces of the heart
The heart is located in the middle of the thorax, with the apex facing toward the left and inferiorly, at the level of the 5th intercostal space. The base of the heart is the posterior part of the heart.
The heart has four surfaces:
Sternocostal surface: The anterior portion formed mostly by the right ventricle
Diaphragmatic surface: The inferior portion formed by the left ventricle and part of the right ventricle
Left pulmonary surface: Made up of the surface of the left ventricle
Right pulmonary surface: Made up by the surface of the right atrium
The heart has four ausculatory areas. Identifying these areas helps you know where to place the diaphragm of a stethoscope so you can hear the heart:
Aortic area: This area is located at the 2nd intercostal space to the right of the sternum.
Pulmonic area: This area is at the 2nd intercostal space, to the left of the sternum.
Tricuspid area: The tricuspid area is over the lower left part of the sternum.
Mitral area: This location is at the 5th intercostal space at the midclavicular line on the left side of the chest wall. This area is where the apex beat can be heard or palpated. The apex beat is created when the apex of the heart hits the thoracic wall when the ventricles contract.
Putting together the four chambers
The interior of the heart is divided into four separate chambers:
Atria: The two atria are called the right atrium and the left atrium. The right atrium receives venous blood from the superior vena cava, inferior vena cava, and the coronary sinus. The left atrium receives blood from the pulmonary veins.
The right atrium has a small pouch called the right auricle, which provides a little more room for blood. The smooth posterior part of the atrium, called the sinus venarum, is where the superior and inferior vena cava and the coronary sinus empty into the chamber. Pectinate muscles form a rough wall, and the interatrial septum lies between the left and right atria. The fossa ovalis is a depression in the wall of the septum. It’s the remnant of the foramen ovale, which is an opening between the two atria. The foramen ovale allows blood to pass from the right atrium to the left atrium of a developing fetus.
The left atrium forms a large part of the base of the heart. Just like the right atrium, it has pectinate muscles and an auricle (the left auricle, of course). It has a thicker wall than the right atrium, and it receives oxygenated blood from the four pulmonary veins.
Ventricles: When the atria contract, blood is forced into the left and right ventricles through atrioventricular orifices. The right ventricle forms the bulk of the anterior surface, and the left ventricle forms the apex and most of the diaphragmatic and left surfaces of the heart.
The right atrioventricular orifice is guarded by the right atrioventricular (tricuspid) valve, which consists of a fibrous ring and three cusps. Tendinous cords called chordae tendineae are attached to the cusps and to the papillary muscles of the ventricle. These cords hold the cusps in place during a ventricular contraction, preventing backflow of blood into the right atrium. The left atrioventricular orifice is guarded by a similar valve called the left atrioventricular (bicuspid; mitral) valve, which has two cusps instead of three.
The interior of the right ventricle is somewhat cone shaped. Its anterior portion, the conus arteriosus, is the location of the entrance to the pulmonary trunk. It’s controlled by the pulmonary (semilunar) valve, which consists of three cusps that keep blood from flowing back into the ventricle. The right ventricle has muscular elevations called the trabeculae carneae and a muscular ridge called the supraventricular crest. Papillary muscles form conical projections.
The interior of the left ventricle has a longer conical cavity, and the walls are much thicker than in the right ventricle. The trabeculae carneae and papillary muscles are larger than those found in the right ventricle. During ventricular contractions, blood is pumped through the aortic (semilunar) valve into the aorta. The aortic valve prevents backflow of blood into the left ventricle.
The ventricles are separated by an interventricular septum. The septomarginal trabecula is a muscular bundle that runs from the interventricular septum to the anterior papillary muscle of the right ventricle.