The Human Heart

The ceaselessly beating heart in the thorax has intrigued people for thousands of years. The ancient Greeks believed the heart was the seat of intelligence. Others thought it was the source of emotions. Although these theories have proved false, emotions certainly affect heart rate. Only when your heart pounds or occasionally skips a beat do you become acutely aware of this dynamic organ. The heart is a muscular double pump with two functions.
  1. Its right side receives oxygen-poor blood from the body tissues and then pumps this blood to the lungs to pick up oxygen and dispel carbon dioxide. The blood vessels that carry blood to and from the lungs form the pulmonary circuit (pulmonos = lung).
  2. Its left side receives the oxygenated blood returning from the lungs and pumps this blood throughout the body to supply oxygen and nutrients to the body tissues. The vessels that transport blood to and from all body tissues and back to the heart form the systemic circuit.



The heart has two receiving chambers, the right atrium and left atrium (atrium= entranceway), that receive blood returning from the systemic and pulmonary circuits. The heart also has two main pumping chambers, the right ventricle and left ventricle (“hollow belly”), that pump blood around the two circuits.

Location and Orientation within The Thorax

The heart’s modest size belies its incredible strength and durability. Only about the size of a fist, this hollow, cone-shaped organ looks enough like the popular valentine image to satisfy the sentimentalists among us. Typically it weighs between 250 and 350 grams-less than a pound.

The heart lies in the thorax posterior to the sternum and costal cartilages and rests on the superior surface of the diaphragm. It is the largest organ in the mediastinum, which is the region between the two lungs (and pleural cavities). The heart assumes an oblique position in the thorax, with its pointed apex lying to the left of the midline and anterior to the rest of the heart.

If you press your fingers between the fifth and sixth ribs just inferior to the left nipple, you may feel the beating of your heart where the apex contacts the thoracic wall. Cone-shaped objects have a base as well as an apex, and the heart’s base is its broad posterior surface.

The heart is said to have four corners defined by four points projected onto the anterior thoracic wall. The second rib is easily palpated just lateral to the sterna angle. Use this landmark to help you locate the four corners of the heart.
  • The superior right point lies where the costal cartilage of the third rib joins the sternum.
  • The superior left point lies at the costal cartilage of the second rib, a finger’s breadth lateral to the sternum.
  • The inferior right point lies at the costal cartilage of the sixth rib, a finger’s breadth lateral to the sternum.
  • Finally, the inferior left point (the apex point) lies in the fifth intercostal space at the midclavicular line-that is, at a line extending inferiorly from the midpoint of the left clavicle.






The imaginary lines that connect these four corner points delineate the normal size and location of the heart. Clinicians must know these normal landmarks, because an enlarged or displaced heart as viewed on an X-ray or other medical image can indicate heart disease or other disease conditions.

Structure of the Heart

Coverings

The pericardium (“around the heart”) is a triple-layered sac that encloses the heart. The outer layer of this sac, called the fibrous pericardium, is a strong layer of dense connective tissue. It adheres to the diaphragm inferiorly, and superiorly it is fused to the roots of the great vessels that leave and enter the heart. The fibrous pericardium acts as a tough outer coat that holds the heart in place and keeps it from overfilling with blood.

Deep to the fibrous pericardium is the double-layered serous pericardium, a closed sac sandwiched between the fibrous pericardium and the heart (“Serous Cavities”). The outer, parietal layer of the serous pericardium adheres to the inner surface of the fibrous pericardium. The parietal layer is continuous with the visceral layer of the serous pericardium, or epicardium, which lies on the heart and is considered a part of the heart wall (discussed shortly). Between the parietal and visceral layers of serous pericardium is a slitlike space, called the pericardial cavity, a division of the embryonic coelom. The epithelial cells of the serous pericardium that line the pericardial cavity produce a lubricating film of serous fluid into the pericardial cavity. This fluid reduces friction between the beating heart and the outer wall of the pericardial sac.

Layers of the Heart Wall

The wall of the heart has three layers: a superficial epicardium, a middle myocardium, and a deep endocardium. All three layers are richly supplied with blood vessels.
  • The epicardium (“upon the heart”) is the visceral layer of the serous pericardium, as previously mentioned. This serous membrane is often infiltrated with fat, especially in older people.
  • The myocardium (“muscle heart”) forms the bulk of the heart. It consists of cardiac muscle tissue and is the layer that actually contracts. Surrounding the cardiac muscle cells in the myocardium are connective tissues that bind these cells together into elongated, circularly and spirally arranged networks called bundles. These bundles function to squeeze blood through the heart in the proper directions: inferiorly through the atria and superiorly through the ventricles. The connective tissues of the myocardium form the cardiac skeleton, which reinforces the myocardium internally and anchors the cardiac muscle fibers.
  • The endocardium (“inside the heart”), located deep to the myocardium, is a sheet of simple squamous epithelium resting on a thin layer of connective tissue. Endocardium lines the heart chambers and covers the heart valves.

Heart Chambers

The four heart chambers are the right and left atria (singular, atrium) superiorly, and the right and left ventricles inferiorly. Internally, the heart is divided longitudinally by a partition called the interatrial septum between the atria and the interventricular septum between the ventricles. Externally, the boundaries of the four chambers are marked by two grooves. The coronary sulcus extends horizontally, circling the boundary between the atria and the ventricles (corona= crown). The anterior interventricular sulcus extends vertically, marking the anterior position of the interventricular septum, and the posterior interventricular sulcus  separates the two ventricles on the heart’s inferior surface. Recall that the heart is oriented obliquely within the thorax; the “posterior” of the heart lies against the diaphragm and is thus its inferior surface.

Right Atrium            

The right atrium forms the entire right border of the human heart. It is the receiving chamber for oxygen-poor blood returning from the systemic circuit. The right atrium receives blood via three veins: the superior vena cava and inferior vena cava and the coronary sinu. Externally, the right auricle, a small flap shaped like a dog's ear (auricle = little ear) projects anteriorly from the superior corner of the atriu. Internally, the right atrium has two parts: a smooth-walled posterior part and an anterior part lined by horizontal ridges called the pectinate muscles (pectin = comb). These two parts of the atrium are separated by a large, C-shaped ridge called the crista terminalis (“terminal crest”). The crista is an important landmark in locating the sites where veins enter the right atrium: The superior vena cava opens into the atrium just posterior to the superior bend of the crista; the inferior vena cava opens into the atrium just posterior to the inferior bend of the crista; and the coronary sinus opens into the atrium just anterior to the inferior end of the crista. Additionally, just posterior to this end of the crista is the fossa ovalis, a depression in the interatrial septum that marks the spot where an opening existed in the fetal heart: the foramen ovale Inferiorly and anteriorly, the right atrium opens into the right ventricle through the tricuspid valve (right atrioventricular valve).

Right Ventricle

The right ventricle forms most of the anterior surface of the heart. It receives blood from the right atrium and pumps it into the pulmonary circuit via an artery called the pulmonary trunk. Internally, the ventricular walls are marked by irregular ridges of muscle called trabeculae carneae (“little beams of flesh”). Additionally, cone-shaped papillary muscles project from the walls into the ventricular cavity (papilla=nipple). Thin, strong bands called chordae tendineae (“tendinous cords”), the “heart strings,” project superiorly from the papillary muscles to the flaps (cusps) of the tricuspid (right atrioventricular) valve. Superiorly, the opening between the right ventricle and the pulmonary trunk contains the pulmonary semilunar valve (or simply, pulmonary valve).

Left Atrium

The left atrium makes up most of the heart’s posterior surface, or base. It receives oxygen-rich blood returning from the lungs through two right and two left pulmonary veins. The only part of the left atrium visible anteriorly is its triangular left auricle. Internally, most of the atrial wall is smooth, with pectinate muscles lining the auricle only. The left atrium opens into the left ventricle through the mitral valve (left atrioventricular valve).

Left Ventricle


The left ventricle forms the apex of the heart and dominates the hearts inferior surface. It pumps blood into the systemic circuit. Like the right ventricle, it contains trabeculae carneae, papillary muscles, chordate tendineae, and the cusps of an atrioventricular (mitral) valve. Superiorly, the left ventricle opens into the stem artery of the systemic circulation (the aorta) through the aortic semilunar valve (or simply, aortic valve).