Position Of The Heart In The Thorax

The heart lies in the protective thorax, posterior to the sternum and costal cartilages, and rests on the superior surface of the diaphragm. The thorax is often referred to as the thoracic cage because of its protective function of the delicate structures within. The heart is located between the two lungs, which occupy the lateral spaces, called the pleural cavities. The space between these two cavities is referred to as the mediastinum ("that which stands in the middle"; Fig. 1).

The mediastinum is divided first into the superior and inferior mediastinum by a midsagittal imaginary line called the transverse thoracic plane. This plane passes through the sternal angle (junction of the manubrium and body of the sternum) and the space between thoracic vertebrae T4 and T5. This plane acts as a convenient landmark because it also passes through the following structures: the bifurcation of the trachea, the superior border of the pericardium, the base of the aorta, and the bifurcation of the pulmonary trunk.

The human heart assumes an oblique position in the thorax, with two-thirds to the left of midline (Figs. 2 and 3). The heart is roughly in a plane that runs from the right shoulder to the left nipple. The base is located below the third rib as it approaches the sternum (note that the sternal angle occurs at the level of the second rib). The base is directed superiorly to the right of midline and posterior. The pointed apex projects to the left of midline and anterior. Thus, the heartbeat can be most easily palpated between the fifth and sixth ribs (just inferior to the left nipple)

from the apex of the heart where it comes into contact with the thoracic wall. Importantly, the heart lies in such an oblique plane that it is often referred to as horizontal. Thus, the anterior side is often referred to as superior and the posterior side as inferior.

Again, the heart is composed of four distinct chambers. There are two atria (left and right) responsible for collecting blood and two ventricles (left and right) responsible for pumping blood. The atria are positioned superior to (posterior to) and to the right of their respective ventricles (Fig. 3). From superior to inferior, down the anterior (superior) surface of the heart runs the anterior interventricular sulcus ("a groove"). This sulcus separates the left and right ventricles. The groove continues around the apex as the posterior interventricular sulcus on the posterior (inferior) surface. Between these sulci, located within the heart, is the interventricular septum ("wall between the ventricles"). The base of the heart is defined by a plane that separates the atria from the ventricles, called the atrioventricular groove or sulcus. This groove appears like a belt cinched around the heart. Because this groove appears as though it might also be formed by placing a crown atop the heart, the groove is also called the coronary (corona = "crown") sulcus. The plane of this sulcus also contains the atrioventricular valves (and the semilunar valves) and a structure that surrounds the valves called the cardiac skeleton. The interatrial ("between the atria") septum is represented on the posterior surface of the heart as the atrial sulcus. Also on the posterior (inferior) side of the heart, the crux cordis ("cross of the heart") is formed from the interatrial sul-cus, posterior interventricular sulcus, and the relatively perpendicular coronary sulcus.

Note that the great arteries, aorta and pulmonary trunk, arise from the base of the heart. The right and left atrial appendages (or auricles, so named because they look like dog ears; auricle = "little ear") appear as extensions hanging off each atria.

The anterior (superior) surface of the heart is formed primarily by the right ventricle. The right lateral border is formed by the right atrium and the left lateral border by the left ventricle. The posterior surface is formed by the left ventricle and the left atrium, which is centered equally on the midline.

The acute angle found on the right anterior side of the heart is referred to as the acute margin of the heart and continues toward the diaphragmatic surface. The rounded left anterior side is referred to as the obtuse margin of the heart and continues posteriorly and anteriorly. Both right and left ventricles contribute equally to the diaphragmatic surface, lying in the plane of the diaphragm.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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