Surface projections of the heart

Borders of the heart

The heart is placed within the middle mediastinum. Laterally and posteriorly it is surrounded by the lungs, while the sternum is located anteriorly. The superior border, or base of the heart, is the portion located opposite the apex where the big blood vessels enter and leave the heart. These aspects explain why the heart and great vessels project onto the middle of the thorax.

Even though the position of the heart can vary depending on the physical build and the position of the diaphragm, we can always identify its borders by following these heart landmarks:

• The superior border of the heart is a line that connects the inferior border of the second left costal cartilage and the superior border of the third right costal cartilage.
• The right border of the heart corresponds to a line that is convex to the right, which extends from the third right costal cartilage to the sixth right costal cartilage.
• The inferior border is marked by a line that joins the inferior end of the right border (six right costal cartilage) to the point where the fifth left intercostal space and midclavicular line intersect. Also, this point of intersection marks the apex of the heart.
• The left border corresponds to a line drawn from the inferior border of the second left costal cartilage to the intersection point between the fifth left intercostal space and midclavicular line. More simply, the line interconnects the left ends of the superior and inferior borders of the heart.

Learn more about heart anatomy using the following resources:

Heart valves

Heart valves are placed between the heart chambers and at the roots of great vessels. Their function is to force blood to flow in only one direction, preventing backflow (regurgitation) due to intra-arterial pressure. In that way, the direction of blood flow is always constant and unidirectional throughout the human body. Therefore, valves are extremely important! Next time you hear an abnormal sound during valve auscultation, realize the impact of the pathology on the patient's entire circulatory system.. 

There are four existing heart valves: 

• The left atrioventricular (mitral) valve between the left atrium and left ventricle. It projects posteriorly to the left side of the sternum, at the level of the left fourth costal cartilage.
• The right atrioventricular (tricuspid) valve between the right atrium and right ventricle.  It is located posteriorly to the right side of the sternum at the level of the right fourth costal cartilage.
• The aortic valve between the left ventricle and aorta. One can find it at the level of the third intercostal space, posterior to the left side of the sternum.
• The pulmonary valve between the right ventricle and pulmonary trunk, projects at the junction of the sternum and  left third costal cartilage.

Already mastered the heart anatomy? Take a quiz to test your knowledge and find out!

Heart auscultation

To complicate matters, the auscultatory areas and the actual locations of the valves are quite different. Valves can be heard away from their anatomical position, further down the stream of the blood. This is because the loudest sound is produced only after the ejected blood passes through the valve. We can hear this sound as the bloodstream ejected during systole hits the vascular wall. As a result, the auscultatory areas of the heart valves are the following:

• Left atrioventricular valve - the left fifth intercostal space at the midclavicular line. Since this auscultation point lies over the apex of the heart, you can simultaneously hear the latter, too. The cardiac apex also represents the point of maximal impulse of the heart. This is where the cardiac impulse can be best palpated during the cardiovascular physical exam.
• Right atrioventricular valve - left fifth intercostal space, parasternally
• Aortic valve - right second intercostal space, parasternally
• Pulmonary valve - left second intercostal space, parasternally

Learn more about heart valve anatomy and valvular heart disease here:

The sound heard during auscultation is caused by the closing of the valves, when they audibly snap shut due to a sudden change of pressure. The normal heartbeat heard with a stethoscope sounds something like: “lub-dub, lub-dub”. The “lub” corresponds to the closure of the atrioventricular valves at the beginning of ventricular systole. It is called the first heart sound (S1). On the other hand, the “dub” is associated with the closure of the aortic and pulmonary valves at the end of the ventricular systole. This is called the second heart sound (S2) and it is best auscultated at the Erb’s point. This landmark is located on the left third intercostal space, parasternally, between the pulmonic and tricuspid areas.

In addition to the physiological S1 and S2 sounds, additional ones can occasionally be heard. These include splits, third (S3) and fourth (S4) heart sounds, as well as murmurs. S3 and S4 are associated with diastole and the cycle of the atria. They are usually physiological in children and skinny adults, but pathological in the general adult population. On the other hand, murmurs are always a sign of cardiovascular dysfunction.

Test your knowledge on the valves of the heart in this interactive quiz.

Heart imaging

So far, we’ve learned about the surface projections of the heart and how this organ is heard when a stethoscope is placed over the thorax. It’s now time to understand how these anatomical cardiac borders and features appear on standard chest X-rays. Such radiological images depict a 2D image of the thoracic wall and contents, including the heart. The X-ray can be performed in the anteroposterior direction (AP), posteroanterior (PA) or from either lateral perspective (left, right).

In a PA image of the thorax, the cardiac silhouette, or shadow, is placed in the middle of the thoracic cavity. The silhouette has two margins, left and right.

The right margin of the heart consists of two lateral convex arches separated by an obtuse angle. In adults, the lower arch belongs to the right atrium, while the upper arch represents the ascending aorta. However, in children, the upper arch is actually flat and is formed by the superior vena cava. This difference originates from the fact that as we get older, the ascending aorta becomes elongated and slightly dilated. These changes are due to the high pressure bloodstream ejected by the left ventricle hitting the walls of the aorta. The right atrium makes an acute angle with the diaphragm, which is called the right cardiophrenic angle.

A sharp knowledge of heart anatomy is essential to understand its projections and imaging. Learn the anatomy of this organ in a fun, engaging and effective way using Kenhub's diagrams, quizzes and labelling worksheets of the heart.

The left margin of the cardiac silhouette has two convex arches separated by a concavity. The upper convex arch, called the aortic knob, is the place where the aortic arch continues as the descending aorta. The concave arch corresponds with the pulmonary trunk and left pulmonary artery. The lower convex arch marks the left ventricle and it ends by merging with the diaphragmatic silhouette. This forms the left cardiophrenic angle. Note that the most lateral point of the lower arch never passes over the left midclavicular line under physiological conditions.

Learn more about normal chest x-ray reading and test your knowledge with a quiz!