12 Adult Heart and Pericardium

Furthermore, the parietal layer of the serous pericardium–and the fibrous pericardium to which it is firmly applied–is a simple bag like shape; but the visceral pericardium–firmly applied to the surface of the heart–follows all its changes in shape. (The visceral serous pericardium, when considered as “part of the heart” rather than as “part of the pericardium”, is called the epicardium.)

PERICARDIAL SINUSES (Netter 212; Moore Fig. 1.46)

Within the pericardial cavity, at the points where the visceral and parietal pericardia are continuous with one another, small chambers or sinuses are located. In Netter 209, and Moore 1.46, the heart has been removed and you are looking toward the posterior wall of the pericardial cavity, the TRANSVERSE PERICARDIAL SINUS (posterior to the aorta and pulmonary trunk; the upper arrow in the cavity) and the OBLIQUE PERICARDIAL SINUS (inferior to and surrounded by the various veins; the tip of the lower arrow in the cavity). These sinuses are simply portions of the pericardial cavity with a distinctive shape that have been dignified with special names.

Borders of the Heart

The anterior surface of the heart is also known as the sternocostal surface for obvious reasons. In Netter 209, notice the ruffled edges of the left (LA) and right (RA) atria. These are the ones to use for orientation.

Note that the anterior surface shows parts of each of the four chambers of the heart:

• right atrium (RA)
• left atrium (LA)
• right ventricle (RV)
• left ventricle (LV)

Also note the three borders of the heart:

• right border – made up of the right atrium
• inferior border – made up of right atrium, right ventricle and left ventricle
• left border – made up of the left ventricle

On Netter 206, also identify the great vessels of the heart:

• superior and inferior venae cavae
• pulmonary trunk and left and right branches
• pulmonary veins (usually 4 in number but this can vary)
• ascending aorta

The last item to identify is the remains of the embryonic connection between the pulmonary trunk and aortic arch, the ligamentum arteriosum. Prior to birth, the lungs were not functional so the blood was shunted into the arterial system at this site. Oxygen exchange in the embryo occurred at the placenta and not the lungs.

On a posterior view of the heart (Netter 211) identify:

• right atrium (RA)
• left atrium (LA)
• right ventricle (RV)
• left ventricle (LV). Notice that most of the left ventricle is posterior.

The left and right ventricles make up the diaphragmatic surface of the heart. This part rests on the fibrous part of the diaphragm.

The left atrium makes up the so-called base of the heart. When the body is in the supine position (lying on its back), the heart rests on its base and the apex of the heart (the tip of the left ventricle) projects up and to the left. The same three borders are seen from the back of the heart: right, inferior and left

Coronary Sulci

When the coronary vessels are removed from the heart, certain sulci (grooves) can be seen and separated the various chambers of the heart. From the anterior view of the heart (Netter 209)., the anterior interventricular and coronary sulci can be seen. The anterior interventricular sulcus separates the right and left ventricles. The anterior part of the coronary sulcus separates the right atrium from the right ventricle.

From the posterior view of the heart (Netter 211), the posterior part of the coronary sulcus and the posterior interventricular sulcus can be seen. From this view, the coronary sulcus can be seen to separate the left and right atria from the left and right ventricles. The posterior interventricular sulcus separated the right ventricle from the left ventricle and if followed inferiorly, it can be seen to be almost continuous with the anterior interventricular sulcus.

Cardiac tamponade (Netter 209) is an acute compression of the heart caused by a rapid accumulation of fluid or blood in the pericardial cavity from wounds to the heart or pericardial effusion. tamponade can be treated by pericardiocentesis. It causes compression of venous return to the heart, resulting in decreased diastolic capacity (ventricular filling), reduced cardiac output with an increased heart rate, increased venous pressure with jugular venous distension and peripheral edema.

Pericardial effusion is an accumulation of fluid in the pericardial space resulting from inflammation caused by acute pericarditis and the accumulated fluid compresses the heart, inhibiting cardiac filling. It has signs of an enlarged heart, a water-bittle appearance in the cardiac silhouette , faint heart sounds and diminished apex beat.

Pericardiocentesis is a surgical puncture of the pericardial cavity for the aspiration of fluid which is necessary to relieve pressure of accumulated fluid on the heart. A needle is inserted into the pericardial cavity through the fifth intercostal space to the left of the sternum. Because of the cardiac notch, the needle misses the pleurae and lungs but penetrates the pericardium. Click here to view a video (from the New England Journal of Medicine) demonstrating the procedure of an emergency pericardiocentesis.

 

OVERVIEW OF THE ANATOMY OF THE HEART (Moore 135-160)

THE ATRIA (Netter 217, 218)

The heart consists of four chambers, right and left atria and right and left ventricles (Radiography of the Chest – Netter 210). The atria (singular atrium) are essentially posterior structures of the heart (the left, much more so than the right) but they both have small structures which are truly anterior — the LEFT and RIGHT AURICLES (they look like ears, hence the name).  The adult auricles are actually formed from the primitive atria; most of the adult atria being formed by gradual incorporation of the walls of the veins draining into them by a process known as intussusception.

(Radiography of the Chest – Netter 210)

One other area of clinically important detail is the interatrial septum, particularly its right aspect. You will remember that, due to the “rotation” of the heart to the left, the right surface of the interatrial septum faces mostly anterior. note: — the smooth posteromedial surface and rough anterolateral and auricular surfaces, as explained above. The FOSSA OVALIS is the site of former communication between the two atria, the foramen ovale, that exists in the fetus. This opening is normally closed in the adult, but in some forms of congenital heart defects, it is open, and allows mixing of the oxygenated and unoxygenated blood in the two atria, to the detriment of the child. — the CORONARY SINUS, which is the last conduit for blood which supplies the heart muscle, empties this blood back into the right atrium.

 

And finally, appreciate that on horizontal section of the adult heart, the chambers are arranged in the following manner. Understanding this is crucial to interpreting the various X-ray views of the heart.

The right atrium has relatively smooth-walled muscles, except for the presence of pectinate muscles. It is larger than the left atrium, but has a thinner wall.

The Sinus venarum is a posteriorly situated, smooth-walled area that is separated from the more muscular atrium proper by the crista terminalis. It develops from the embryonic sinus venosus and receives the superior vena cava, inferior vena cava, coronary sinus and anterior cardiac veins.

The pectinate muscles are prominent ridges of atrial myocardium located in the interior of both auricles and the right atrium. The crista terminalis is a vertical muscular ridge running anteriorly along the right atrial wall from the opening of the superior vena cava to the opening of the inferior vena cava, providing the origin of the pectinate muscles. It represents the junction between the primitive sinus venarum and the right atrium proper and is indicated externally by the sulcus terminalis.

The left atrium is smaller and has thicker walls than the right atrium, but its walls are smooth, except for a few pectinate muscles in the auricle. It is the most posterior of the four chambers lying anterior to the esophagus and shows no structural borders on a posterioranterior radiograph. It receives oxygenated blood through four pulmonary veins.

THE VENTRICLES (Netter 217, 218)

The ventricles are the working part of the heart, and it is their vigorous contractions that move the blood through the pulmonary (from the right ventricle to the pulmonary artery) and the systemic (from the left ventricle to the aorta) circulations.

The walls of most of both ventricles are trabeculated (covered with small muscular ridges of myocardium known as trabeculae carnae,  – analogous to the pectinate muscle sof the atria). Separating the two ventricular chambers is the interventricular septum.

All the muscle of the heart is known collectively as the myocardium. On its inside surface — that surface exposed to the blood — is the thin layer of the endocardium; on the outside of the heart is the thin layer of the epicardium. The myocardial fibers are arranged in spirals so that the blood is not simply expressed from a shrinking bag, but rather wrung out of a twisting, shrinking ventricle.

The LEFT VENTRICLE works against the high systolic pressure of the systemic circulation (which is normally 120 mmHg) and its wall is quite thick, usually about 1.5 cm. Its cavity is conical in shape and lies posterior and to the left of the right ventricle. The RIGHT VENTRICLE, which works against the relatively low systolic pressure of the pulmonary circulation (normally about 25 mmHg) is only 1/3 as thick as the left. Its cavity is flattened in a crescentric shape against the convexly bulging septum, and lies anterior and to the right of the left ventricle.

TRICUSPID AND MITRAL VALVES (Netter 217, 218, 219, 220, 221)

The heart valves come in two sets of two; the two atrioventricular valves (so called because they’re in between the atria and the ventricles) and the semilunar valves (so-called because of the shape of the free edges of the valve cusps).

The A-V VALVES are both large valves whose job is to prevent blood from rushing back into the atria upon ventricular contraction. The free edges of the valve cusps protrude into the ventricles, and are prevented from prolapsing back into the atria by their firm attachment to the ventricular wall through the CHORDAE TENDINEAE and the PAPILLARY MUSCLES.

In the pictures below, note that the papillary muscles are named roughly after whatever surface of the ventricular chamber that they arise from- anterior wall (A), posterior wall (P), or septum (S).

The right A-V valve is the TRICUSPID, and it has three cusps (= leaflets): ANTERIOR, POSTERIOR, and SEPTAL; and three corresponding papillary muscles: ANTERIOR, POSTERIOR, and SEPTAL. The left A-V valve is the MITRAL, and it has two cusps (something like a bishop’s hat or mitre)- ANTERlOR and POSTERIOR.

 

Clinical Note: Certain diseases can result in scarring and adhesion at the commissures particularly in the mitral valve. This results in a narrowing of the valves effective orifice, or STENOSIS, which can have severe effects on the dynamics of blood flow through the heart. Mitral valve prolapse is a condition in which the valve everts into the left atrium and thus fails to close properly when the left ventricle contracts. It may produce chest pain, shortness of breath, palpitations and cardiac arrhythmia. A cardiac murmor is a characteristic sound generated by turbulence of blood through an orifice of the heart.

SEMILUNAR VALVES

The SEMILUNAR VALVES are two smaller valves located in the arterial outflow vessels which prevent blood in the arteries from rushing back into the ventricles when the ventricles relax. The semilunar valve of the right heart is the PULMONIC valve and the semilunar valve of the left heart is the AORTIC valve. Both have three cusps as shown on the next page.

Notice that the S-L valves differ from the A-V valves in their mechanism. The A-V valves are big and floppy, and require the firm attachment to the ventricular wall. The S-L valves are small and with arterial back pressure, the lunules are forced against each other by the blood filling the sinuses.

 

SKELETON OF THE HEART (Netter 219, 220; Moore 136)

If the valves just sat in a ring of muscle, the bases of the cusps would move all around as that muscle contracted and relaxed, and the valves would leak badly. The heart therefore has a  tough fibrous skeleton of connective tissue — which is nothing more than four interconnected rings, one for each valve, that give the valve cusps firm and unvarying foundation. All of the heart musculature arises from and inserts into these fibrous rings, thus making them truly a skeleton.

The coronary vessels (Netter 214, 215, 216) form the all-important vasculature which supplies the heart muscle. The myocardium is quite sensitive to the lack of oxygen, and if a portion is deprived of oxygenated blood (= ischemia) by the occlusion of part of the coronary circulation, it soon stops contracting and within 15 minutes is irreversibly damaged, and dies (the heart attack, or myocardial infarct).  Only the brain is more sensitive than the heart to the lack of oxygen, and it is irreversibly damaged by anoxia in about 4 minutes). Since about 25% of the deaths annually in the United States are by fatal heart attacks, this is a crucial bit of anatomy. Luckily, it is very simple.

The CORONARY ARTERIES are usually two, a left and a right. They arise from the corresponding left and right aortic sinuses.

There are three major CORONARY VEINS (Netter 211); the small, middle, and great cardiac veins, which all empty into the CORONARY SINUS–found on the posterior aspect of the heart–which then drains into the right atrium.

Coronary refers to arterial vessels whereas cardiac refers to venous vessels.

The right and left coronary arteries arise from right and left coronary sinuses of the aortic valve .

The left coronary artery (Netter 214, 215, 216)

• passes anterior between the pulmonary trunk and the tip of the auricle of the left atrium. It divides on the anterior aspect of the heart into the anterior interventricular branch (1) and the circumflex branch (2).
• (1) descends in the anterior interventricular sulcus to the inferior margin of the heart and continues into the posterior interventricular sulcus on the diaphragmatic surface. It supplies the anterior aspects of the right and left ventricles and the anterior 2/3 of the interventricular septum.
• (2) runs to the left in the atrioventricular sulcus between the left atrium and ventricle. It gives off a marginal branch for the lateral margin of the left ventricle and continues onto the posterior aspect of the heart. It forms an anastomosis with the arteries (derived from the right coronary artery) in the posterior interventricular sulcus.

The right coronary artery (Netter 214, 215, 216)

• runs in the coronary sulcus between the right atrium and right ventricle.
• gives off a nodal artery which passes onto the posterior aspect of the right atrium and supplies the area of the sinoatrial (SA) node.
• then gives off a marginal branch (smaller than the left one) to supply the right ventricle.
• terminates in the posterior interventricular sulcus as the posterior interventricular artery, supplying mainly the posterior aspect of the right and left ventricles as well as the posterior 1/3 of the interventricular septum.

The cardiac veins (Netter 214)

• accompany coronary arteries and their branches.
• lie superficial to the arteries in the sulci.
• Most of the veins drain into the coronary sinus.

The coronary sinus

• is derived from the left sinus venosus (the primitive receiving chamber of the developing heart).
• lies in the coronary sulcus between the left margin of the heart and the posterior interventricular sulcus. It drains into the right atrium by an opening to the left of the entrance of the inferior vena cava.

The great cardiac vein

• forms in the anterior interventricular sulcus and travels with the anterior interventricular (left anterior descending artery)
• joins the coronary sinus near the left margin of the heart.

The middle cardiac vein

• occupies the posterior interventricular sulcus and travels with the posterior interventricular artery
• enters the coronary sinus near the entrance to the right atrium.

The small cardiac vein

• follows the right marginal branch of the right coronary artery
• joins the coronary sinus near the junction with the middle cardiac vein.

The oblique vein drains from the left atrium into the coronary sinus along with posterior ventricular veins draining the diaphragmatic surface of the left ventricle.

The anterior cardiac veins

• drain the anterior surface of the right ventricle.
• open directly into the right atrium.

Venae cordis minimae or thebesian veins

• are tiny veins draining the heart wall.
• open directly into the chambers.

Clinical Considerations:

Myocardial infarction is a necrosis of the myocardium because of local ischemia resulting from vasospasm or obstruction of the blood supply, most commonly by a thrombus in the coronary arteries. Symptoms are severe chest pain or pressure for a prolonged period (more than 30 minutes), congestive heart failure, and murmur of mitral regurgitation. Angina pectoris is characterized by attacks of chest pain originating in the heart and felt beneath the sternum, in many cases radiating to the left shoulder and down the arm. It is caused by insufficient supply of oxygen to the heart muscle because of coronary artery disease or exertion.

 

A cross section of the heart at the T8 level can be viewed by clicking here

 

STERNALCOSTAL PROJECTIONS OF HEART

   Within the thoracic cavity, the heart forms the following surface relationship outlines:

Base –  left side – 3rd costal cartilage  right side – 3rd intercostal space

Right margin –  right border of sternum

Inferior margin – right side – 6th costal cartilage to   5th interspace on (apex of heart)

Left margin  left 5th interspace (apex) to   3rd costal cartilage on the left

These indices are approximates since the heart will change shape and vary slightly in position depending on inspiration/expiration, lying down/ standing, etc.

Valve                                      Area of Auscultation Pulmonary semilunar:                      2nd intercostal space on the LEFT Aortic semilunar  :                              2nd intercostal space on the RIGHT Mitral :                                                 5th intercostal space on the LEFT approximately  in  midclavicular line Tricuspid:                                             5th intercostal space on the LEFT approximately over xiphoid process of sternum

To hear typical and atypical heart sounds, go to this website created by the University of Washington’s Department of Medicine

 

THE CONDUCTION SYSTEM OF THE HEART (Netter 222; Moore 148-149)

Control of the rate and integration of atrial and ventricular contractions is achieved by the NODES and CONDUCTION FIBERS of the heart. All these structures are composed of modified myocardial muscle. These structures are:

Sino-atrial node (S-A Node) — is a small lump of fibers found in the anterior portion of the junction of the superior vena cava and the right atrium. This node has the property of spontaneous and rhythmical depolarization, such that 60 – 80 times a minute, its cells suddenly change their electrical state. This depolarization spreads through the atria, resulting in the atrial contraction then the impulse arrives at the

Atrioventricular Node (A-V Node) –which is a second small lump of specialized myocardial fibers found in the postero-inferior part of the interatrial septum next to the opening of the coronary sinus into the right atrium. Here the impulse is delayed briefly by special properties of the A-V node fibers, and then enters the next unit in this chain, the

Atrioventricular bundle (Bundle of His) –which is the sole ‘electrical’ connection of the atria to the ventricles. This short bundle pierces the fibrous skeleton, passes just posterior to the membranous portion of the interventricular septum, and when it reaches the muscular septum, divides into

Left and right bundle branches — which descend into their respective ventricles subendocardially along the appropriate left and right surfaces of the septum. Finally, the impulse passes to the last unit of this system, the:

 

Basically, the S-A node forms the impulses at just the right rate, the A-V node slows them down just the right amount, and the bundle of His and the bundle branches are high speed connecting lines which deliver the impulse almost instantaneously to all the Purkinje fibers; all, together, resulting in a properly timed and integrated heart action.   Both nodes are subject to control by autonomic fibers; the sympathetic fibers resulting in an increased automaticity (- faster rate) of the S-A node; the parasympathetic fibers resulting in decreased automaticity. At the A-V node, sympathetic stimulation increases the speed of impulse conduction; parasympathetic decreases it. In the normal heart, control is predominantly parasympathetic; that is, the heart, with all nerves cut, likes to run faster than 60-80 beats a minute–therefore it is controlled basically by parasympathetic slowing. However, during exercise and stress, the sympathetic innervation becomes crucial.

Purkinje fiber network–which is a very fine and widespread reticulum which delivers the impulse to the ventricular myocardium, resulting in ventricular contraction.

Clinical Note: Damage to the conduction system causes heart block, which interferes with the ability of the ventricles to receive the atrial impulses. A delay or disruption of the electrical signals produces an irregular and slower heartbeat, reducing the heart’s efficiency in maintainiing adequate circulation. Note that most commonly, the SA and AV nodes are supplied by atrial branches from the right coronary artery, so that one of the first signs of impending heart disease or myocardial infarction might be an irregular heart beat.

 

CARDIAC PLEXUSES (Netter 223, 224; Moore 150)

The strength and frequency of the heart beat is controlled by the autonomic nervous system. Both parasympathetic and sympathetic parts of the autonomic nervous system are involved in the control of the heart.

The sympathetic fibers arise from segments T1-T3 (sometimes T4) of the spinal cord and are distributed through the middle cervical and cervico-thoracic (or stellate) ganglia and the first four ganglia of the thoracic sympathetic chain. The sympathetic fibers pass into the cardiac plexus and from there to the SA node and the cardiac muscle. The effect of the sympathetic nerves at the SA node is an increase in heart rate. The effect on the muscle is an increase in rise of pressure within the ventricle, thus increasing stroke volume.

The vagus nerve (Cranial Nerve CN X) provides the parasympathetic control to the heart. The effect of the vagus at the SA node is the opposite of the sympathetic nerves, it decreases the heart rate. It also decreases the excitability of the junctional tissue around the AV node and this results in slower transmission. Strong vagal stimulation here may produce AV block

The autonomic innervation for the heart reflects its early embryologic stage then the heart develops in the neck region of the embryo.  Due to differential growth, the heart descends into the thoracic cavity carrying its nerve supply along with it.

Contributions for the sympathetic component arise from the superior, middle, and inferior cervical ganglia on both sides. Efferents (motor fibers) pass to the superficial and deep cardiac plexuses; the superficial located in the arch of the aorta and the deep located in the area of the tracheal bifurcation.

Contributions from the vagus nerves also enter the plexuses from both the right and left sides.

The right vagal and right sympathetic branches end chiefly in the region of the sinoatrial node and left vagal and sympathetic branches end chiefly in the region of the atrioventricular node.