IMS III Cardiovascular Anatomy and Histology F Gndoan (2014) 1

INTRODUCTION

The aim of this lecture is to facilitate the understanding of normal cardiac anatomy and establish a knowledgebase prior to learning the disease states and clinical approach to heart disease. This document is an in depth review of cardiovascular anatomy and histology. It includes the descriptions of anatomic relationships of the heart in the thoracic cavity, external and internal anatomic features of the heart, valvular anatomy, coronary artery anatomy, cardiac and vascular histology, and anatomy and histology of the conduction system. The following sources were used in preparation of this syllabus and the PowerPoint presentation:

Van Mierop LHS. Illustrations by FH Netter. Anatomy of the heart. CIBA Clinical Symposia, volume 17 (3). 1965.

Van Praagh R. Congenital heart disease: Embryology, anatomy, and approach to diagnosis. Syllabus from Harvard Medical School and The Childrens Hospital, Boston.

Bharati S, Lev M. The pathology of congenital heart disease. 1996. Saremi F, Achenbach S, Arbustini E, Narula J. Revisiting cardiac anatomy. A computed-

tomography-based atlas and reference. 2011. Mescher AL. Junqueiras basic histology. Dr. Calvin Oyers lecture notes and PowerPoint presentation Robbins and Cotran Pathologic Basis of Disease, 8th edition Personal slide collection

Recommended reading: Chapter 12 Robbins & Cotran Pathologic Basis of Disease and the cardiovascular histology notes from last year.

THE HEART

Steadily pumps blood through the body and provides the tissues with oxygen and nutrients and facilitates the removal of waste products

Average weight in adults: 250-300 g in females, 300-350 g in males The average right ventricular wall thickness (free wall) is 0.3-0.5 cm; left ventricular wall

thickness is 1.3-1.5 cm. In general, increased heart weight or ventricular wall thickness indicates hypertrophy,

whereas enlarged chamber size implies dilation. The heart has 4 chambers: 2 atria and 2 ventricles. Each ventricle has an inflow and

outflow tract with a valve at each end. These valves maintain the unidirectional blood flow through the heart. Briefly, systemic deoxygenated blood return to right atrium (RA) via inferior vena cava (IVC) and superior vena cava (SVC). During diastole, tricuspid valve opens and the blood fills the right ventricle. Eventually the RV pressure exceeds RA, tricuspid closes, pulmonary valve opens and the blood is ejected to the pulmonary trunk towards the lungs. The blood reaches the lungs by pulmonary arteries and it gets oxygenated. The oxygenated blood returns to left atrium (LA) through the pulmonary veins. During diastole, mitral valve opens and the blood fills the left ventricle (LV), then mitral valve closes. During systole, it is ejected into the aorta and goes into the systemic circulation.

Of course, in reality the both sides of the heart contract simultaneously. You will learn the cardiac cycle and its reflection to auscultation and EKG findings later in the course.

Anatomic Relationships

The central space between the two pleural cavities is the mediastinum. Arbitrarily, the mediastinum is divided into superior, anterior, middle, and posterior portions. The heart is located in the middle mediastinum with one-third of its mass to the right of the midline, and with its own long axis directed from the right shoulder towards the left hip. Anteriorly, the sternum and the costal cartilages cover the heart. Posteriorly, the heart lies on the esophagus and the tracheal bifurcation, and bronchi that extend into the lung. Once the chest plate is removed, thymus gland could be easily identified in children. Thymus gland is located in the superior mediastinum. It reaches its maximal size in children of about 2 years of age. Gradually after puberty, it almost disappears leaving a small pad composted mostly of fat tissue. Removing of the thymus reveals the brachiocephalic (innominate) veins that join each other on the right to form the superior vena cava. Its absence suggests a persistent left superior vena cava. The heart is located in the pericardial sac. Similar to pleura, pericardium has two layers. Visceral pericardium, also called epicardium, overlies the heart and the proximal portions of the great vessels. Inferior portion of

the parietal pericardium is adherent to the middle, tendinous part of the diaphragm. Most of the lateral and anterior portions are contiguous with, but not normally adherent to, the pleura.

Position Of The Heart

The apex of the heart points anteriorly, inferiorly, and about 45 degrees to the left. You should be familiar with the radiographic borders of the heart in different projections. On anterior projection, the right cardiac border is formed by the right atrium (RA). The inferior border is made by the

right ventricle (RV) and extends horizontally along the diaphragm to the cardiac apex. The left border slopes upwards from the apex and is formed by the left ventricle (LV). At the top of the left border, the left atrial appendage contributes to the heart silhouette. The pulmonary trunk and aorta emerge from the superior border of the silhouette, with the aorta in a rightward position. On the lateral projection, anterior cardiac border is the RV, while the posterior cardiac border is composed of both the LV and the LA. As a result, marked hypertrophy of RV, particularly in children, is evidenced by a prominence of the left anterior chest and an easily palpable thrust or heave over the precordium. By far the greater part of the

LV lies in a posterior position, and when this chamber is hypertrophied, the apex beat is found to be more forceful than usual and displaced downwards and outwards.

EXTERNAL EXAMINATION OF THE HEART

The right atrial surface is separated from the RV by the right atrioventricular groove in which the right coronary artery is located. The anterior interventricular groove separates the RV from LV. The descending left coronary artery lies in this groove. The amount of fat located in these grooves increases with age and nutritional status of the individual. Location of the anterior interventricular sulcus indicates the location of the interventricular septum. SVC and IVC, which is not illustrated here, enter the RA. Removing of the thymus reveals the brachiocephalic (innominate) veins that join each other on the right to form the superior vena cava. Its absence suggests a persistent left superior vena cava.

At the posterior aspect of the heart, there is a shallow sulcus between the SVC and the RA, which is known as sulcus terminalis. The sinoatrial node (pacemaker) resides in this sulcus. Coronary sinus, a venous channel into which most of the cardiac veins enter, is located in the posterior portion of the left atrioventricular groove (coronary sinus) that separates the LA from LV. Coronary sinus enters the RA.

The diaphragmatic surfaces of the right and left ventricles are separated by the posterior interventricular groove, which is in continuity with the anterior interventricular groove just to the right of the cardiac apex. As a result, the apex of a normal heart is formed entirely by the LV. The junction point of the coronary sulcus and the posterior interventricular sulcus is the crux of the heart, where all 4 chambers intersect.

The great vessels

The pulmonary trunk originates from the RV, leaves the pericardium and bifurcates into its 2 main branches, right and left pulmonary arteries. The bifurcation lies on the roof of the LA, the left pulmonary artery coursing immediately toward the left lung. The right pulmonary artery runs behind the ascending aorta and the proximal SVC and above the right pulmonary veins to the right lung.

Anatomy of the heart by multislice computed tomography. Faletra FF, Pandian NG, Ho SY

IMS III Cardiovascular Anatomy and Histology F Gndoan (2014) 3

Intrapericardial portion of the ascending aorta is located to the right of the pulmonary trunk and its base is largely covered by the right atrial appendage. The aortic arch crosses the pulmonary artery bifurcation after giving off its three main branches: the brachiocephalic (innominate), the left common carotid, and the left subclavian arteries. Variations in this pattern are not uncommon and usually are of little significance.

The right pulmonary veins, usually two but occasionally three, leave the right lung, cross the right atrium posteriorly and enter the right side of the LA. The two left pulmonary veins enter the left side of LA, sometimes by a large common stem. The posterior wall of the LA forms the anterior wall of the oblique pericardial sinus.

CHAMBERS OF THE HEART

Right Atrium (RA)

The RA resembles Snoopy looking to his left. The right atrial appendage (RAA), Snoopys nose, is quite broad. On the other hand, LAA is long and thin, resembling a pointing finger. Normally SVC and IVC return to the RA. The RA consists of two parts: a posterior, smooth-walled portion derived from the embryonic sinus venosus to where SVC, IVC and coronary sinus enter, and a very thin-walled, trabeculated part, which constitutes the primitive atrial component. A ridge of muscle called crista terminalis, which is most prominent above, next to the SVC orifice, separates the two parts. The position of crista terminalis corresponds to sulcus terminalis externally. From the lateral aspect of the crista terminalis, a large number of pectinate muscles run laterally and more or less parallel to each other along the free wall of the atrium. In between these pectinate muscles, the atrial wall

is paper-thin and translucent. A fold of tissue (Eustachian valve) guards the anterior border of the inferior vena caval ostium. The coronary sinus enters the RA medial to IVC. Its orifice also may or may not be guarded by a valve-like fold called thebesian valve. The posteromedial wall of the RA is formed by the atrial septum. The right atrial septal surface displays the superior limbic band of septum secundum. Central, ovoid portion of the septum appears thin and fibrous forming a shallow depression in the septum corresponding to fossa ovalis. During fetal life it is patent (foramen ovale)

and acts as a unidirectional flap valve, allowing blood to pass directly from RA to LA. At birth, the LA pressure increases and exceeds that of the RA and functionally closes the valve. In about 80%

of adults the valve is structurally closed and probe patent in the remainder.

Right ventricle (RV)

The diaphragmatic or inferior surface of the RV makes an acute angle with the anterior surface, thereby forming the acute margin of the heart. Tricuspid valve forms the inflow tract of the RV, whereas pulmonary trunk forms the outflow tract. The cusps of the pulmonary artery and the leaflets of the tricuspid valve are widely separated by conal or infundibular musculature. Presence of conus is a characteristic of RV. The pulmonary trunk arises superiorly from the conus arteriosus of the RV and

bifurcates into right and left pulmonary arteries just after leaving the pericardial cavity. The tricuspid valve consists of an anterior, a medial (septal), and one or two posterior cusps. The depth of the commissures between the cusps is variable, but almost never reach the annulus, so that the cusps are only incompletely separated from each other. A number of papillary muscles

anchor the tricuspid valve leaflets (cusps) to the RV wall by means slender, fibrous strands called chordae tendineae.

Left atrium (LA)

LA is mainly a smooth walled sac. The pectinate muscles are confined to the left atrial appendage. On the right, two, and occasionally three, pulmonary veins enter. On the left, there are two (sometimes only one) pulmonary veins. The wall of the LA is distinctly thicker than RA. The valve of foramen ovale is seen from the left side of the atrial septum. Most of the ventricular septum is muscular. A small area of the septum below the commissure between the right and posterior aortic valve cusps is thin and membranous (membranous septum).

Left ventricle (LV)

The left ventricular margin of the heart is known as the obtuse margin (>90). The mitral valve forms the inflow portion of the LV. The mitral (bicuspid) valve actually is made up of four cusps: two large ones, the anterior (aortic) and posterior (mural) cusps, and two small commissural cusps. Similar to tricuspid valve, the commissures are never complete.

The average thickness of the LV is about three times that of the RV. Absence of conus is a feature of the LV. As a result, the mitral valve and aortic valve are located adjacent to each other and are separated only by a fibrous band (aortic-mitral fibrous continuity).

SEMILUNAR VALVES

The arterial or semilunar valves of the aorta and pulmonary trunk consist of equally sized three pocket-like cusps. There is no distinct, circular ring of fibrous tissue at the base of the arteries. The arterial wall expands into three dilated pouches, the sinuses of Valsalva, the walls of which are much thinner than those of the aorta and pulmonary trunk.

THE CORONARY ARTERIES

Right and left cusps of the aortic valve have the ostia of right and left coronary arteries, respectively. The posterior cusp of the aortic valve is the noncoronary cusp.

The heart and proximal portions of the great vessels receive their blood from supply from two coronary arteries. Cardiac myocytes rely almost exclusively on oxidative phosphorylation to meet the energy needs. Since oxidative phosphorylation requires oxygen, cardiac myocytes are extremely vulnerable to ischemia. A constant supply of oxygenated blood is essential for cardiac

function. Coronary arteries run along the external surface of the heart (epicardial) initially and then penetrate the myocardium (intramural arteries) to provide a rich network of capillaries. The left coronary artery bifurcates into the left anterior descending coronary artery and the left circumflex coronary artery (LCX). The left anterior descending coronary artery (LAD), courses downward in the anterior interventricular groove extending all the way to the apex, and ascends a short distance up the posterior interventricular groove. Important branches of the LAD include the first diagonal, which supplies muscle in the

anterior wall and the first septal perforator, which supplies much of the muscular septum. The LCX gives rise to the obtuse marginal branch, which supplies the lateral wall of the left ventricle. In most of us the LCX is small and not of much importance distal to the branching of the obtuse marginal. The right coronary artery (RCA) courses to the right in the AV groove, inferior to the right atrial appendage. It rounds the acute margin to reach the crux in the majority of cases (the meeting place of all 4 chambers posteriorly), and it gives off a variable number of branches to the anterior right ventricular wall. A usually well-developed and large branch (marginal branch) runs along the acute margin of the heart. The posterior interventricular (descending) branch descends along the posterior interventricular groove, not quite reaching the apex. Small parallel branches from the marginal and posterior descending arteries largely supply the diaphragmatic part of the RV. Posterior descending branch generally crosses the crux, giving off a small but

IMS III Cardiovascular Anatomy and Histology F Gndoan (2014) 5

important branch to the atrioventricular node. It terminates in a number of branches to the left ventricular wall. Right atrial branch of the right coronary artery is of great importance. This branch originates at the right coronary artery shortly after its takeoff and ascends along the anteromedial wall of the right atrium. It enters the upper part of the atrial septum and reappears

as the superior vena caval branch (nodal artery).

Particularly in the evaluation of angiocardiograms it is important to remember that variations in the branching pattern of the coronaries are extremely common. In about 67% of the cases the right coronary artery is dominant, crosses the crux and supplies part of the left ventricular wall and ventricular septum. In 15% of the cases the left coronary artery is dominant, in which case its circumflex branch crosses the crux, giving off the posterior interventricular branch and supplying all of the LV, ventricular septum, and part of

the right ventricular wall. In about 18% of the cases, both coronary arteries reach the crux, a so-called balanced coronary arterial pattern. In 40% of the cases, the superior vena caval branch is a continuation of a large anterior atrial branch of the left coronary artery rather than of the anterior atrial branch of the right coronary artery. It is also quite common for the first, second, or even the third branch of the right coronary artery to originate independently from the right sinus of Valsalva, rather than from the parent artery.

CARDIAC HISTOLOGY

The cardiac wall is composed of 3 layers: internal endocardium, the middle myocardium, and the external epicardium.

There are 3 different muscle types: skeletal, cardiac and smooth muscle. Both skeletal and cardiac muscles are striated. The skeletal muscle fibers are multinucleated and their nuclei are peripheral. In contrast, cardiac muscle is composed of irregular branched cells bound together longitudinally by intercalated disks and each cardiac muscle cell has only one or two centrally located nuclei.

Cardiac muscle cells contain numerous mitochondria, reflecting the need for continuous aerobic metabolism. Atrial muscle cells contain membrane-bound granules that are aggregated at the nuclear poles. These granules are most abundant in the right atrium. The atrial granules contain the precursor of a polypeptide hormone, atrial natriuretic factor. ANF targets the kidneys (natriuresis and diuresis).

Ventricular myocytes are arranged circumferentially in a spiral orientation and contract during systole and relax during diastole. The contractile unit is the sarcomere composed of principally myosin, thin filaments containing actin, and regulatory proteins such as troponin and tropomyosin. Striated appearance of the myocytes is due to presence of strings of sarcomeres. During contraction, myosin filament pulls the neighboring actin filaments toward the center of the sarcomere, leading to the shortening of the myocyte.

THE CONDUCTION SYSTEM

The heart rate and rhythm are regulated by specialized excitatory and conducting myocytes responsible for initiating and conducting electrical impulses to the myocardium. Key components of the conduction system are 1) the sinoatrial (SA) node, pacemaker of the heart; 2) atrioventricular (AV) node; 3) the AV or His bundle; and 4) the right and left bundle branches. SA node normally sets the pace. The AV node delays the transmission of signals from the atria to the ventricles to ensure atria contraction precedes ventricular contraction. The autonomic nervous

system controls the rate of firing of the SA node.

The SA node surrounds the SA node artery in the sulcus terminalis near the junction of the crest of the right atrial appendage with the SVC. The cells responsible for initiating and conducting are modified fibers that are smaller than contractile myofibers.

The AV node is located in the lower atrial septum in the triangle of Koch which is formed by: 1) the origin of the septal leaflet of the tricuspid valve; 2) the thebesian valve of the coronary sinus; and 3) the Eustachian valve of the IVC with its anterior

extension which is known as the tendon of Todaro. The AV node is in continuity with the AV bundle (Bundle of His), which runs along the superior ridge of the ventricular septum. As this bundle approaches the membranous septum it gives off fibers, which go onto the left side of the ventricular septum as the left bundle branch (LBB). Rather than being a bundle, this is a series of fibers going down the left side as a waterfall. After the fibers to the LBB are dispatched, the remaining fibers constitute the right bundle branch (RBB). Distally fibers of the bundle branches become larger than ordinary cardiac muscle fibers and acquire a distinctive appearance. These Purkinje fibers have one or two central nuclei and their cytoplasm is rich in mitochondria and glycogen.

BLOOD VESSELS

The circulatory system is composed of heart, arteries, capillaries, and veins. Large elastic arteries leave the heart and branch to form muscular arteries. These arteries branch further and enter organs, where they branch further to form arterioles. Eventually arterioles branch into capillaries, the site of exchange between blood and surrounding tissue. Capillaries then merge and form venules, then merge further into small and then medium-sized veins. These veins leave organs; form larger veins, which eventually bring the deoxygenated blood back to the heart.

The cellular composition of blood vessels is the same throughout the CV system. The arteries have pulsatile flow and higher blood pressures, thus their walls are thicker than veins. The three concentric layers are intima, media, and adventitia. Arterial wall thickness gradually diminishes as the vessels become smaller, but the ratio of wall thickness to lumen diameter becomes greater. The capillaries have small diameters and they lack media.

LABELING EXERCISE

Identify the regions of the human heart using the terms provided next to the illustration.

Aorta

Sinoatrial node

Aortic valve

Pulmonary valve

Left ventricle

Atrioventricular node

Mitral valve

Tricuspid valve

Purkinje fibers

Bundle of His

Bundle branches