Heart is a hollow muscular organ, which is somewhat pyramidal in shape. It lies within the pericardium in the mediastinum. It lies free within the pericardium except at its base where it is connected to great blood vessels.
Surfaces of heart:
Because of its shape, the heart has three surfaces: anterior, inferior and posterior. Often the surfaces are referred to as: sternocostal (anterior), diaphragmatic (inferior) and base (posterior). The apex of the heart is directed downward, forward and to the left.
- Anterior (Sternocostal) surface: It is formed mainly by the right atrium and right ventricle. They are separated from each other by the vertical atrioventricular groove. The right border of the anterior surface is formed by the right atrium while the left border is formed by left atrium and part of left auricle.
- Inferior (Diaphragmatic) surface: It is formed mainly by the right and left ventricles separated by the posterior interventricular groove. The inferior surface of the right atrium into which the inferior vena cava opens, also forms part of this surface.
- The base of the heart (posterior surface): It is formed mainly by the left atrium, into which the four pulmonary veins drain. It lies opposite to the apex. Often, the beginners think of the diaphragmatic surface of the heart as its base because of the fact that the heart rests on it, however, it should be kept in mind that the heart does not rest on its base. It rests on the diaphragmatic surface which is not the base. The posterior surface is called the base because it lies opposite to the apex of the pyramidal shaped heart.
Apex of the heart: It is formed by the left ventricle and is directed downward, forward and the left. It lies at the level of the fifth intercostals space, about 3.5 inches from the midline. The apex beat can be palpated in the region of apex of the heart.
Borders of the heart:
Because of pyramidal nature of its shape, the heart has three borders: right, left and lower. Right border is formed by the right atrium. The left border is formed by the left auricle and left ventricle. The lower border is formed by right ventricle, however, some part of it is also formed by the right atrium.
Wall of the heart:
As it was stated earlier, the heart is a hollow muscular chamber. It has strong wall that are composed of three main layers. The bulk of the wall of the heart is formed by cardiac muscles called the endocardium. On the outer side, the endocardium is covered with visceral layer of serous pericardium, known as epicardium. On the inner side, the endocardium is line with a layer of endothelium known as endocardium.
Chambers of the heart:
Human heart is not a simple hollow pump. It has been divided by vertical septa into four chambers: two atria (right and left) and two ventricles (right and left). The atria lie superior to the ventricles. In anatomic position, the right atrium lies anterior to the left atrium and the right ventricle lies anterior to the left ventricle.
It consists of two regions: the main concavity and a small outpouching called auricle. At the region of junction between these two parts, on the outer side, there is a vertical groove called sulcus terminalis, which on the inner side forms a ridge known as crista terminalis. The main part of the atrium lies posterior to crista terminalis and is derived embryologically from sinus venosus. The part of the atrium, which lies in front of crista terminalis, is roughened by bundles of muscle fibers, the musculi pectinati. This anterior part is derived embryologically from primitive atrium.
Openings in the right atrium: There are four openings in the right atrium that are described below:
- Opening for superior vena cava: It lies in the upper part and has no valves
- Opening for inferior vena cave: It lies in the lower part and is guarder by a rudimentary, and nonfunctioning valve.
- Opening for the coronary sinus: It lies between the opening for inferior vena cave and the atrioventricular orifice. It is also guarded by a rudimentary, nonfunctioning valve.
- Right atrioventricular orifice: It lies anterior to the opening for inferior vena cava and is guarded by the tricuspid valve.
The walls of right ventricle are much thicker as compared to those of right atrium. They show several internal projecting ridges, which are formed of muscle bundles. These ridges are known as trabeculae corneae and they give the walls a spongy appearance. They are of three types:
- Type 1: First type of trabeculae consists of papillary muscles, which project inward. They are attached by their bases to the ventricular wall and their apices are attached by fibrous chords, known as chordae tendinae, to the cusps of the tricuspid valve.
- Type 2: Second type consists of muscle fibers attached to the ventricular walls in the same way as the first type but they are free in the middle. One of them, known as the moderator band, crosses the entire ventricular cavity from septal to anterior wall.
- Type 3: It is simply composed of prominent ridges.
Openings in the right ventricle: There are two openings in the right ventricle: the right atrioventricular orifice (guarded by tricuspid valve) and the opening for the pulmonary trunk (guarded by the pulmonary valve).
Tricuspid valve: It consists of three cusps each of which is formed by a fold of endocardium with a little amount of connective tissue enclosed. The bases of all three cusps are attached to the fibrous ring of the skeleton of heart and their free edges are attached to chordae tendinae. Chordae tendinae connect them to the papillary muscles, which prevent the cusps from being forced into the atrium of turning inside out during ventricular contraction.
Pulmonary valve: It guards the pulmonary orifice that leads to pulmonary trunk. It also consists of three cusps with similar formation, however, in this case the cusps are semilunar in shape. The curved lower margins and sides of each cusp are attached to the arterial wall and their open mouths are directed into the pulmonary trunk. No chordae tendinae or papillary muscles are associated with this valve. External to each cusp, the wall of pulmonary trunk bulges out to form a sinus.
Similar to right atrium, it consists of a main cavity and the left auricle. In anatomic position of the heart, it is situated behind the right atrium and forms greater part of the base of heart. The interior of the left atrium is smooth but the auricle possesses muscular ridges as was the case with right atrium.
Openings in the left atrium: There are a total of five openings in the left atrium, four of which are for the pulmonary veins and one is the left atrioventricular orifice. The openings of the pulmonary veins are not guarded by any valve, however, the left atrioventricular orifice is guarded by bicuspid valve.
It is the strongest chamber of the heart. Its walls are three times thicker than those of the right ventricle. The reason for extra thick walls is that the left ventricle has to deal with high pressures. The pressure inside the left ventricle is about six times higher than that inside the right ventricle. In cross section, the right ventricle is circular and consequently the right ventricle is crescentic. It is because of the bulging of the interventricular septum into the right ventricle.
Openings in the left ventricle: There are two openings in the left ventricle: the left atrioventricular orifice (guarded by mitral valve, also known as bicuspid valve) and the aortic opening (guarded by aortic valve).
Mitral valve: It consists of two cusps, which have the structure similar to the cusps of tricuspid valve. The anterior of the two cusps is larger and intervenes between atrioventricular and aortic orifices. The attachment of chordae tendinae and papillary muscles is also similar to that of tricuspid valve.
Aortic valve: It is precisely similar to the pulmonary valve. Behind each cusp the aortic wall bulges to form an aortic sinus.
Relative structure of chambers of the heart:
The atrial portion of heart is relatively thin walled and is divided into right and left atria by the interatrial septum. This septum runs from the anterior wall of heart backward and to the right. The ventricular portion of the heart has thick wall. It is divided into right and left ventricles by the interventricular spetum. This septum is placed obliquely and its position is indicated by anterior and posterior interventricular grooves. The lower part of the septum is thick and formed of muscular tissue while the upper part is thin and membranous.
Skeleton of heart:
The skeleton of the heart is not an actual bony skeleton. It just consists of fibrous rings that surround right and left atrioventricular, aortic and pulmonary orifices. It is continuous with the membranous part of the interventricular septum. It separates the muscular wall of the atria from that of the ventricles and forms the basis of electrical discontinuity between them. The skeleton also supports the bases of the valve cusps and prevents them from stretching and becoming incompetent.
Conducting system of the heart:
It consists of specialized cardiac muscle present in the sinuatrial node, atrioventricular node and atrioventricular bundle along with its right and left terminal branches and Purkinje fibers (specialized cardiac muscle fibers that form the conducting system of the heart).
- Sinuatrial node: It is located in the wall of the right atrium in the upper part of the sulcus terminalis just to the right of the opening of superior vena cava. The sinuatrial node gives origin spontaneously to rhythmical impulses that spread in all direction through the cardiac muscle of the atria. As a result, the atrial muscle contracts.
- Atrioventricular node: It is strategically placed in the lower part of the right atrium just above the attachment of the septal cusp of the tricuspid valve. Through this node, the cardiac impulse is conducted from atria to the ventricles. The speed of conduction of impulse through the atrioventricular node is very slow, which allows sufficient time for the atria to empty their blood into the ventricles completely.
- Atrioventricular bundle: It is also known as “the bundle of His”. It is the only pathway that connects the myocardium of the atria to the myocardium of the ventricles electrically. Thus it is the only route for transmission of impulse from atria into the ventricles. The bundle descends through the fibrous skeleton of the heart to reach the inferior border of the membranous part of the ventricular septum. When it reaches the muscular part of the septum, it divides into two branches, one for each ventricle. The right bundle branch passes to the right ventricle and the left bundle branch passes to the left. After this point, they become continuous with the fibers of Purkinje plexus.
Function of conducting system of heart: The conducting system of heart is responsible not only for generating rhythmical cardiac impulses but also for conducting these impulses rapidly throughout the myocardium of the heart. Thus it aids in coordinated and efficient contraction of different chambers of the heart.
The activities of the conducting system of heart can be influenced by the autonomic nerve supply of heart. The parasympathetic nerves slow the rhythm and diminish the speed of conduction, while the sympathetic nerves have the opposite effect.
Impulses from the sinuatrial node have been show to travel to the atrioventricular node more rapidly than they can pass through the muscle of the heart. This can be explained by presence of specialized pathways in the atrial wall, which have a structure in between that of the Purkinje fibers and ordinary muscle cells. These specialized pathways are called internodal pathways and there are three of them in the atrial wall.
- Anterior internodal pathway: It leaves the anterior end of the SA node and passes anterior to the superior vena cava to end in the AV node.
- Middle internodal pathway: It leaves the posterior end of the SA node and passes posterior to the superior vena cava to end in the AV node.
- Posterior internodal pathway: It leaves the posterior part of the SA node and descends through the crista terminalis to end in the AV node.
Arterial supply of the heart:
Arterial supply of the heart consists of right and left coronary arteries. These arteries arise from the ascending aorta immediately above the aortic valve. The coronary arteries along with their major branches are distributed over the surface of the heart.
Right coronary artery: It arises from the anterior aortic sinus of the ascending aorta and runs forward between the pulmonary trunk and right auricle. After reaching the surface of the heart, it descends almost vertically into the right atrioventricular groove. After reaching the inferior border, it continues posteriorly and anastomose with the left coronary artery.
Branches of right coronary artery:
- Right conus artery: It supplies the anterior surface of pulmonary conus and upper part of the right ventricle on anterior side.
- Anterior ventricular branches: They are two or three in number and all of them supply the anterior surface of the right ventricle.
- Posterior ventricular branches: They are usually two in number and both of them supply the diaphragmatic surface of the right ventricle.
- Posterior interventricular artery: It is also known as posterior descending artery. It runs towards the apex of the heart in the posterior interventricular groove and gives of branches to the right and left ventricles. It also supplies the posterior part of the ventricular septum (not the apical part, which receives blood from the anterior interventricular branch of left coronary artery) and the atrioventricular node.
- The atrial branches: They supply the anterior and lateral surfaces of the right atrium. They also supply the sinuatrial node. In 35% individuals, the atrial branches arise from the left coronary artery.
Left coronary artery:
It is usually larger than the right coronary artery and supplies the major part of heart. It arises from the left posterior aortic sinus of the ascending aorta and passes forward between pulmonary trunk and left auricle. Then it enters the atrioventricular groove and divides into anterior interventricular branch and circumflex branch.
Branches of left coronary artery:
- Anterior interventricular branch: It is also known as the anterior descending branch. It runs downward in the anterior interventricular groove to the apex of the heart. After reaching the apex, it passes around to enter the posterior interventricular groove and anastomoses with the terminal branches of the right coronary artery. In 33% individuals, it ends in the apex of the heart. Anterior interventricular branch supplies the right and left ventricles along with the anterior part of the ventricular septum.
- Circumflex artery: It is the same size as the anterior interventricular artery. It winds around the left margin of the heart in the atrioventricular groove. It is further divided into a number of branches that supply various parts of heart as described below.
- Left marginal artery: It supplies the left margin of the left ventricle till the apex.
- Anterior ventricular and posterior ventricular branches: They supply the left ventricle
- Atrial branches: They supply the left atrium.
Summary of the overall arterial supply of heart in most individuals:
Right coronary artery: It supplies
- The entire right ventricle except for the small to the right
- Variable part of the diaphragmatic surface of right ventricle
- Posteroinferior third of the ventricular septum
- Right atrium
- Part of the left atrium
- Sinuatrial node
- Atrioventricular node
- Atrioventricular bundle
- A part of the Left bundle branch of atrioventricular bundle
Left coronary artery: It supplies
- Most of the left ventricle
- Small area of the right ventricle that is not supplied by right coronary artery
- Anterior two thirds of the ventricular septum
- Most of the left atrium
- Right bundle branch
- Left bundle branch
Venous drainage of the heart:
Most blood from the heart wall drains into the right atrium through the coronary sinus. This sinus lies in the posterior part of the anterior interventricular groove and is a continuation of the great cardiac vein. It opens into the right atrium to the left of the inferior vena cava. The small and middle cardiac veins are tributaries of the coronary sinus. The remainder of the blood is returned to right atrium through anterior cardiac vein and by other small veins that open directly into the heart chambers.
Nerve supply of the heart:
The heart is innervated by sympathetic and parasympathetic fibers through the cardiac plexus situated below the arch of aorta. The sympathetic supply arises from the cervical and upper thoracic portions of the sympathetic trunk. The parasympathetic supply comes from the vagus nerve.
Sympathetic fibers, which are postganglionic, terminate on the sinuatrial node, atrioventricular node, cardiac muscle fibers, and coronary arteries. Activation of these nerves causes cardiac acceleration, increased force of contraction and dilation of coronary arteries, all of which are meant to increase blood supply to the body.
The parasympathetic fibers, which are also postganglionic, terminate on sinuatrial node, atrioventricular node and coronary arteries. Cardiac muscle fibers do not receive the parasympathetic nerve fibers. Activation of theses nerves cause reduction in rate of heart beats, and constriction of the coronary arteries. The force of contraction is not affected much by the parasympathetic activation.
Afferent fibers from the heart run with the parasympathetic nerves and the vagus nerve. Sensory impulses from the heart are carried in these afferent fibers.