Human heart

The heart is a hollow muscular organ of a somewhat conical form; it lies between the lungs in the middle mediastinum and is enclosed in the pericardium (Fig. 490). It is placed obliquely in the chest behind the body of the sternum and adjoining parts of the rib cartilages, and projects farther into the left than into the right half of the thoracic cavity, so that about one-third of it is situated on the right and two-thirds on the left of the median plane. Every day, the heart pumps about 2,000 gallons (7,600 liters) of blood, beating about 100,000 times

Size of human heart

The heart, in the adult, measures about 12 cm. in length, 8 to 9 cm. in breadth at the broadest part, and 6 cm. in thickness. Its weight, in the male, varies from 280 to 340 grams; in the female, from 230 to 280 grams. The heart continues to increase in weight and size up to an advanced period of life; this increase is more marked in men than in women.

Position of heart

The heart is a hollow muscular organ of a somewhat conical form; it lies between the lungs in the middle mediastinum and is enclosed in the pericardium It is placed obliquely in the chest behind the body of the sternum and adjoining parts of the rib cartilages, and projects farther into the left than into the right half of the thoracic cavity, so that about one-third of it is situated on the right and two-thirds on the left of the median plane.

Front view of heart and lungs

The heart is demarcated by:

A point 9 cm to the left of the midsternal line (apex of the heart) The seventh right sternocostal articulation The upper border of the third right costal cartilage 1 cm from the right sternal line The lower border of the second left costal cartilage 2.5 cm from the left lateral sternal line.

Structure of the Heart Cell

The cells of the heart are called cardiac myocytes, or cardiomyocytes. Scientists consider heart cells a part of the muscle cell family, though with unique differences in mitochondria, intercalated disks and t-tubes, as well as in cellular growth.

Under a Microscope Heart muscle cells build to form involuntary striated muscles strands, which form the

walls of the heart, called the myocardium. Under a microscope, the striated muscle strands look like long chain links.

Mitochondria Unlike other muscle cells, heart cells are highly resistant to fatigue because they have

more mitochondria organelles than any other cell in the body. Mitochondria are a cell's "digestive system," which breaks down nutrients into cell energy. This continuous supply of energy helps stop muscle fatigue.

Intercalated disks Intercalated disks synchronize the contractions of all cardiomyocytes. Under a

microscope they appear white and act as a double membrane to separate heart cells. These disks regulate the passage of positive and negative electrons. As electric currents repel and attract, it causes electron depolarization, which regulates heartbeat contractions.

T-Tubules T-tubules, or transverse tubules, are plasma membranes that surround each cell and

organize them into pairs to create the striated muscles strands used to build the myocardium. Other muscle strands in the body are grouped using four cells.

Cell Growth In 2009, scientists discovered that the cardiomyocytes of the human heart actually

generate new heart cells. This discovery allows researchers to find ways of encouraging the heart to repair itself.

Outer Coverings of the Heart:

The Pericardium - a double sac of serous membrane surrounding the heart

(A). Parietal pericardium - a loose fitting outer membrane consisting of two layers: The fibrous layer - composed of tough, white fibrous tissue covering the heart and anchoring it to the diaphragm,

sternum and large blood vessels. The serous layer - a thin inner membrane composed of a thin fibrous layer on top of a simple squamous epithelium. This

layer folds back over and adheres to the heart forming the visceral pericardium. (B).Visceral pericardium - this layer is also called the epicardium. It is well integrated with the muscular wall of the

heart. It is often infiltrated with fat.

Pericardial cavity - is a fluid-filled cavity located between the parietal and visceral membranes. The serous portions of the parietal and visceral membranes face the cavity and produce the pericardial fluid. This fluid prevents the heart and lungs from rubbing against each other during their actions. Pericarditis is an inflammation of the pericardium. It can produce painful adhesions between the membranes.

The Heart Wall - three layers

(A). The Epicardium (described above)

(B). The Myocardium - the muscular wall of the heart composed of cardiac muscle and a reinforcing internal network of fibrous connective tissue called the "skeleton of the heart". This connective tissue serves two primary functions:It provides anchorage for the cardiac muscle and the atrioventricular valves. The portion of the skeleton anchoring the A-V valves is called the coronary trigone.

The elastic component of the skeleton provides the recoil that assists in filling the chambers following systole.

(C). The Endocardium - lines the chambers of the heart. In the chambers it consists of a simple squamous epithelium overlying a delicate layer of loose connective tissue. It lines the dense connective tissue of the cusps of the A-V valves. It is continuous with the endothelium of the blood vessels. Inflammation of this layer is called endocarditis.

CARDIAC CHAMBER

There are four cavities in the heart. The two upper chambers of the heart are called atrium, the bottom chambers are called ventricles. The right side of the heart receives deoxygenated blood from all parts of the body except for the lungs. The left side of the heart receives oxygenated blood from the lungs and pumps it to the rest of the body.

Right atrium

The superior vena cava and inferior vena cava drain systemic venous blood into the posterior wall of the right atrium. The internal wall of the right atrium is composed of a smooth posterior portion (into which the vena cavae and coronary sinus drain) and a ridge like muscular anterior portion. The coronary sinus drains coronary venous blood into the antero-inferior portion of the right atrium. The thebesian valve is located at the orifice of the coronary sinus. The limbus of the fossa ovalis is located on the medial wall of the right atrium and circumscribes the septum primum of the fossa ovalis anteriorly, posteriorly, and superiorly.

The right auricle is separated from the right atrium by a shallow posterior vertical indentation on the right atrium (ie, the sulcus terminalis) and, internally, by a vertical crest (ie, the crista terminalis). The crista terminalis separates the right atrium into trabeculated and nontrabeculated portions.

Right Atrium receives blood via:

a. Superior vena cava - returns blood from the head, shoulders, arms and neck.

b. Inferior vena cava - returns blood from the lower body

c. Coronary sinus - returns blood from the coronary circulation

Left atrium

The 4 pulmonary veins drain into the left atrium. The flap valve of the fossa ovalis is located on the septal surface of the left atrium. The appendage of the left atrium is consistently narrow and long; recognition of this appendage is the most reliable way to differentiate the left atrium from the right atrium. The left atrial appendage is the only trabeculated structure in the left atrium because, unlike the right atrium, the left atrium has no crista terminalis.

Left Atrium receives blood via four pulmonary veins. These vessels enter the left atrium posteriorly bringing oxygenated blood back to the heart from the lungs

Right ventricle

The right ventricle receives blood from the right atrium across the tricuspid valve, which is located in the large anterolateral (ie, sinus) portion of the right ventricle. The right ventricle discharges blood into the pulmonary artery across the pulmonic (semilunar) valve located in the outflow tract (infundibulum). The inflow tract (sinus) and outflow tract (infundibulum) of the right ventricle are widely separated. Internally, both the sinus area and infundibulum contain coarse trabeculations.

The septal portion of the right ventricle has 3 components:

(1) the inflow tract, which supports the tricuspid valve;

(2) the trabecular wall, which typifies the internal appearance of the right ventricle; and

(3) the outflow tract, which itself is subdivided into 3 components, namely, the conal septum, septal band division, and trabecular septum.

Of these 3 subdivisions, the conal septum is clinically significant because it can be malpositioned in patients with congenital disorders .The tricuspid valve is supported by a large anterior papillary muscle, which arises from the anterior free wall and the moderator band, and by several small posterior papillary muscles, which attach posteriorly to the septal band.

The pulmonary trunk carries blood from the right ventricle

Left ventricle

The left ventricle receives blood from the left atrium via the mitral (ie, bicuspid) valve and ejects blood across the aortic valve in the aorta. The left ventricle can be divided into 2 primary portions, namely, the large sinus portion containing the mitral valve and the small outflow tract that supports the aortic (semilunar) valve. Inflow and outflow portions are closely juxtaposed, unlike in the right ventricle, in which the tricuspid and pulmonic valves are widely separated.

The free wall and apical half of the septum contain fine internal trabeculations. The septal surface is divided into a trabeculated portion (sinus) and a smooth portion (outflow). The sinus area just beneath the mitral valve is termed the inlet septum; the remainder of the sinus area is termed the trabecular septum. The outflow tract is located anterior to the anterior mitral leaflet and is part of the atrioventricular (AV) septum. Both the right half of the anterior mitral valve leaflet and the right aortic cusp attach to the septum. (In the right ventricle, only the septal tricuspid leaflet attaches to the septum.)

The left half of the anterior mitral leaflet is in direct fibrous contact with the aortic valve at the aortic-mitral annulus. The conal septum of the right ventricle is positioned opposite the aortic valve. The mitral valve is supported by 2 large papillary muscles (ie, anterolateral, posteromedial) attached to the free wall. The anterior papillary muscle is attached to the anterior portion of the left ventricular wall, and the posterior papillary muscle arises more posteriorly from the ventricle's inferior wall.

The aorta carries blood from the left ventricle.

The Septum

The right and left sides of r heart are divided by an internal wall of tissue called the septum. The area of the septum that divides the two upper chambers (atria) of your heart is called the atrial or interatrial septum. The area of the septum that divides the two lower chambers (ventricles) of heart is called the ventricular or interventricular septum.

Ventricular septum

The ventricular septum is divided into a muscular section (inferior) and a membranous section (superior). The muscular portion comprises the left and right ventricular walls. The membranous septum, also termed the pars membranacea, is a fibrous structure partially separating the left ventricular outflow tract from the right atrium and ventricle.

Atrioventricular septum

The atrioventricular (AV) septum, located behind the right atrium and left ventricle, is divided into 2 portions: a superior portion (membranous) and an inferior portion (muscular). Inside the left ventricle, the muscular component comprises part of the outlet septum. The AV node lies in the atrial septum, juxtaposed to the membranous and muscular portions of the AV septum.

THE HEART VALVES

The heart consists of four chambers, two atria (upper chambers) and two ventricles (lower chambers). There is a valve through which blood passes before leaving each chamber of the heart. The valves prevent the backward flow of blood.

These valves are actual flaps that are located on each end of the two ventricles (lower chambers of the heart). They act as one-way inlets of blood on one side of a ventricle and one-way outlets of blood on the other side of a ventricle. Each valve actually has three flaps, except the mitral valve, which has two flaps.

The four heart valves include the following:

tricuspid valve: located between the right atrium and the right ventricle pulmonary valve: located between the right ventricle and the pulmonary artery mitral valve: located between the left atrium and the left ventricle aortic valve: located between the left ventricle and the aorta.

Tricuspid valve

The AV valve of the right ventricle has anterior, posterior, and septal leaflets. The orifice is larger than the mitral orifice and is triangular. The tricuspid valve leaflets and chordae are more fragile than those of the mitral valve. The anterior leaflet, largest of the 3 leaflets, often has notches. The posterior leaflet, smallest of the 3 leaflets, is usually scalloped. The septal leaflet usually attaches to the membranous and muscular portions of the ventricular septum.

A major factor to consider during surgery is the proximity of the conduction system to the septal leaflet. The membranous septum lies beneath the septal leaflet, where the His bundle penetrates the right trigone beneath the interventricular membranous septum. Moreover, the portion of the septal leaflet between the membranous septum and the commissure may form a flap valve over some ventricular septal defects (VSDs).

Pulmonary valve

As with the aortic valve, the pulmonary valve has 3 cusps, with a midpoint nodule at the free end and lunulae on either side; a sinus is located behind each cusp. Compared with the aortic valve, the pulmonary valve has thinner cusps, no associated coronary arteries, and no continuity with the corresponding (anterior) tricuspid valve leaflet. The term used for each cusp reflects its relationship to the aortic valve, namely, right, left, and nonseptal

Mitral valve

The AV valve of the left ventricle is bicuspid. The AV valve has a large anterior leaflet (septal or aortic) and a smaller posterior leaflet (mural or ventricular). The anterior leaflet is triangular with a smooth texture. The posterior leaflet has a scalloped appearance. The chordae tendineae to the mitral valve originate from the 2 large papillary muscles of the left ventricle and insert primarily on the leaflet's free edge.

Aortic valve

The aortic valve has 3 leaflets composed of fragile cusps and the sinuses of Valsalva. Thus, the valve apparatus is composed of 3 cuplike structures that are in continuity with the membranous septum and the mitral anterior leaflet. The free end of each cusp has a stronger consistency than the cusp. The midpoint of each free edge contains the fibrous nodulus arantii, which bisects the thin crescent-shaped lunula on either side.

The aortic sinuses of Valsalva are 3 dilations of the aortic root that arise from the 3 closing cusps of the aortic valve. The right and left sinuses give rise to the right and left coronary arteries; the noncoronary sinus has no coronary artery. The sinus of Valsalva walls are much thinner than the aortic wall, which is a factor of surgical significance; therefore, aortotomies are typically performed away from this region.

The aortic annulus is situated anteriorly to the mitral valve annulus and the anterior leaflet of the mitral valve. The cranial portion of the left atrium is interposed between the aortic and mitral valves. Anteriorly, the aortic annulus is related to the ventricular septum and right ventricular outflow tract. The His bundle courses beneath the right and noncoronary aortic valve cusps of the membranous ventricular septum. Thus, incision of the aortic annulus or septal myocardium anterior to the right coronary sinus should not interfere with the conduction system.

How do the heart valves function?

As the heart muscle contracts and relaxes, the valves open and shut, letting blood flow into the ventricles and atria at alternate times.

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The following is a step-by-step illustration of how the valves function normally in the left ventricle:

After the left ventricle contracts, the aortic valve closes and the mitral valve opens, to allow blood to flow from the left atrium into the left ventricle.

As the left atrium contracts, more blood flows into the left ventricle. When the left ventricle contracts again, the mitral valve closes and the aortic valve opens, so blood flows into the aorta

Muscular structure of the heart

The muscular structure of the heart consists of bands of fibers, which present an exceedingly intricate interlacement. They comprise

(a) the fibers of the atria,

(b) the fibers of the ventricles,

(c) the atrioventricular bundle of His.

Fibers of the atria

The fibers of the atria are arranged in two layers—a superficial, common to both cavities, and a deep, proper to each. The superficial fibers are most distinct on the front of the atria, across the bases of which they run in a transverse direction, forming a thin and incomplete layer. Some of these fibers run into the atrial septum. The deep fibers consist of looped and annular fibers. The looped fibers pass upward over each atrium, being attached by their two extremities to the corresponding atrioventricular ring, in front and behind. The annular fibers surround the auriculæ, and form annular bands around the terminations of the veins and around the fossa ovalis.

Fibers of the ventricles

The fibers of the ventricles are arranged in a complex manner, and various accounts have been given of their course and connections; the following description is based on the work of McCallum. (*94 They consist of superficial and deep layers, all of which, with the exception of two, are inserted into the papillary muscles of the ventricles. The superficial layers consist of the following: (a) Fibers which spring from the tendon of the conus arteriosus and sweep downward and toward the left across the anterior longitudinal sulcus and around the apex of the heart, where they pass upward and inward to terminate in the papillary muscles of the left ventricle; those arising from the upper half of the tendon of the conus arteriosus pass to the anterior papillary muscle, those from the lower half to the posterior papillary muscle and the papillary muscles of the septum. (b) Fibers which arise from the right atrioventricular ring and run diagonally across the diaphragmatic surface of the right ventricle and around its right border on to its costosternal surface, where they dip beneath the fibers just described, and, crossing the anterior longitudinal sulcus, wind around the apex of the heart and end in the posterior papillary muscle of the left ventricle. (c) Fibers which spring from the left atrioventricular ring, and, crossing the posterior longitudinal sulcus, pass successively into the right ventricle and end in its papillary muscles.The deep layers are three in number; they arise in the papillary muscles of one ventricle and, curving in an S-shaped manner, turn in at the longitudinal sulcus and end in the papillary muscles of the other ventricle. The layer which is most superficial in the right ventricle lies next the lumen of the left, and vice versa. Those of the first layer almost encircle the right ventricle and, crossing in the septum to the left, unite with the superficial fibers from the right atrioventricular ring to form the posterior papillary muscle. Those of the second layer have a less extensive course in the wall of the right ventricle, and a correspondingly greater course in the left, where they join with the superficial fibers from the anterior half of the tendon of the conus arteriosus to form the papillary muscles of the septum. Those of the third layer pass almost entirely around the left ventricle and unite with the superficial fibers from the lower half of the tendon of the conus arteriosus to form the anterior papillary muscle. Besides the layers just described there are two bands which do not end in papillary muscles. One springs from the right atrioventricular ring and crosses in the atrioventricular septum; it then encircles the deep layers of the left ventricle and ends in the left atrioventricular ring. The second band is apparently confined to the left ventricle; it is attached to the left atrioventricular ring, and encircles the portion of the ventricle adjacent to the aortic orifice.

Atrioventricular bundle of His

The atrio-ventricular bundle of His is the only direct muscular connection known to exist between the atria and the ventricles. Its cells differ from ordinary cardiac muscle cells in being more spindle-shaped. They are, moreover, more loosely arranged and have a richer vascular supply than the rest of the heart muscle. It arises in connection with two small collections of spindle-shaped cells, the sinoatrial and atrioventricular nodes. The sinoatrial node is situated on the anterior border of the opening of the superior vena cava; from its strands of fusiform fibers run under the endocardium of the wall of the atrium to the atrioventricular node. The atrioventricular node lies near the orifice of the coronary

sinus in the annular and septal fibers of the right atrium; from it the atrioventricular bundle passes forward in the lower part of the membranous septum, and divides into right and left fasciculi. These run down in the right and left ventricles, one on either side of the ventricular septum, covered by endocardium. In the lower parts of the ventricles they break up into numerous strands which end in the papillary muscles and in the ventricular muscle generally. The greater portion of the atrioventricular bundle consists of narrow, somewhat fusiform fibers, but its terminal strands are composed of Purkinje fibers.

Coronary Circulation - supplies oxygenated blood to the myocardium and returns blood back to the heart. Right

and left coronary arteries supply oxygenated blood to the myocardial wall. They branch from the aorta just above the semilunar valve. The coronary sinus receives blood from the small cardiac, middle cardiac, great cardiac and posterior vein. This deoxygenated blood is then returned to the right atrium.

The Conduction System of the Heart - is designed to spread the waves of depolarization and

repolarization rapidly through the myocardium. The system consists of modified cardiac muscle cells called Purkinje cells. These cells are organized into:

The sinoatrial node - is an accumulation of Purkinje cells located medial to the opening of the superior vena cava in the posterior wall of the right atrium. These cells depolarize at a rate of 70 to 80 times per minute. This is a faster rate of depolarization than any other portion of the heart and determines the normal heart rate, i.e., sinus rhythm. For this reason, the S-A node is referred to as the "pacemaker".

The atrioventricular node is located in the right atrium medial to the tricuspid valve. The A-V node receives the wave of depolarization about 50msec after it leaves the S-A node. However, the passage of the wave through the A-V node slows down and takes about three times longer to pass through the node (150 msec). This delay is crucial for the normal functioning of the heart. It permits the atrial myocardium to finish its contraction before the ventricular contractions begin.

The Bundle of His receives the wave of depolarization from the A-V node. The bundle passes into the interventricular septum. Here the bundle divides into the right and left bundle branches. These branches continue to divide forming the Purkinje fibers which carry the wave to every part of the ventricular myocardium. The fibers making up the bundle and bundle branches have a wider diameter and more numerous gap junctions than typical cardiac cells. As a result, they carry the wave at a great speed throughout the ventricular myocardium (about 175 msec). The total time elapsed from the origin of the wave in the S-A node to the arrival of the wave in the ventricular myocardium is 225/1000 of one second. At this time the atria have finished their contraction and the ventricles will begin their contraction.

SUMMARY OF EVENTS OCCURING DURING THE CARDIAC CYCLE

STAGE I - Atrial and Ventricular Diastole

Both the upper and lower chambers are filling. The atriaoventricular valves are open. The semilunar valves are closed.

STAGE II - Atrial Systole; Ventricular Diastole

The atria are contracting forcing extra blood into the ventricles which become distended. Each ventricle now contains about 120 ml of blood. The atrioventricular valves are open.

The semilunar valves are closed.

STAGE III - This stage is divided into two phases:

Phase 1 -The Isovolumetric phase (Atrial Diastole; Ventricular Systole) 1. The atria have finished their contraction and are beginning to fill. The ventricle are beginning to contract. 2. Due to the rising blood pressure in the ventricles, the atrioventricular valves close. This action produces the first

heart sound "lub". The semilunar valves remain closed. 4. The volume of the blood in the ventricles remains the same during this phase (isovolumetric).

Phase 2 - The Ejection phase (Atrial Diastole; Ventricular Systole finishes) The atria continue to fill. The ventricles finish their systole. This creates a high enough pressure on the blood in the ventricles to overcome

the downward force of blood in the pulmonary trunk and aorta. This open the semilunars and allow about 70 ml of blood to be ejected from each ventricle.

The atrioventricular valves remain closed.

STAGE IV - Atrial Diastole continues; Ventricular Diastole begins

The atria continue to fill. The ventricles enter diastole and begin to fill. This produces a decrease in ventricular blood pressure. As a result, the

blood in the pulmonary trunk and aorta begins to flow back down towards the ventricles. The cusps of the similunars fill with this blood and close the valve. This action produces the second heart sound "dup". The recoil of the descending blood against the cusps of the semilunars produces the slight, transient rise in aortic blood pressure called the "dicrotic notch".

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