CH 21 The Heart

Coronary Artery Disease and Myocardial Infarction

*Coronary Artery Disease (CAD)* is the leading cause of death for men and women in the United States. It occurs when cardiac arteries become hardened and narrowed due to the buildup of cholesterol, calcium, and plaque. This narrowing of coronary arteries, termed *atherosclerosis*, creases blood supply to the heart muscle and leads to *cardiac ischemia*. Cardiac ischemia often causes heart pain known as *angina*, which is usually manifested as a crushing, smothering chest discomfort that may radiate to the back, neck, jaw, or left arm. If a blood clot forms at the site of a plaque and suddenly blocks the artery complete, a *myocardial infarction (MI)*, or heart attack, can occur. The insufficient blood supply causes necrotic damage, called an *infarct*. The affected heart muscle then scars, which affects the heart's ability to contract properly. This impairs the cardiac conduction system and can lead to arrhythmias (heart beat irregularities) and heart failure. Treatment involves restoring blood flow using a thrombolytic (clot dissolver) or coronary angioplasty (surgical repair of blood vessel). Sometimes atherosclerosis develops so slowly that collateral circulation (alternative blood supply growing around a blockage) will develop and can save some cardiac muscle in the event of a sudden coronary thrombosis (stationary blood clot). Many causes of CAD are preventable, including smoking, obesity, hypetension, hypercholesterolemia (high blood cholesterol), diabetes, and sedentary lifestyle. Percutaneous (through a puncture) *balloon angioplasty* involves threading a catheter past an area of atherosclerosis, inflating a balloon to compress the plaque against the vessel wall, then inserting a metal stent, or sleeve, to keep the area open. A surgical procedure in which damaged sections of the coronary arteries are replaced with new venous graftings, known as *coronary artery bypass graft (CABG)*, can also be done. Such procedures typically use the great saphenous vein of the leg or the internal thoracic artery of the chest wall for the graft.

Heart Murmurs

A *heart murmur* is a soft sound - such as a whooshing or swishing - made by turbulent blood flow in or near the heart during a heartbeat. Most murmurs are "innocent," meaning they have no clinical significance. Pregnant women often develop innocent murmurs because their blood volume and cardiac output increase while their cardiac valves remain the same size. Congenital cardiac defects, including atrial septal and ventricular septal defects (holes in the interatrial septum and interventricular septum), cause blood to flow in the wrong direction through the septum during systole, generating a murmur. Stenotic valves (valves that do not open all the way) or incompetent valvular heart disease, such as valvular stenosis and valvular insufficiency, occur when the valves do not function properly. *Valvular stenosis* is the narrowing of the valve opening that reduces the amount of blood flow. *Valvular insufficiency is a regurgitation of blood that results from incomplete valve closure.

Structure of the Heart Wall

A section through the wall of the heart reveals three distinct layers: (1) an outer epicardium, (2) a middle myocardium, and (3) an inner endocardium. 1) The *visceral layer of the serous pericardium (epicardium)* covers the surface of the heart. The epicardium has two layers: a mesothelium and an underlying, supporting layer of areolar tissue. The *parietal layer of the serous pericardium* consits of an outer dense fibrous layer and an inner mesothelium. 2) The *myocardium* is cardiac muscle tissue that forms the atria and ventricles. Associated with the myocardium are the cardiac muscle cells, connective tissues, blood vessels, and nerves. The atrial myocardium is quite thin and is organized into layers forming figure eights as they pass from one atrium to the other. The ventricular myocardium is much thicker, and its muscle orientation changes from layer to layer. The most superficial ventricular muscles wrap around both ventricles. Deeper layers spiral around and between the individual muscles. 3) The *endocardium* covers the inner surfaces of the heart, including those of the heart valves. This simple squamous epithelium is continuous with the endothelium of the attached great vessels.

Orientation and Superficial Anatomy of the Heart

Advertisements and cartoons often show the heart at the center of the chest. However, a midsagittal section does not cut the heart in half. This is because the heart (1) lies slightly to the left of the midline, (2) sits at an angle to the longitudinal axis of the body, and (3) is rotated to the left. The heart is located within the mediastinum, between the two lungs. Because the heart lies slightly to the left of midline, the cardiac notch within the medial surface of the left lung is deeper than the cardiac notch in the medial surface of the right lung. The *base* of the heart is the broad, superior portion of the heart, where it is attached to the major arteries and veins of the systemic and pulmonary circuits. The base of the hear begins at the origins of the major vessels and the superior surfaces of the two atria. Thinking back to our balloon analogy, the base of the heart corresponds to your wrist. The base sits posterior to the sternum, approximately at the third costal cartilage. The *apex* of the heart is the inferior, point tip of the heart and is formed mainly by the left ventricle. It points laterally. The apex reaches the fifth intercostal space and extends to the left of the midline. The heart is rotated slightly to the left. Therefore, the base forms the *superior border* of the heart. The *right border* of the heart is formed by the right atrium. The left ventricle and a small portion of the left atrium form the *left border*. The left border extends to the apex, where it meets the *inferior border*. The apex is formed mainly by the left ventricle, and the inferior border is formed mainly by the inferior wall of the right ventricle. The *anterior surface*, or sternocostal surface, of the heart faces the anterior thoracic wall and consists mostly of the wall of the right ventricle and some of the left ventricle. The *posterior surface*, at the base, if formed by the left atrium and a small portion of the right atrium. The *diaphragmatic surface* of the heart is composed mainly of the posterior, inferior surfaces of the right and left ventricles. External grooves, or sulci, of the heart show the approximate borders of the four internal chambers of the heart. A shallow *interatrial groove* separated the two atria. The deeper *coronary sulcus* marks the border between the atria and the ventricles. On the anterior surface the *anterior interventricular sulcus* separates the left and right ventricles. The *posterior interventricular sulcus* separates the left and right ventricles on the posterior surface. A considerable amount of adipose tissue is usually found in the connective tissue of the epicardium at the coronary sulcus and the interventricular sulci. In fresh or preserved hearts, this fat must be removed to expose the underlying grooves. These sulci also contain coronary arteries and veins - the arteries and veins supplying blood to the cardiac muscle. The atria and ventricles have very different functions. The atria receive venous blood that continues flowing into the ventricles. The ventricles propel blood to the peripheral tissues and the lungs. These functional differences are linked to external and internal structural differences between the right and left sides of the heart. Because of the heart sits at an angle, the right atrium is anterior, inferior, and to the right of the left atrium. The left atrium curves posteriorly and forms most of the posterior surface of the heart superior to the coronary sulcus. Both atria have relatively thin, muscular walls. When the atria are not filled with blood, the anterior portion of each atrium deflates and becomes a rather lumpy and wrinkled flap. This is called an *auricle* because it reminded early anatomists of the external ear.

Intercalated Discs

Cardiac muscle cells are connected to neighboring cells at specialized cell junctions called *intercalated discs*. Intercalated discs are unique to cardiac muscle tissue. The arrangement of these specialized cell-to-cell junctions and the extensive interlocking of the adjacent cardiac plasma membranes give intercalated discs a jagged appearance. Features of intercalated discs include the following: - The plasma membranes of two cardiac muscle cells are bound together by desmosomes. This locks the cells together and prevents them from separating during contractions. - Intercalated discs possess a specialized junction termed a fascia adherens. Actin filaments in cardiac muscle cells anchor firmly to the plasma membrane at the fascia adherens within the intercalated disc. As a result, the intercalated discs ties together the actin filaments of the adjacent cells, and the two muscle cells "pull together" with maximum efficiency. - Cardiac muscle cells are also connected by gap junctions (communicating junctions). Ions and small molecules move between cells at gap junctions, creating a direct electrical connection between the two muscle cells. As a result, the stimulus for contraction - an action potential - moves from one cardiac muscle cell to another as if the sarcolemmae were continuous. *Because cardiac muscle cells are mechanically, chemically, and electrically connected to one another, cardiac muscle tissue functions like a single, enormous muscle cell.* The contraction of any one cell will trigger the contraction of several others, and the contraction will spread throughout the myocardium. For this reason, cardiac muscle has been called a *functional syncytium*.

Endocarditis

Endocarditis indicates inflammation of the endocardium. Endocarditis is almost always the result of a bacterial infection, but may also be the result of a fungal infection. Hearts with cardiac birth defects or damaged or abnormal valves are particularly susceptible to endocarditis. Endocarditis begins when bacteria enter the bloodstream and settle on the endocardium. This can happen following dental surgery or in the hospital following placement of a central venous access line. Another common cause is unsterile self-injection of drugs. Endocarditis can destroy heart valves, requiring their surgical replacement.

The Heart Introduction

Every living cell relies on the surrounding interstitial fluid as a source of oxygen & nutrients and as a place to dispose of wastes. Continuous exchange between the interstitial fluid and the circulating blood stabilizes the levels of gases, nutrients, and wastes in the interstitial fluid. This constant blood supply is essential for homeostasis. If blood flow stops, oxygen and nutrients in the blood are quickly depleted, wastes cannot be discarded, and hormones and white blood cells cannot reach their targets. Therefore, all functions of the cardiovascular system depend on the *heart*, because it is the heart that keeps blood moving. This muscular organ beats approximately 100,000 times each day. Each year, your heart pumps more than 1.5 million gallons of blood - enough to fill 200 train tank cars. To appreciate your heart's pumping abilities, turn on a facet in your kitchen and open it all the way. You would have to leave the facet on for 45 years to deliver an amount of water equal to the volume of blood pumped by the heart in an average lifetime. The nervous system closely monitors and regulates the performance of the heart to ensure that gas, nutrient, and waste levels in peripheral tissues remain within normal limits, whether you are sleeping peacefully, reading a book, or running a marathon. As a result of the nervous system's monitoring and regulation, the volume of blood pumped by the heart varies widely, ranging from 5 to 30 liters per minute.

The Left Coronary Artery

In most individuals, the lumen of the *left coronary artery LCA* is larger in diameter than the lumen of the right coronary artery. It supplies blood to (1) most of the left ventricle, (2) a small segment of the right ventricle, (3) most of the left atrium, and (4) the anterior two-thirds of the interventricular septum. As the left coronary artery reaches the anterior surface of the heart, it fofrms the anterior interventricular branch and the circumflex branch. The *anterior interventricular branch*, or the left anterior descending branch, is a large artery running along the anterior surface within the anterior interventricular sulcus. This artery supplies the anterior ventricular myocardium and the anterior two-thirds of the interventricular septum. Often, small branches of the anterior interventricular branch of the left coronary artery are continuous with those of the posterior interventricular branch of the right coronary artery. The *circumflex branch* curves to the left within the coronary sulcus. As it circles toward the posterior surface of the heart, it gives rise to one or more diagonal branches that supply portions of the left ventricule with blood. In most individuals the left coronary artery forms the *left marginal branch. Typically, this vessel reaches the apex of the heart and supplies much of the left ventricle. Upon reaching the posterior surface of the heart, the right coronary artery forms the *posterior left ventricular branch*. This branch is small and quite variable; in some individuals it is totally absent. If this branch is very small or absent, it is often replaced by the *posterior interventricular branch* of the circumflex artery. In addition, distal portions of the circumflex branch often meet and fuse with small branches of the right coronary artery. As mentioned previously, interconnections between coronary arteries are visible in various locations on the ventricles of the heart. Such interconnections are called *anastomoses*. Because the arteries are interconnected in this way, blood supply to the ventricular muscle remains relatively constant, regardless of pressure fluctuations within the left and right coronary arteries.

Myocarditis

Myocarditis is an inflammation of the myocardium. Myocarditis is commonly caused by viral infection and can affect young, healthy people. Myocarditis can also result from autoimmune disease, environmental toxins, alcohol, certain medications, and chemotherapy agents and radiation frequently used in breast cancer therapy. If enough cardiac muscle cells are damaged, chronic cardiomyopathy with impaired pumping power can result. Blood clots may form in the heart, possibly leading to heart attack or stroke. Chronic cardiomyopathy may require a heart transplant.

The Left Atrium

Oxygen enters the bloodstream at the pulmonary capillaries. The oxygen-rich (oxygenated) blood flows from the pulmonary capillaries into small veins. These ultimately unite to form four pulmonary veins, usually two for each lung. These *left* and *right pulmonary veins* empty into the posterior portion of the left atrium. The left atrium differs from the right atrium in that (1) the left atrium is more cuboidal in shape; (2) the left auricle is longer, narrower, and more hook-shaped; and (3) all of the pectinate muscles of the left atrium are contained within the left auricle. As blood flows from the left atrium into the left ventricle, it passes through the *left atrioventricular (AV) valve*, also known as the *mitral* or *bicuspid* valve. This valve has two cusps compared to the three seen in the right AV valve. The left AV valve permits the flow of oxygen-rich blood from the left atrium into the left ventricle, but prevents blood flow in the reverse direction.

The Right Ventricle

Oxygen-poor venous blood travels from the right atrium into the right ventricle. In doing so, the blood passes through an opening guarded by three fibrous flaps. These flaps, or *cusps*, form the *right atrioventricular (AV) valve*, or *tricuspid valve*. On one side, the cusps are attached to the cardiac skeleton of the heart. Their free edges are attached to connective tissue fibers called *chordae tendinae*. These fibers arise from the *papillary muscles* - cone-shaped muscular projections of the inner surface of the right ventricle. The chordae tendinae limit the movement of the cusps when the valve closes. This prevents backflow of blood from the right ventricle into the right atrium. The internal surface of the right ventricle contains a series of irregular muscular ridges called the *trabeculae carnea*. The *moderator band* is a muscular ridge that extends hroizontally from the inferior portion of the interventricular septum and connects to the anterior papillary muscle. The superior end of the right ventricle tapers to the *conus arteriosus*, a smooth-walled, cone-shaped pouch. The conus arteriosus ends at the *pulmonary valve* (pulmonary semilunar valve). This valve consits of three thick semilunar (half moon-shaped) cusps. As blood is pumped out of the right ventricle, it passes through this valve and enters the *pulmonary trunk*. the pulmonary trunk is the first vessel of the pulmonary circuit. The pulmonary valve prevents the backflow of blood into the right ventricle when that chamber relaxes. From the pulmonary trunk, blood flows into both the *left* and *right pulmonary arteries*. These vessels branch repeatedly within the lungs before supplying the pulmonary capillaries, where gas exchange occurs.

Pericarditis

Pericarditis refers to infection or inflammation of the pericardium surrounding the heart. The most frequent cause is a viral infection. Bacterial pericarditis can be a complication of tuberculosis. Other possible causes include cancer, which can invade the pericardium, and kidney failure, which causes uremic pericarditis. Pericarditis can also be due to severe chest trauma, with bleeding into the pericardial sac.

The Coronary Veins

The *great cardiac vein* and *middle cardiac vein* collect blood from smaller veins draining the myocardial capillaries. They deliver venous blood to the *coronary sinuses*, a large, thin-walled vein lying in the posterior portion of the coronary sulcus. The coronary sinus drains into the right atrium inferior to the opening of the inferior vena cava. Cardiac veins that empty into the great cardiac vein or the coronary sinus include (1) the *posterior vein of the left ventricle*, draining the area served by the circumflex branch of the left coronary artery, (2) the middle cardiac vein*, draining the area supplied by the posterior interventricular branch of the left coronary artery, and (3) the *small cardiac vein*, which receives blood from the posterior surfaces of the right atrium and ventricle. The *anterior cardiac veins* drain the anterior surface of the right ventricle. These vessels empty directly into the right atrium, bypassing the coronary sinus.

Internal Anatomy and Organization of the Heart

The *interatrial septum* separates the atria, and the *interventricular septum* separates the ventricles. Blood flows from each atrium into the ventricle of the same side. The *valves* are folds of endocardium extending into the openings between the atria and ventricles. These valves open and close to prevent the backflow of blood, maintaining a one-way flow of blood from the atria to the ventricles. The atria collect blood returning to the heart and then deliver it to the attached ventricle. The functional demands placed on the right and left atria are very similar, and the two chambers look almost identical. However, the demands placed on the right and left ventricles are very different. As a result, there are significant anatomical differences between the two ventricles.

The Right Coronary Artery

The *right coronary artery (RCA)* branches off the ascending aorta and turns to the right. This vessel lies within the coronary sulcus and passes between the right auricle and the pulmonary trunk. 60% of the time, the right coronary artery is the dominant coronary artery - the coronary artery that gives off a posterior interventricular branch. Although variations occur, the branches of the right coronary artery typically supply blood to (1) the right atrum, (2) a portion of the left atrium, (3) the interatrial septum, (4) the entire right ventricle, (5) a variable portion of the left ventricle, (6) the postero-inferior one-third of the interventricular septum, and (7) portions of the conducting system (sinoatrial node) of the heart. The right coronary artery gives off *atrial branches* as it curves across the anterior surface of the heart. These branches supply the right atrium and a portion of the left atrium with blood. Near the right border of the heart, the *right marginal branch* is formed. This vessel extends toward the apex of the heart along the anterior surface of the right ventricle. The right marginal branch supplies the right atrium, interatrial septum, and right ventricle with blood. As the right coronary artery continues across the posterior surface of the heart it gives off the *posterior interventricular branch*. This branch, or posterior descending artery, continues toward the apex of the heart within the posterior interventricular sulcus. This branch supplies blood to the interventricular septum and adjacent portions of the ventricles. A small branch near the base of the right coronary artery penetrates the atrial wall to reach the sinoatrial (SA) node, also known as the *cardiac pacemaker*. A small branch to the atrioventricular (AV) node, another part of the conducting system of the heart, originates from the right coronary artery near the posterior interventricular branch.

The Structure and Function of the Heart Valves

The atriventricular (AV) valves are located between the atria and the ventricles. Each AV valve has four components: (1) a ring of connective tissue attached to teh cardiac skeleton of the heart, (2) connective tissue cussps, which close the opening between the heart chambers, (3) chordae tendinae that attach the margins of the cusps to papillary muscles, and (4) the papillary msucles that tense the chordae tendinae. Tension in the papillary muscles and chordae tendinae keeps the cusps from swinging farther and opening into the atria. Thus, the chordae tendinae and papillary muscles are essential to prevent the *regurgitation*, or backflow, or blood into the atria each time the ventricles contract. The *pulmonary valve* is located at the junction between the right ventricle and the pulmonary artery, and the *aortic valve* is located at the junction between the left ventricle and the ascending aorta. These valves lack chordae tendinae because the three symmetrical cusps support one another like the legs of a tripod. Serious valvular abnormalities interfere with cardiac function, and the timing and intensity of the related heart sounds provide useful diagnostic information. Health professionals use an instrument, called a *stethoscope*, to listen to normal and abnormal heart sounds. Heart sounds may be muffled because they must pass through the pericardium, surrounding tissues, and chest wall. As a result, stethoscope placement does not always resound to the position of the valve being listened to.

Structural Differences between the Right and Left Ventricles

The best way to view anatomical differences between the right and left ventricles is in three-dimensional or sectional views. The lungs are close to the heart, and the pulmonary blood vessels are short and wide. As a result, the right ventricle does not need to work very hard to push blood through the pulmonary circuit. Accordingly, the muscular wall of the right ventricle is relatively thin. In sectional view it resembles a pouch attached to the massive wall of the left ventricle. Contraction of the right ventricle moves it toward the wall of the left ventricle, which compresses the blood within the right ventricle. The rising pressure fores the blood through the pulmonary valve and into the pulmonary trunk. This contraction moves blood very efficiently with minimal effort, but it develops relativity low pressure. Low pressure is all that is needed to move blood around the pulmonary circuit. Higher pressures would actually be dangerous because the pulmonary capillaries are very delicate. Pressures as high as those in systemic capillaries would damage the pulmonary vessels and force fluid into the alveoli of the lungs. An identical pumping arrangement would not work for the left ventricle. Four to six times more force must be generated to push blood through the systemic circuit. The left ventricle, which has an extremely thick muscular wall, is round in cross section. When the left ventricle contracts, it shortens and narrows, and (1) the distance between the base and apex decreases, and (2) the diameter of the left ventricle chamber decreases. Imagine the effects of simultaneously squeezing and rolling up the end of a toothpaste tube and you have the idea. The pressure generated is more than enough to force open the aortic valve and eject blood into the ascending aorta. As the powerful left ventricle contracts, it also bulges into the right ventricular cavity. This action improves the pumping efficiency of the right ventricle. Individuals with severe damage to the right ventricle may survive because the contraction of the left ventricle helps push blood into the pulmonary circuit.

The Cardiac Skeleton

The connective tissues of the heart include large numbers of reticular, collagen, and elastic fibers. Each cardiac muscle cell is wrapped in a strong, elastic sheath, and adjacent cells are tied together by fibrous cross-links, or "struts." In turn, each muscle layer has a fibrous wrapping, and fibrous sheets separate the superficial and deep muscle layers. These connective tissue layers are continuous with bands of dense connective tissue that (1) encircle the bases of the pulmonary trunk and aorta, (2) encircle the valves of the heart, (3) connect the fibrous rings surrounding the openings for the heart valves, and (4) extend into the cardiac muscle that separates the atria and ventricles. This extensive connective tissue network is called the *cardiac skeleton* of the heart. Functions of the cardiac skeleton include: - Stabilizing the positions of the muscle cells and valves in the heart. - Providing physical support for the cardiac muscle cells and the blood vessels and nerves within the myocardium. - Distributing the forces of contraction. - Reinforcing the valves and helping prevent over expansion of the heart. - Providing the elasticity that returns the heart to its original shape after each contraction. - Physically isolating the atrial muscle cells form the ventricular muscle cells.

The Coordination of Cardiac Contractions

The function of any pump is to (1) develop pressure and (2) move a volume of fluid in a specific direction at an acceptable speed. The heart works in cycles of contraction (systole) and relaxation (diastole), and the pressure within each chamber rises and falls within each cycle. The AV, aortic, and pulmonary valves help ensure a one-way flow of blood. Blood will flow out of an atrium only when the AV valve is open and atrial pressure is greater than ventricular pressure. Likewise, blood will flow from a ventricle into the aorta or pulmonary trunk only when the aortic or pulmonary valve is open and ventricular pressure is greater than arterial pressure. The proper functioning of the heart depends on the proper timing of atrial and ventricular contractions. The pacemaking and conduction systems of the heart provide this required timing. Unlike skeletal muscle, cardiac muscle tissue contracts on its own, without neural or hormonal stimulation; even a heart removed for a heart transplant will continue to beat unless it is kept chilled in a preservation solution. This ability to generate and conduct impulses is called autorhythmicity. Neural or hormonal stimuli alter the basic rhythm of the contractions. Cardiac contractions are coordinated by specialized conducting cells. These specialized cardiac muscle cells do not contract like the other cells within the heart. There are two distinct populations of these cells: (1) *Nodal cells* establish the rate of cardiac contraction, and (2) *conducting cells* distribute the contractile stimulus to the myocardium.

An Overview of the Cardiovascular System

The heart is a small organ - roughly the size of your fist. It has four muscular chambers, the right and left *atria* and right and left *ventricles*. These four chambers work together, pumping blood through a network of blood vessels that connect the heart to peripheral tissues. The network of vessels is divided into two circuits: the pulmonary circuit and the systemic circuit. The *pulmonary circuit* carries carbon dioxide-rich blood from the heart to the gas exchange surfaces of the lungs and returns the oxygen-rich blood to the heart. The *systemic circuit* transports oxygen-rich blood from the heart to the rest of the body's cells and returns carbon dioxide-rich blood back to the heart. The right atrium receieves blood from the systemic circuit, and the right ventricle pumps blood into the pulmonary circuit. The left atrium receives blood from the pulmonary circuit, and the left ventricle pumps blood into the systemic circuit. With each heartbeat, the atria contract first, followed by the ventricles. The two ventricles contract at the same time and eject equal volumes of blood into the pulmonary and systemic circuits. Each circuit begins and ends at the heart, and blood flows through these circuits in sequence. Thus, blood returning to the heart from the systemic circuit must complete the pulmonary circuit before re-entering the systemic circuit. The blood vessels of both circuits are arteries, veins, and capillaries. *Arteries* transport blood away from the heart; *veins* return blood to the heart. *Capillaries are small, thin-walled vessels connecting the smallest arteries and veins. Capillaries are called exchange vessels because their thin walls permit exchange of nutrients, dissolved gases, and wastes between the blood and surrounding tissues.

The Pericardium

The heart is located near the anterior chest wall, directly posterior to the sternum. The mediastinum contains great vessels, which are attached at the base of the heart, as well as the thymus, esophagus, and trachea. The *pericardium* surrounds the heart and is composed of two parts: an outer fibrous pericardium and an inner serous pericardium. The *fibrous pericardium* is composed of a dense network of collagen fibers that stabilize the position of the heart and associated vessels within the mediastinum. The lining of the pericardium is the *serous cardicardium*. This two-layered membrane is composed of an outer *parietal layer* and an inner *visceral layer*. The visceral layer is also known as the *epicardium*. The potential, fluid-filled space between these two serous layers is the *pericardial cavity*. The pericardial cavity normally contains up to 50 mL of *pericardial fluid*, secreted by the pericardial membranes. This fluid acts as a lubricant, reducing friction between the opposing visceral and parietal surfaces as the heart beats. To visualize the relationship between the heart and the pericardial cavity, imagine pushing your fist toward the center of a large, partially inflated balloon. The balloon represents the pericardium, and your fist represents the heart.

Coronary Blood Vessels

The heart works continuously, and cardiac muscle cells require reliable supplies of oxygen and nutrients. The *coronary circulation* supplies blood to the muscle tissue of the heart. During maximum exertion, the oxygen demand rises considerably, and blood flow to the heart may increase to nine times resting levels. The coronary circulation includes an extensive network of coronary blood vessels. Considerable variations often occur between individuals. The descriptions given here are considered the typical pattern. The left and right *coronary arteries* originate at the base of the ascending aorta, within the aortic sinuses. They are the first branches from this vessel. Blood pressure here is the highest found anywhere in the systemic circuit, and this pressure guarantees a continuous flow of blood to meet the demands of active cardiac muscle tissue.

Cardiac Muscle Tissue

The histological characteristics of cardiac muscle tissue give the heart its unique functional properties. Cardiac muscle cells are relatively small, averaging 10-20 um in diameter and 50-100 mm in length. A typical cardiac muscle cell has a single, centrally placed nucleus. Although they are much smaller, cardiac muscle cells resemble skeletal muscle fibers. Each cardiac muscle cell contains organized myofibrils, and the arrangement of their sarcomeres produces striations. However, cardiac muscle cells differ from skeletal muscle fibers in several important respects: - Cardiac muscle cells are almost totally dependent on aerobic respiration to obtain the energy needed to contract. Therefore, the sarcoplasm of a cardiac muscle cell contains hundreds of mitochondria and large reserves of myoglobin to store oxygen. The sarcoplasm of cardiac muscle cells also contains large amounts of glycogen and lipid inclusions as energy reserves. - The T tubules of cardiac muscle cells are shorter than those of skeletal muscle cells. In addition, T tubules do not form triads with the sarcoplasmic reticulum. - Cardiac muscle has a larger number of blood vessels, even more than in red skeletal muscle tissue. -Cardiac muscle cells contract without nervous system stimulation. - Cardiac muscle cells are interconnected by specialized junctions called intercalated discs.

The Cardiac Cycle

The plasma membrane of nodal cells process unique qualities that allow these cells to spontaneously depolarize to threshold. In addition, nodal cells possess intercellular junctions that electrically couple these cells to one another, to the conducting fibers, and to cardiac muscle cells. When a nodal cell depolarizes, it generates an action potential. The action potential travels through the conducting system of the heart and reaches all the cardiac muscle tissue, causing a contraction and a *cardiac cycle*, or a complete heartbeat. In this way, nodal cells determine the heart rate. Not all nodal cells depolarize at the same rate. The normal rate of contraction is determined by which nodal cells reach threshold first. The impulse they produce brings all other nodal cells to threshold. These rapidly depolarizing cells are called *pacemaker cells*. They are found in the *sinoatrial node (SA node)*, or *cardiac pacemaker*. The SA node is located in the posterior wall of the right atrium, near the entrance of the superior vena cava. These pacemaker cells depolarize spontaneously and rapidly, generating 80-100 action potentials per minute. Each time the SA node generates an impulse, it produces a heartbeat. Therefore, the resting heart rate is 80-100 beats per minute (bpm). However, any factor that changes either the SA node's resting potential or the rate of spontaneous depolarization alters the heart rate. For example, nodal cell activity is affected by the activity of the autonomic nervous system. When parasympathetic neurons release acetylcholine (ACh), the rate of spontaneous depolarization slows, and the heart rate decreases. In contrast, when sympathetic neurons release norepinephrine (NE), the rate of depolarization increases, and the heart rate increases. Under normal resting conditions, parasympathetic activity reduces the heart rate from the inherent nodal rate of 80-100 impulses per minute to a more leisurely 70-80 beats per minute. A number of clinical problems result from abnormal pacemaker cell function. *Bradycardia* indicates a slower-than-normal heart rate, whereas *tachycardia* refers to a faster-than-normal heart rate. In clinical practice the definition varies depending on the normal resting heart rate and conditioning of the individual. The cells of the SA node are electrically connected to those of the larger *atrioventricular node (AV node)* through conducting fibers in the atrial wall.

The Right Atrium

The right atrium receives oxygen-poor (deoxygenated) venous blood from the systemic circuit by the *superior vena cava* and the *inferior vena cava*. The superior vena cava opens into the posterior, superior portion of the right atrium. It receives venous blood from the head, neck, upper limbs, and chest. The inferior vena cava opens into the posterior, inferior portion of the right atrium. It receives venous blood from the tissues and organs of the abdominal and pelvic cavities and the lower limbs. The veins of the heart itself, called coronary veins, collect blood from the heart wall and deliver it to the coronary sinus. the coronary sinus opens into the posterior wall of the right atrium, inferior to the opening of the inferior vena cava. The *pectinate muscles* extend along the inner surface of the right auricle and across the anterior wall of the right atrium. The *interatrial septum* separates the right and left atria. From the fifth week of embryonic development until birth, there is an oval opening in this septum. This opening is called the *foramen ovale*. The foramen ovale allows blood to flow directly from the right atrium to the left atrium while the lungs are developing. At birth, the lungs expand and begin functioning, and the foramen ovale closes. Within 3 months, it is permanently sealed. A small depression called the *fossa ovalis* remains at this site in the adult heart. Occasionally, the foramen ovale remains patent (open). As a result, blood passes from the left atrium into the right atrium and recirculates into the pulmonary circuit. This reduces the efficiency of the systemic circulation and elevates blood pressure in the pulmonary vessels. If not corrected, this leads to cardiac enlargement, fluid buildup in the lungs, and eventual heart failure.

Autonomic Control of Heart

The sympathetic and parasympahetic divisions of the autonomic nervous system (ANS) innervate the heart through the cardiac plexus. Both the symapthetic and parasympathetic divisions of the ANS innervate the SA and AV nodes. These divisions also innervate the atrial and ventricular cardiac muscle cells and smooth muscles in the walls of the cardiac blood vessels. - NE release increases heart rate and contraction force by stimulating the beta receptors on nodal cells and contractile cells. - ACh release decreases heart rate and contraction force by stimulating the muscarinic receptors on nodal cells and contracile cells. The cardiac centers of the medulla oblongata contain ANS centers for cardiac control. Stimulation of the *cardioacceleratory center* activates synmapthetic neurons; the nearby *cardioinhibitory center* activates parasympathetic neurons. Teh cardiac centers receive inputs from higher centers, especially from the parasympathetic and sympathetic headquarters in the hypothalmus.

The Left Ventricle

The wall of the left ventricle is approximately three times thicker than the wall of the right ventricle. Contractions of the left ventricle must produce enough pressure to push the blood through the entire systemic circuit. The right ventricle, in contrast, has a relatively thin wall. It only has to develop enough pressure to push blood into the lungs and then back to the heart, a total distance of only about 30 cm (1 ft). The internal organization of the left ventricle closely resembles that of the right ventricle. However, (1) its trabeculae carneae are more prominent than they are in the right ventricle; (2) there is no moderator band; and (3) since the left AV valve has only two cusps, there are two large papillary muscles rather than three. Blood leaving the left ventricle passes through the *aortic valve* (aortic semilunar valve) into the *ascending aorta*. The arrangement of the cusps in the aortic valve is similar to that in the pulmonary valve. Small, saclike dilations of the base of the ascending aorta occur next to each cusp of the aortic valve. These sacs, called *aortic sinuses*, prevent the individual cusps from sticking to the wall of the aorta when the valve opens. The right and left coronary arteries, which deliver blood to the myocardium, originate at the aortic sinuses. The aortic valve prevents the backflow of blood into the left ventricle once it has been pumped out of the heart and into the systemic circuit. From the ascending aorta, blood flows into the *aortic arch* and then into the *descending aorta*. The pulmonary trunk is attached to the aortic arch by the *ligamentum arteriosum*, a fibrous band of connective tissue that is left over from an important fetal blood vessel that once linked the pulmonary and systemic circuits.