HEART

The heart is located at the center of the chest. The muscle mass is greater on the left side and the apex of the heart is pointed slightly to the left.

HUMAN HEART

The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients, and also assists in the removal of metabolic wastes. The heart is located in the middle compartment of the mediastinum in the chest. In humans, other mammals and birds the heart is divided into four chambers: upper left and right atria; and lower left and right ventricles.Commonly the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart.In a healthy heart blood flows one way through the heart due to heart valves, which prevent backflow. The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium; myocardium; and endocardium.

The heart pumps blood through both circulatory systems, blood low in oxygen from the systemic circulation enters the right atrium from the superior and inferior vena cavae and passes to the right ventricle. From here it is pumped into the pulmonary circulation, through the lungs where it receives oxygen and gives off carbon dioxide. Oxygenated blood then returns to the left atrium, passes through the left ventricle and is pumped out through the aorta to the systemic circulation-where the oxygen is used and metabolized to carbon dioxide.In addition the blood carries nutrients from the liver and gastrointestinal tract to various organs of the body, while transporting waste to the liver and kidneys.Normally with each heartbeat, the right ventricle pumps the same amount of blood into the lungs as the left ventricle pumps out into the body. Veins transport blood to the heart, while arteries transport blood away from the heart. Veins normally have lower pressures than arteries.The heart contracts at a rate of around 72 beats per minute, at rest. Exercise temporarily increases this rate, but lowers resting heart rate in the long term, and is good for heart health.                       

Cardiovascular diseases (CVD) were the most common cause of death globally in 2008, accounting for 30% of cases.Of these deaths more than three quarters were due to coronary artery disease and stroke. Risk factors include: smoking, being overweight, not enough exercise, high cholesterol, high blood pressure, and poorly controlled diabetes among others.Diagnosis of CVD is often done by listening to the heart-sounds with a stethoscope, ECG or by ultrasound.Diseases of the heart are primarily treated by cardiologists, although many specialties of medicine may be involve.

 Some of the vital hearty terms 

Heart wall

Layers of the heart wall, including visceral and parietal pericardium.

The heart wall is made up of the inner endocardium, middle myocardium and outer epicardium. These are surrounded by a double-membraned sac called the pericardium.

The innermost layer of the heart is called the endocardium. It is made up of a lining of simple squamous epithelium, and covers heart chambers and valves. It is continuous with the endothelium of the veins and arteries of the heart, and is joined to the myocardium with a thin layer of connective tissue.The endocardium, by secreting endothelins, may also play a role in regulating the contraction of the myocardium. 

The middle layer of the heart wall is the myocardium, which is a layer of involuntary striated muscle tissue surrounded by a framework of collagen. It is also supplied with blood vessels, and nerve fibers by way of the epicardium that help to regulate the heart rate.

The pericardium surrounds the heart. It consists of two membranes: an inner serous membrane called the epicardium, and an outer fibrous membrane. These enclose the pericardial cavity, the pericardial cavity contains fluid which lubricates the surface of the heart.

Chambers

Heart being dissected showing right and left ventricles, from above

The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The atria are connected to the ventricles by the atrioventricular valves and separated from the ventricles by the coronary sulcus. There is an ear-shaped structure in the upper right atrium called the right atrial appendage, or auricle, and another in the upper left atrium, the left atrial appendage. The right atrium and the right ventricle together are sometimes referred to as the right heart and this sometimes includes the pulmonary artery. Similarly, the left atrium and the left ventricle together are sometimes referred to as the left heart. These are separated by the posterior interventricular sulcus.

The cardiac skeleton is made of dense connective tissue and this gives structure to the heart. It forms the atrioventricular septum which separates the atria from the ventricles, and the fibrous rings which serve as bases for the four heart valves.The cardiac skeleton also provides an important boundary in the heart’s electrical conduction system since collagen cannot conduct electricity. The interatrial septum separates the atria and the interventricular septum separates the ventricles.The interventricular septum is much thicker than the interatrial septum, since the ventricles need to generate greater pressure when they contract.

Valves

With the atria and major vessels removed, all four valves are clearly visible.

The heart, showing valves, arteries and veins. The white arrows shows the normal direction of blood flow.

Frontal section showing papillary muscles attached to the tricuspid valve on the right and to the mitral valve on the left via chordae tendineae.

All four heart valves lie along the same plane. The valves ensure unidirectional blood flow through the heart and prevent backflow. Between the right atrium and the right ventricle is the tricuspid valve. This consists of three cusps (flaps or leaflets), made of endocardium reinforced with additional connective tissue. Each of the three valve-cusps is attached to several strands of connective tissue, the chordae tendineae (tendinous cords), sometimes referred to as the heart strings. They are composed of approximately 80 percentcollagenous fibers with the remainder consisting of elastic fibers and endothelium. They connect each of the cusps to a papillary muscle that extends from the lower ventricular surface. These muscles control the opening and closing of the valves. The three papillary muscles in the right ventricle are called the anterior, posterior, and septal muscles, which correspond to the three positions of the valve cusps.

Between the left atrium and left ventricle is the mitral valve, also known as the bicuspid valve due to its having two cusps, an anterior and a posterior medial cusp. These cusps are also attached via chordae tendinae to two papillary muscles projecting from the ventricular wall.

The tricuspid and the mitral valves are the atrioventricular valves. During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight. However, as the ventricle contracts, so do the papillary muscles. This creates tension on the chordae tendineae, helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria.

The semilunar pulmonary valve is located at the base of the pulmonary artery. This has three cusps which are not attached to any papillary muscles. When the ventricle relaxes blood flows back into the ventricle from the artery and this flow of blood fills the pocket-like valve, pressing against the cusps which close to seal the valve. The semilunar aortic valve is at the base of the aorta and also is not attached to papillary muscles. This too has three cusps which close with the pressure of the blood flowing back from the aorta.

Right heart

The two major systemic veins, the superior and inferior venae cavae, and the collection of veins that make up the coronary sinus which drains the myocardium, empty into the right atrium. The superior vena cava drains blood from above the diaphragm and empties into the upper back part of the right atrium. The inferior vena cava drains the blood from below the diaphragm and empties into the back part of the atrium below the opening for the superior vena cava. Immediately above and to the middle of the opening of the inferior vena cava is the opening of the thin-walled coronary sinus.

In the wall of the right atrium is an oval-shaped depression known as the fossa ovalis, which is a remnant of an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the pulmonary circuit. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern. Most of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface has prominent ridges of pectinate muscles, which are also present in the right atrial appendage.

The atria receive venous blood on a nearly continuous basis, preventing venous flow from stopping while the ventricles are contracting. While most ventricular filling occurs while the atria are relaxed, they do demonstrate a contractile phase when they actively pump blood into the ventricles just prior to ventricular contraction. The right atrium is connected to the right ventricle by the tricuspid valve.

When the myocardium of the ventricle contracts, pressure within the ventricular chamber rises. Blood, like any fluid, flows from higher pressure to lower pressure areas, in this case, toward the pulmonary artery and the atrium. To prevent any potential backflow, the papillary muscles also contract, generating tension on the chordae tendineae. This prevents the flaps of the valves from being forced into the atria and regurgitation of the blood back into the atria during ventricular contraction.

The walls of the right ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator band reinforces the thin walls of the right ventricle and plays a crucial role in cardiac conduction. It arises from the lower part of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle.

When the right ventricle contracts, it ejects blood into the pulmonary artery, which branches into the left and right pulmonary arteries that carry it to each lung. The upper surface of the right ventricle begins to taper as it approaches the pulmonary artery. At the base of the pulmonary artery is the pulmonary semilunar valve that prevents backflow from the pulmonary artery.[7]

Left heart

After gas exchange in the pulmonary capillaries, blood returns to the left atrium high in oxygen via one of the four pulmonary veins. Only the left atrial appendagecontains pectinate muscles. Blood flows nearly continuously from the pulmonary veins back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. This atrial contraction accounts for approximately 20 percent of ventricular filling. The left atrium is connected to the left ventricle by the mitral valve.

Although both sides of the heart will pump the same amount of blood, the muscular layer is much thicker in the left ventricle compared to the right, due to the greater force needed here. Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The left ventricle is the major pumping chamber for the systemic circuit; it ejects blood into the aorta through the aortic semilunar valve.

Cardiac muscle

The swirling pattern of myocardium helps the heart pump effectively

Cardiac muscle tissue has autorhythmicity, the unique ability to initiate a cardiac action potential at a fixed rate – spreading the impulse rapidly from cell to cell to trigger the contraction of the entire heart. This autorhythmicity is still modulated by the endocrine and nervous systems.

There are two types of cardiac muscle cell: cardiomyocytes which have the ability to contract easily, and modified cardiomyocytes the pacemaker cells of the conducting system. The cardiomyocytes make up the bulk (99%) of cells in the atria and ventricles. These contractile cells respond to impulses of action potential from the pacemaker cells and are responsible for the contractions that pump blood through the body. The pacemaker cells make up just (1% of cells) and form the conduction system of the heart. They are generally much smaller than the contractile cells and have few of the myofibrils or myofilaments which means that have limited contractibility. Their function is similar in many respects to neurons.

It is the contraction of the myocardium that pumps blood through the heart and into the major arteries.The muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart. They form a figure 8 pattern around the atria and around the bases of the great vessels. Deeper ventricular muscles also form a figure 8 around the two ventricles and proceed toward the apex. More superficial layers of ventricular muscle wrap around both ventricles. This complex swirling pattern allows the heart to pump blood more effectively than a simple linear pattern would.

Surface features

Inside the pericardium, the chambers and a series of sulci are visible. Major coronary blood vessels are located in these sulci. The deep coronary sulcus is located between the atria and ventricles. Between the left and right ventricles are two additional sulci that are not as deep as the coronary sulcus. On the front and back of the heart's surface are the anterior and posterior interventricular sulci.[7] These two grooves separate the ventricles.

Coronary circulation

Arterial supply to the heart (red), with other areas labelled (blue).

Main article: Coronary circulation

Cardiomyocytes like all other cells need to be supplied with oxygen, nutrients and a way of removing metabolic wastes. This is achieved by the coronary circulation. The coronary circulation cycles in peaks and troughs relating to the heart muscle relaxing or contracting.

Coronary arteries supply blood to the heart and the coronary veins remove the deoxygenated blood. There is a left and a right coronary artery supplying the left and right hearts respectively, and the septa. Smaller branches of these arteries anastomose, which in other parts of the body serve to divert blood due to a blockage. In the heart these are very small and cannot form other interconnections with the result that a coronary artery blockage can cause a myocardial infarction and with it, tissue damage.

The great cardiac vein receives the major branches of the posterior, middle, and small cardiac veins and drains into the coronary sinus a large vein that empties into the right atrium. The anterior cardiac veins drain the front of the right ventricle and drain directly into the right atrium.

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