Heart

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Heart
left main coronary artery[c]
VeinSuperior vena cava, inferior vena cava,[d] right and left pulmonary veins,[e] great cardiac vein, middle cardiac vein, small cardiac vein, anterior cardiac veins[f]
NerveAccelerans nerve, vagus nerve
Identifiers
Latincor
Greekkardía (καρδία)
MeSHD006321
TA98A12.1.00.001
TA23932
Anatomical terminology]

The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system.[1] The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to the lungs.[2] In humans, the heart is approximately the size of a closed fist and is located between the lungs, in the middle compartment of the chest, called the mediastinum.[3]

In humans, other mammals, and birds, the heart is divided into four chambers: upper left and right

vertebrates, the heart has an asymmetric orientation, almost always on the left side. According to one theory, this is caused by a developmental axial twist in the early embryo.[8][9]

The heart pumps blood with a

Exercise temporarily increases the rate, but lowers it in the long term, and is good for heart health.[12]

cardiologists, although many specialties of medicine may be involved in treatment.[14]

Structure

Human heart during an autopsy
Computer generated animation of a beating human heart
Computer-generated animation of a beating human heart
Cardiology video

Location and shape

Real-time MRI of the human heart
The human heart is in the middle of the thorax, with its apex pointing to the left.[16]

The human heart is situated in the

articulation with the costal cartilages.[7]

The largest part of the heart is usually slightly offset to the left side of the chest (though occasionally it may be

lungs, the left lung is smaller than the right lung and has a cardiac notch in its border to accommodate the heart.[7]
The heart is cone-shaped, with its base positioned upwards and tapering down to the apex.[7] An adult heart has a mass of 250–350 grams (9–12 oz).[18] The heart is often described as the size of a fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness,[7] although this description is disputed, as the heart is likely to be slightly larger.[19] Well-trained athletes can have much larger hearts due to the effects of exercise on the heart muscle, similar to the response of skeletal muscle.[7]

Chambers

Heart being dissected showing right and left ventricles, from above

The heart has four chambers, two upper

anterior longitudinal sulcus and the posterior interventricular sulcus.[20]

The fibrous cardiac skeleton 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.[22] 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.[7] The interventricular septum is much thicker than the interatrial septum since the ventricles need to generate greater pressure when they contract.[7]

Valves

With the atria and major vessels removed, all four valves are clearly visible.[7]
The heart, showing valves, arteries and veins. The white arrows show 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.[7]

The heart has four valves, which separate its chambers. One valve lies between each atrium and ventricle, and one valve rests at the exit of each ventricle.[7]

The valves between the atria and ventricles are called the atrioventricular valves. Between the right atrium and the right ventricle is the

chordae tendinae and three papillary muscles named the anterior, posterior, and septal muscles, after their relative positions.[23] The mitral valve lies between the left atrium and left ventricle. It is also known as the bicuspid valve due to its having two cusps, an anterior and a posterior cusp. These cusps are also attached via chordae tendinae to two papillary muscles projecting from the ventricular wall.[24]

The papillary muscles extend from the walls of the heart to valves by cartilaginous connections called chordae tendinae. These muscles prevent the valves from falling too far back when they close.[25] During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight. As the heart chambers contract, 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.[7] [g][23]

Two additional semilunar valves sit at the exit of each of the ventricles. The 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.[7]

Right heart

The right heart consists of two chambers, the right atrium and the right ventricle, separated by a valve, the tricuspid valve.[7]

The right atrium receives blood almost continuously from the body's two major

right atrial appendage.[7]

The right atrium is connected to the right ventricle by the tricuspid valve.

pulmonary trunk, into which it ejects blood when contracting. The pulmonary trunk branches into the left and right pulmonary arteries that carry the blood to each lung. The pulmonary valve lies between the right heart and the pulmonary trunk.[7]

Left heart

The left heart has two chambers: the left atrium and the left ventricle, separated by the mitral valve.[7]

The left atrium receives oxygenated blood back from the lungs via one of the four

left atrial appendage. Like the right atrium, the left atrium is lined by pectinate muscles.[26] The left atrium is connected to the left ventricle by the mitral valve.[7]

The left ventricle is much thicker as compared with the right, due to the greater force needed to pump blood to the entire body. Like the right ventricle, the left also has

heart muscle; the left coronary artery is above the left cusp of the valve, and the right coronary artery is above the right cusp.[7]

Wall

Layers of the heart wall, including visceral and parietal pericardium

The heart wall is made up of three layers: the inner

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

endothelins, may also play a role in regulating the contraction of the myocardium.[7]

The swirling pattern of myocardium helps the heart pump effectively

The middle layer of the heart wall is the myocardium, which is the cardiac muscle—a layer of involuntary striated muscle tissue surrounded by a framework of collagen. The cardiac muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart, with the outer muscles forming a figure 8 pattern around the atria and around the bases of the great vessels and the inner muscles, forming a figure 8 around the two ventricles and proceeding toward the apex. This complex swirling pattern allows the heart to pump blood more effectively.[7]

There are two types of cells in cardiac muscle:

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.[7]

There are specific

CDH2 and PKP2. Other proteins expressed are MYH7 and LDB3 that are also expressed in skeletal muscle.[29]

Pericardium

The pericardium is the sac that surrounds the heart. The tough outer surface of the pericardium is called the fibrous membrane. This is lined by a double inner membrane called the serous membrane that produces pericardial fluid to lubricate the surface of the heart.[30] The part of the serous membrane attached to the fibrous membrane is called the parietal pericardium, while the part of the serous membrane attached to the heart is known as the visceral pericardium. The pericardium is present in order to lubricate its movement against other structures within the chest, to keep the heart's position stabilised within the chest, and to protect the heart from infection.[31]

Coronary circulation

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

Heart tissue, like all cells in the body, needs to be supplied with

lymphatic vessels. Blood flow through the coronary vessels occurs in peaks and troughs relating to the heart muscle's relaxation or contraction.[7]

Heart tissue receives blood from two arteries which arise just above the aortic valve. These are the

left circumflex artery. The left anterior descending artery supplies heart tissue and the front, outer side, and septum of the left ventricle. It does this by branching into smaller arteries—diagonal and septal branches. The left circumflex supplies the back and underneath of the left ventricle. The right coronary artery supplies the right atrium, right ventricle, and lower posterior sections of the left ventricle. The right coronary artery also supplies blood to the atrioventricular node (in about 90% of people) and the sinoatrial node (in about 60% of people). The right coronary artery runs in a groove at the back of the heart and the left anterior descending artery runs in a groove at the front. There is significant variation between people in the anatomy of the arteries that supply the heart [32] The arteries divide at their furthest reaches into smaller branches that join at the edges of each arterial distribution.[7]

The

posterior cardiac vein (draining the back of the left ventricle), the middle cardiac vein (draining the bottom of the left and right ventricles), and small cardiac veins.[33] The anterior cardiac veins drain the front of the right ventricle and drain directly into the right atrium.[7]

Small lymphatic networks called

inferior tracheobronchial node. The right vessel travels along the right atrium and the part of the right ventricle sitting on the diaphragm. It usually then travels in front of the ascending aorta and then ends in a brachiocephalic node.[34]

Nerve supply

Autonomic innervation of the heart

The heart receives nerve signals from the vagus nerve and from nerves arising from the sympathetic trunk. These nerves act to influence, but not control, the heart rate. Sympathetic nerves also influence the force of heart contraction.[35] Signals that travel along these nerves arise from two paired cardiovascular centres in the medulla oblongata. The vagus nerve of the parasympathetic nervous system acts to decrease the heart rate, and nerves from the sympathetic trunk act to increase the heart rate.[7] These nerves form a network of nerves that lies over the heart called the cardiac plexus.[7][34]

The vagus nerve is a long, wandering nerve that emerges from the

positively charged ions.[7] Norepinephrine binds to the beta–1 receptor.[7]

Development

Development of the human heart during the first eight weeks (top) and the formation of the heart chambers (bottom). In this figure, the blue and red colors represent blood inflow and outflow (not venous and arterial blood). Initially, all venous blood flows from the tail/atria to the ventricles/head, a very different pattern from that of an adult.[7]

The heart is the first functional organ to develop and starts to beat and pump blood at about three weeks into

embryogenesis. This early start is crucial for subsequent embryonic and prenatal development
.

The heart derives from

cardiogenic region. Two endocardial tubes form here that fuse to form a primitive heart tube known as the tubular heart.[37] Between the third and fourth week, the heart tube lengthens, and begins to fold to form an S-shape within the pericardium. This places the chambers and major vessels into the correct alignment for the developed heart. Further development will include the formation of the septa and the valves and the remodeling of the heart chambers. By the end of the fifth week, the septa are complete, and by the ninth week, the heart valves are complete.[7]

Before the fifth week, there is 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 lungs. 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. A depression in the surface of the right atrium remains where the foramen ovale was, called the fossa ovalis.[7]

The

fetal stage) it starts to decelerate, slowing to around 145 (±25) bpm at birth. There is no difference in female and male heart rates before birth.[40]

Physiology

Blood flow

Blood flow through the valves
Blood flow through the heart
Video explanation of blood flow through the heart

The heart functions as a pump in the

systemic circulation to and from the body and the pulmonary circulation to and from the lungs. Blood in the pulmonary circulation exchanges carbon dioxide for oxygen in the lungs through the process of respiration. The systemic circulation then transports oxygen to the body and returns carbon dioxide and relatively deoxygenated blood to the heart for transfer to the lungs.[7]

The

capillaries. As these pass by alveoli carbon dioxide is exchanged for oxygen. This happens through the passive process of diffusion
.

In the left heart, oxygenated blood is returned to the left atrium via the pulmonary veins. It is then pumped into the left ventricle through the mitral valve and into the aorta through the aortic valve for systemic circulation. The aorta is a large artery that branches into many smaller arteries, arterioles, and ultimately capillaries. In the capillaries, oxygen and nutrients from blood are supplied to body cells for metabolism, and exchanged for carbon dioxide and waste products.[7] Capillary blood, now deoxygenated, travels into venules and veins that ultimately collect in the superior and inferior vena cavae, and into the right heart.

Cardiac cycle

The cardiac cycle as correlated to the ECG

The cardiac cycle is the sequence of events in which the heart contracts and relaxes with every heartbeat.[11] The period of time during which the ventricles contract, forcing blood out into the aorta and main pulmonary artery, is known as systole, while the period during which the ventricles relax and refill with blood is known as diastole. The atria and ventricles work in concert, so in systole when the ventricles are contracting, the atria are relaxed and collecting blood. When the ventricles are relaxed in diastole, the atria contract to pump blood to the ventricles. This coordination ensures blood is pumped efficiently to the body.[7]

At the beginning of the cardiac cycle, the ventricles are relaxing. As they do so, they are filled by blood passing through the open mitral and tricuspid valves. After the ventricles have completed most of their filling, the atria contract, forcing further blood into the ventricles and priming the pump. Next, the ventricles start to contract. As the pressure rises within the cavities of the ventricles, the mitral and tricuspid valves are forced shut. As the pressure within the ventricles rises further, exceeding the pressure with the aorta and pulmonary arteries, the aortic and pulmonary valves open. Blood is ejected from the heart, causing the pressure within the ventricles to fall. Simultaneously, the atria refill as blood flows into the right atrium through the superior and inferior vena cavae, and into the left atrium through the pulmonary veins. Finally, when the pressure within the ventricles falls below the pressure within the aorta and pulmonary arteries, the aortic and pulmonary valves close. The ventricles start to relax, the mitral and tricuspid valves open, and the cycle begins again.[11]

Cardiac output

The x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure.[7]

Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle (stroke volume) in one minute. This is calculated by multiplying the stroke volume (SV) by the beats per minute of the heart rate (HR). So that: CO = SV x HR.[7] The cardiac output is normalized to body size through body surface area and is called the cardiac index.

The average cardiac output, using an average stroke volume of about 70mL, is 5.25 L/min, with a normal range of 4.0–8.0 L/min.

echocardiogram and can be influenced by the size of the heart, physical and mental condition of the individual, sex, contractility, duration of contraction, preload and afterload.[7]

Frank-Starling mechanism. This states that the force of contraction is directly proportional to the initial length of muscle fiber, meaning a ventricle will contract more forcefully, the more it is stretched.[7][41]

Afterload, or how much pressure the heart must generate to eject blood at systole, is influenced by vascular resistance. It can be influenced by narrowing of the heart valves (stenosis) or contraction or relaxation of the peripheral blood vessels.[7]

The strength of heart muscle contractions controls the stroke volume. This can be influenced positively or negatively by agents termed

noradrenaline and dopamine.[43] "Negative" inotropes decrease the force of contraction and include calcium channel blockers.[42]

Electrical conduction

Transmission of a cardiac action potential through the heart's conduction system

The normal rhythmical heart beat, called

right atrium near to the junction with the superior vena cava.[44] The electrical signal generated by the sinoatrial node travels through the right atrium in a radial way that is not completely understood. It travels to the left atrium via Bachmann's bundle, such that the muscles of the left and right atria contract together.[45][46][47] The signal then travels to the atrioventricular node. This is found at the bottom of the right atrium in the atrioventricular septum, the boundary between the right atrium and the left ventricle. The septum is part of the cardiac skeleton, tissue within the heart that the electrical signal cannot pass through, which forces the signal to pass through the atrioventricular node only.[7] The signal then travels along the bundle of His to left and right bundle branches through to the ventricles of the heart. In the ventricles the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the heart muscle.[48]

Conduction system of the heart

Heart rate

The prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarisation and repolarisation. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarisation and contraction of the cell; there is no resting potential.[7]

The normal

resting heart rate is called the sinus rhythm, created and sustained by the sinoatrial node, a group of pacemaking cells found in the wall of the right atrium. Cells in the sinoatrial node do this by creating an action potential. The cardiac action potential is created by the movement of specific electrolytes into and out of the pacemaker cells. The action potential then spreads to nearby cells.[49]

When the sinoatrial cells are resting, they have a negative charge on their membranes. A rapid influx of

potassium channels open, allowing potassium to leave the cell. This causes the cell to have a negative resting charge and is called repolarisation. When the membrane potential reaches approximately −60 mV, the potassium channels close and the process may begin again.[7]

The ions move from areas where they are concentrated to where they are not. For this reason sodium moves into the cell from outside, and potassium moves from within the cell to outside the cell. Calcium also plays a critical role. Their influx through slow channels means that the sinoatrial cells have a prolonged "plateau" phase when they have a positive charge. A part of this is called the

troponin complex to enable contraction of the cardiac muscle, and separate from the protein to allow relaxation.[50]

The adult resting heart rate ranges from 60 to 100 bpm. The resting heart rate of a

newborn can be 129 beats per minute (bpm) and this gradually decreases until maturity.[51] An athlete's heart rate can be lower than 60 bpm. During exercise the rate can be 150 bpm with maximum rates reaching from 200 to 220 bpm.[7]

Influences

The normal sinus rhythm of the heart, giving the resting heart rate, is influenced by a number of factors. The cardiovascular centres in the brainstem control the sympathetic and parasympathetic influences to the heart through the vagus nerve and sympathetic trunk.[52] These cardiovascular centres receive input from a series of receptors including baroreceptors, sensing the stretching of blood vessels and chemoreceptors, sensing the amount of oxygen and carbon dioxide in the blood and its pH. Through a series of reflexes these help regulate and sustain blood flow.[7]

Baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself. Baroreceptors fire at a rate determined by how much they are stretched,[53] which is influenced by blood pressure, level of physical activity, and the relative distribution of blood. With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation. As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation.[7] There is a similar reflex, called the atrial reflex or Bainbridge reflex, associated with varying rates of blood flow to the atria. Increased venous return stretches the walls of the atria where specialized baroreceptors are located. However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase heart rate. The opposite is also true.[7] Chemoreceptors present in the carotid body or adjacent to the aorta in an aortic body respond to the blood's oxygen, carbon dioxide levels. Low oxygen or high carbon dioxide will stimulate firing of the receptors.[54]

Exercise and fitness levels, age, body temperature,

thyroid hormones can increase the heart rate. The levels of electrolytes including calcium, potassium, and sodium can also influence the speed and regularity of the heart rate; low blood oxygen, low blood pressure and dehydration may increase it.[7]

Clinical significance

Diseases

heart attack
.

Ischemic heart disease

Coronary artery disease, also known as ischemic heart disease, is caused by

high blood pressure, uncontrolled diabetes, smoking and high cholesterol can all increase the risk of developing atherosclerosis and coronary artery disease.[55][57]

Heart failure

Heart failure is defined as a condition in which the heart is unable to pump enough blood to meet the demands of the body.

arrhythmias.[60]

Cardiomyopathies