Editing Heart (section)

==Structure==
[[File:Humhrt2.jpg|thumbnail|200px|right|Human heart during an [[autopsy]]]]
[[File:CG Heart.gif|alt=Computer generated animation of a beating human heart|thumb|Computer-generated animation of a beating human heart]]
[[File:MC1 Cardiology 1.webm|thumb|Cardiology video]]
{{See also|Anatomy of the human heart}}

===Location and shape===
[[File:Real-time MRI - Thorax.ogv|thumb|[[Real-time MRI]] of the human heart]]
[[File:Heart near.png|thumb|left|upright|The human heart is in the middle of the [[thorax]], with its apex pointing to the left.<ref>{{cite web |url=http://www.bartleby.com/107/284.html |title=Gray's Anatomy of the Human Body – 6. Surface Markings of the Thorax |publisher=Bartleby.com |access-date=2010-10-18 |url-status=live |archive-url=https://web.archive.org/web/20101120145802/http://bartleby.com/107/284.html |archive-date=20 November 2010}}</ref>]]

The human heart is situated in the [[mediastinum]], at the level of [[thoracic vertebrae]] [[Fifth thoracic vertebra|T5]]–[[Eighth thoracic vertebra|T8]]. A double-membraned sac called the [[pericardium]] surrounds the heart and attaches to the mediastinum.<ref>{{cite book|last1=Dorland's|title=Dorland's Illustrated Medical Dictionary|date=2012|publisher=Elsevier|isbn=978-1-4160-6257-8|page=1461|edition=32nd}}</ref> The back surface of the heart lies near the [[vertebral column]], and the front surface, known as the sternocostal surface, sits behind the [[sternum]] and [[costal cartilage|rib cartilages]].<ref name="CNX2014"/> The upper part of the heart is the attachment point for several large blood vessels—the [[venae cavae]], [[aorta]] and [[pulmonary trunk]]. The upper part of the heart is located at the level of the third costal cartilage.<ref name="CNX2014"/> The lower tip of the heart, the '''apex''', lies to the left of the sternum (8 to 9&nbsp;cm from the [[midsternal line]]) between the junction of the fourth and fifth ribs near their [[articulation (anatomy)|articulation]] with the costal cartilages.<ref name="CNX2014"/>

The largest part of the heart is usually slightly offset to the left side of the chest ([[levocardia]]). In a rare congenital disorder ([[dextrocardia]]) the heart is offset to the right side and is felt to be on the left because the [[left heart]] is stronger and larger, since it pumps to all body parts. Because the heart is between the [[human lung|lungs]], the left lung is smaller than the right lung and has a cardiac notch in its border to accommodate the heart.<ref name="CNX2014"/>
The heart is cone-shaped, with its '''base''' positioned upwards and tapering down to the apex.<ref name="CNX2014"/> An adult heart has a mass of 250–350 grams (9–12&nbsp;oz).<ref>{{cite web|last1=Bianco|first1=Carl|title=How Your Heart Works|url=http://health.howstuffworks.com/human-body/systems/circulatory/heart1.htm|website=HowStuffWorks|access-date=14 August 2016|url-status=dead|archive-url=https://web.archive.org/web/20160729035858/http://health.howstuffworks.com/human-body/systems/circulatory/heart1.htm|archive-date=29 July 2016|date=April 2000}}</ref> The heart is often described as the size of a fist: 12&nbsp;cm (5&nbsp;in) in length, 8&nbsp;cm (3.5&nbsp;in) wide, and 6&nbsp;cm (2.5&nbsp;in) in thickness,<ref name="CNX2014"/> although this description is disputed, as the heart is likely to be slightly larger.<ref>{{cite journal |last1=Ampanozi |first1=Garyfalia |last2=Krinke |first2=Eileen |last3=Laberke |first3=Patrick |last4=Schweitzer |first4=Wolf |last5=Thali |first5=Michael J. |last6=Ebert |first6=Lars C. |title=Comparing fist size to heart size is not a viable technique to assess cardiomegaly |journal=Cardiovascular Pathology|date=7 May 2018 |volume=36 |pages=1–5 |doi=10.1016/j.carpath.2018.04.009 |pmid=29859507 |s2cid=44086023 |issn=1879-1336}}</ref> Well-trained [[athlete]]s can have much larger hearts due to the effects of exercise on the heart muscle, similar to the response of skeletal muscle.<ref name="CNX2014"/>

===Chambers===
[[File:Heart diseccion.jpg|thumb|Heart being dissected showing right and left ventricles, from above]]
The heart has four chambers, two upper [[atrium (heart)|atria]], the receiving chambers, and two lower [[ventricle (heart)|ventricles]], the discharging chambers. The atria open into the ventricles via the [[atrioventricular valve]]s, present in the [[atrioventricular septum]]. This distinction is visible also on the surface of the heart as the [[coronary sulcus]].{{sfn|Gray's Anatomy|2008|pp=960–962}} There is an ear-shaped structure in the upper right atrium called the [[Atrium (heart)#Structure|right atrial appendage]], or auricle, and another in the upper left atrium, the [[Atrium (heart)#Structure|left atrial appendage]].{{sfn|Gray's Anatomy|2008|pp=964–967}} The right atrium and the right ventricle together are sometimes referred to as the [[right heart]]. Similarly, the left atrium and the left ventricle together are sometimes referred to as the [[left heart]].{{sfn|Gray's Anatomy|2008|p=960}} The ventricles are separated from each other by the [[interventricular septum]], visible on the surface of the heart as the [[anterior longitudinal sulcus]] and the [[posterior interventricular sulcus]].{{sfn|Gray's Anatomy|2008|pp=960–962}}

The [[dense connective tissue|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 valve]]s.<ref>{{cite book|last1=Pocock|first1=Gillian|title=Human Physiology|date=2006|publisher=Oxford University Press|isbn=978-0-19-856878-0|page=264}}</ref> 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.<ref name="CNX2014"/> The interventricular septum is much thicker than the interatrial septum since the ventricles need to generate greater pressure when they contract.<ref name="CNX2014"/>

====Valves====
{{Main|Heart valves}}
{{multiple image
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| caption1 = With the atria and major vessels removed, all four valves are clearly visible.<ref name="CNX2014"/>
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| caption2 = The heart, showing valves, arteries and veins. The white arrows show the normal direction of blood flow.
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[[File:2010 Chordae Tendinae Papillary Muscles.jpg|thumb|Frontal section showing [[papillary muscle]]s attached to the [[tricuspid valve]] on the right and to the [[mitral valve]] on the left via [[chordae tendineae]].<ref name="CNX2014"/>]]
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.<ref name=CNX2014/>

The valves between the atria and ventricles are called the atrioventricular valves. Between the right atrium and the right ventricle is the [[tricuspid valve]]. The tricuspid valve has three cusps,{{sfn|Gray's Anatomy|2008|pp=966–967}} which connect to [[chordae tendinae]] and three [[papillary muscle]]s named the anterior, posterior, and septal muscles, after their relative positions.{{sfn|Gray's Anatomy|2008|pp=966–967}} 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.{{sfn|Gray's Anatomy|2008|p=970}}

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.<ref>{{cite web|author1=University of Minnesota|title=Papillary Muscles|url=http://www.vhlab.umn.edu/atlas/left-ventricle/papillary-muscles/index.shtml|website=Atlas of Human Cardiac Anatomy|access-date=7 March 2016|url-status=live|archive-url=https://web.archive.org/web/20160317203756/http://www.vhlab.umn.edu/atlas/left-ventricle/papillary-muscles/index.shtml|archive-date=17 March 2016}}</ref> 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.<ref name="CNX2014"/>{{efn|Note the muscles do '''not''' cause the valves to open. The pressure difference between the blood in the atria and the ventricles does this.}}{{sfn|Gray's Anatomy|2008|pp=966–967}}

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.<ref name="CNX2014"/>

====Right heart====
{{anchor|Right heart}}
The right heart consists of two chambers, the right atrium and the right ventricle, separated by a valve, the [[tricuspid valve]].<ref name="CNX2014"/>

The right atrium receives blood almost continuously from the body's two major [[vein]]s, the [[Superior vena cava|superior]] and [[Inferior vena cava|inferior]] [[Vena cava|venae cavae]]. A small amount of blood from the coronary circulation also drains into the right atrium via the [[coronary sinus]], which is immediately above and to the middle of the opening of the inferior vena cava.<ref name="CNX2014"/> In the wall of the right atrium is an oval-shaped depression known as the [[fossa ovalis (heart)|fossa ovalis]], which is a remnant of an opening in the fetal heart known as the [[Foramen ovale (heart)|foramen ovale]].<ref name="CNX2014"/> 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 muscle]]s, which are also present in the [[right atrial appendage]].<ref name="CNX2014"/>

The right atrium is connected to the right ventricle by the tricuspid valve.<ref name="CNX2014"/> 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.<ref name="CNX2014"/> The right ventricle tapers into the [[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.<ref name="CNX2014"/>

====Left heart====
{{anchor|left heart}}
The left heart has two chambers: the left atrium and the left ventricle, separated by the [[mitral valve]].<ref name="CNX2014"/>

The left atrium receives oxygenated blood back from the lungs via one of the four [[pulmonary vein]]s. The left atrium has an outpouching called the [[left atrial appendage]]. Like the right atrium, the left atrium is lined by [[pectinate muscles]].<ref>{{Cite web|url=http://medical-dictionary.thefreedictionary.com/pectinate+muscle|title=pectinate muscle|publisher=The Free Dictionary|access-date=2016-07-31|archive-date=23 August 2018|archive-url=https://web.archive.org/web/20180823131849/https://medical-dictionary.thefreedictionary.com/pectinate+muscle|url-status=live}}</ref> The left atrium is connected to the left ventricle by the mitral valve.<ref name="CNX2014"/>

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 [[trabeculae carneae]], but there is no [[moderator band]]. The left ventricle pumps blood to the body through the aortic valve and into the aorta. Two small openings above the aortic valve carry blood to the [[heart muscle]]; the [[left coronary artery]] is above the left cusp of the valve, and the [[right coronary artery]] is above the right cusp.<ref name="CNX2014"/>

===Wall===
{{Further|Cardiac muscle}}
[[File:2004 Heart Wall.jpg|thumb|Layers of the heart wall, including visceral and parietal pericardium]]

The heart wall is made up of three layers: 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.<ref name="CNX2014"/> The endocardium, by secreting [[endothelins]], may also play a role in regulating the contraction of the myocardium.<ref name="CNX2014"/>

[[File:2006 Heart Musculature.jpg|200px|thumbnail|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.<ref name="CNX2014"/>

There are two types of cells in cardiac muscle: [[cardiomyocyte|muscle cells]] which have the ability to contract easily, and [[cardiac pacemaker|pacemaker cells]] of the conducting system. The muscle cells make up the bulk (99%) of cells in the atria and ventricles. These contractile cells are connected by [[intercalated disc]]s which allow a rapid response to impulses of [[cardiac action potential|action potential]] from the pacemaker cells. The intercalated discs allow the cells to act as a [[syncytium]] and enable the contractions that pump blood through the heart and into the [[great arteries|major arteries]].<ref name="CNX2014"/> The pacemaker cells make up 1% of cells and form the conduction system of the heart. They are generally much smaller than the contractile cells and have few [[myofibril]]s which gives them limited contractibility. Their function is similar in many respects to [[neuron]]s.<ref name="CNX2014"/> 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.<ref name="CNX2014"/>

There are specific [[Bioinformatics#Gene and protein expression|proteins expressed]] in cardiac muscle cells.<ref>{{Cite web|url=https://www.proteinatlas.org/humanproteome/heart|title=The human proteome in heart |series=The Human Protein Atlas|website=www.proteinatlas.org|access-date=2017-09-29|archive-date=9 November 2018|archive-url=https://web.archive.org/web/20181109191813/http://www.proteinatlas.org/humanproteome/heart|url-status=live}}</ref><ref>{{Cite journal|last1=Uhlén|first1=Mathias|last2=Fagerberg|first2=Linn|last3=Hallström|first3=Björn M.|last4=Lindskog|first4=Cecilia|last5=Oksvold|first5=Per|last6=Mardinoglu|first6=Adil|last7=Sivertsson|first7=Åsa|last8=Kampf|first8=Caroline|last9=Sjöstedt|first9=Evelina|s2cid=802377|date=2015-01-23|title=Tissue-based map of the human proteome|journal=Science|language=en|volume=347|issue=6220|pages=1260419|doi=10.1126/science.1260419|issn=0036-8075|pmid=25613900}}</ref> These are mostly associated with muscle contraction, and bind with [[actin]], [[myosin]], [[tropomyosin]], and [[troponin]]. They include [[MYH6]], [[ACTC1]], [[TNNI3]], [[CDH2]] and [[Plakophilin-2|PKP2]]. Other proteins expressed are [[MYH7]] and [[LDB3]] that are also expressed in skeletal muscle.<ref>{{Cite journal|last1=Lindskog|first1=Cecilia|last2=Linné|first2=Jerker|last3=Fagerberg|first3=Linn|last4=Hallström|first4=Björn M.|last5=Sundberg|first5=Carl Johan|last6=Lindholm|first6=Malene|last7=Huss|first7=Mikael|last8=Kampf|first8=Caroline|last9=Choi|first9=Howard|date=2015-06-25|title=The human cardiac and skeletal muscle proteomes defined by transcriptomics and antibody-based profiling|journal=BMC Genomics|volume=16|issue=1 |pages=475|doi=10.1186/s12864-015-1686-y|pmid=26109061|pmc=4479346|issn=1471-2164 |doi-access=free }}</ref>

=== Pericardium ===
{{Main|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.{{sfn|Gray's Anatomy|2008|p=959}} 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.<ref>{{Cite book|title=Principles of human anatomy|last=J.|first=Tortora, Gerard|date=2009|publisher=J. Wiley|others=Nielsen, Mark T. (Mark Thomas)|isbn=978-0-471-78931-4|edition=11th|location=Hoboken, NJ|oclc=213300667}}</ref>

===Coronary circulation===
[[File:Coronary arteries.svg|thumb|Arterial supply to the heart (red), with other areas labelled (blue).]]
{{main|Coronary circulation}}
Heart tissue, like all cells in the body, needs to be supplied with [[oxygen]], [[nutrient]]s and a way of removing [[metabolic waste]]s. This is achieved by the coronary circulation, which includes [[artery|arteries]], [[vein]]s, and [[lymph vessel|lymphatic vessels]]. Blood flow through the coronary vessels occurs in peaks and troughs relating to the heart muscle's relaxation or contraction.<ref name="CNX2014"/>

Heart tissue receives blood from two arteries which arise just above the aortic valve. These are the [[left main coronary artery]] and the [[right coronary artery]]. The left main coronary artery splits shortly after leaving the aorta into two vessels, the [[left anterior descending]] and 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.{{sfn|Davidson's|2010|p=525}} The arteries divide at their furthest reaches into smaller branches that join at the edges of each arterial distribution.<ref name="CNX2014"/>

The [[coronary sinus]] is a large vein that drains into the right atrium, and receives most of the venous drainage of the heart. It receives blood from the [[great cardiac vein]] (receiving the left atrium and both ventricles), 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 vein]]s.{{sfn|Gray's Anatomy|2008|p=981}} The [[anterior cardiac veins]] drain the front of the right ventricle and drain directly into the right atrium.<ref name="CNX2014"/>

Small lymphatic networks called [[plexus]]es exist beneath each of the three layers of the heart. These networks collect into a main left and a main right trunk, which travel up the groove between the ventricles that exists on the heart's surface, receiving smaller vessels as they travel up. These vessels then travel into the atrioventricular groove, and receive a third vessel which drains the section of the left ventricle sitting on the diaphragm. The left vessel joins with this third vessel, and travels along the pulmonary artery and left atrium, ending in the [[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.{{sfn|Gray's Anatomy|2008|p=982}}

===Nerve supply===
[[File:2032 Automatic Innervation.jpg|thumb|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 nervous system|Sympathetic nerves]] also influence the force of heart contraction.{{sfn|Davidson's|2010|p=526}} Signals that travel along these nerves arise from two paired [[cardiovascular centre]]s 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.<ref name="CNX2014"/> These nerves form a network of nerves that lies over the heart called the [[cardiac plexus]].<ref name="CNX2014"/>{{sfn|Gray's Anatomy|2008|p=982}}

The vagus nerve is a long, wandering nerve that emerges from the [[brainstem]] and provides parasympathetic stimulation to a large number of organs in the thorax and abdomen, including the heart.{{sfn|Gray's Anatomy|2008|p=945}} The nerves from the sympathetic trunk emerge through the T1–T4 [[thoracic ganglia]] and travel to both the sinoatrial and atrioventricular nodes, as well as to the atria and ventricles. The ventricles are more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation causes the release of the neurotransmitter [[norepinephrine]] (also known as [[noradrenaline]]) at the [[neuromuscular junction]] of the cardiac nerves{{Citation needed|date=October 2024}}. This shortens the repolarisation period, thus speeding the rate of depolarisation and contraction, which results in an increased heart rate. It opens chemical or ligand-gated sodium and calcium ion channels, allowing an influx of [[cation|positively charged ions]].<ref name="CNX2014"/> Norepinephrine binds to the [[beta-1 adrenergic receptor|beta–1 receptor]].<ref name="CNX2014"/>