{{Short description|Muscular organ responsible for pumping blood through the body}}
{{Infobox anatomy
| Name = Heart
| Latin = cor
| Image = Heart anterior exterior view.jpg
| Caption = Anterior view of the human heart
| System = [[Circulatory system]]
| Location = [[Thoracic cavity]], between the lungs, anterior to vertebral column, posterior to sternum
| Function = Pumping blood to maintain circulation
| Precursor = [[Mesoderm]]
| Artery = [[Coronary arteries]]
| Vein = [[Cardiac veins]]
| Nerve = [[Vagus nerve]], [[cardiac nerves]]
}}
[[File:Human-heart.png|right|thumb]]
[[File:Human-heart.png|right|thumb]]
[[File:Human-circulation.png|right|thumb]]
[[File:Diagram of the human heart (cropped).svg|'''Diagram of the human heart showing major vessels'''|right|thumb]]
The '''heart''' is a hollow, muscular organ that pumps [[blood]] throughout the body's [[blood vessels]] by repeated, rhythmic contractions. It is present in all animals possessing a [[circulatory system]], notably the [[vertebrate]] species. The term "cardiac," meaning "related to the heart," is derived from the Greek καρδιά (''kardia''), meaning "heart". The medical specialty dedicated to the heart, its disorders, and abnormalities is known as '''[[cardiology]]'''.
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== Structure ==
The '''heart''' is a hollow [[muscular]] organ that pumps [[blood]] throughout the [[blood vessel]]s to various parts of the body by repeated, rhythmic contractions.<ref>{{cite web|url=https://www.boundless.com/biology/circulatory-system/facilitating-exchange-with-cells-throughout-the-body/evolutionary-adaptations-of-the-circulatory-system/ |title=Evolutionary adaptations of the circulatory system – Facilitating Exchange with Cells Throughout the Body |publisher=Boundless.com |date= |accessdate=2013-10-01}}</ref> It is found in all [[animal]]s with a [[circulatory system]], which includes the [[vertebrate]]s.<ref>{{cite web |url=http://www.seymoursimon.com/index.php/science_dictionary/heart_/ |title=Heart |work=SeymourSimon.com |date= }}</ref>
The structure and complexity of the heart can vary considerably among different animal groups.
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=== Vertebrate heart ===
The adjective '''''cardiac''''' means "related to the heart" and comes from the [[Greek language|Greek]] καρδιά, ''kardia'', for "heart". [[Cardiology]] is the [[Specialty (medicine)|medical speciality]] that deals with [[Cardiovascular disease|cardiac diseases]] and abnormalities.
The vertebrate heart is primarily composed of specialized [[cardiac muscle]] tissue, [[connective tissue]], and a lining of [[endothelium]]. Cardiac muscle is a specialized, involuntary striated muscle, uniquely adapted to sustain continuous rhythmic contractions throughout an organism’s lifespan.
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=== Variations among animals ===
The vertebrate heart is principally composed of [[cardiac muscle]] and [[connective tissue]]. Cardiac muscle is an involuntary striated muscle tissue specific to the heart and is responsible for the heart's ability to pump blood.
[[Cephalopods]], such as octopuses and squids, possess three hearts—two "gill hearts" for pumping blood through the gills, and one systemic heart that circulates blood throughout the body. Vertebrates typically have a single heart located in the anterior portion of the [[body cavity]], dorsal to the digestive organs.
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In certain primitive vertebrates, like jawless fishes ([[hagfish]]), there is an additional heart-like structure located in the tail region, unique among vertebrates.
The average [[human heart]], [[Cardiac cycle|beating]] at 72 beats per minute, will beat approximately 2.5 billion times during an average 66 year lifespan, and pumps approximately 4.7-5.7 litres of blood per minute. It weighs approximately {{convert|250|to|300|g|oz|0}} in females and {{convert|300|to|350|g|oz|0}} in males.<ref>{{cite book |last=Kumar |last2=Abbas |last3=Fausto |year=2005 |title=Robbins and Cotran Pathologic Basis of Disease |edition=7th |location=Philadelphia |publisher=Elsevier Saunders |page=556 |isbn=0-7216-0187-1 }}</ref>
==Structure== <!--T:6-->
== Human heart ==
The structure of the heart can vary among the different [[animal]] [[species]]. (See [[Circulatory system]].) [[Cephalopod]]s have two "gill hearts" and one "systemic heart". In vertebrates, the heart lies in the anterior part of the body cavity, dorsal to the gut. It is always surrounded by a [[pericardium]], which is usually a distinct structure, but may be continuous with the [[peritoneum]] in jawless and cartilaginous fish. [[Hagfish]], uniquely among vertebrates, also possess a second heart-like structure in the tail.<ref name="VB" />
In humans, the heart is a muscular organ approximately the size of a closed fist, weighing around 250–300 grams (9–11 oz) in females and 300–350 grams (11–12 oz) in males. Located within the [[thoracic cavity]], it sits anterior to the vertebral column and posterior to the [[sternum]], slightly left of the body's midline.
===In humans=== <!--T:7-->
=== Pericardium ===
The adult human heart has a mass of between 250 and 350 grams and is about the size of a fist.<ref name="manual">{{cite book |author=MacDonald, Matthew |title=Your Body: The Missing Manual |publisher=Pogue Press |location=Sebastopol, CA |year=2009 |isbn=0-596-80174-2}}</ref> It is located [[Anterior and posterior|anterior]] to the vertebral column and [[Anterior and posterior|posterior]] to the [[sternum]].<ref name="StarrEvers2009">{{cite book|author1=Cecie Starr|author2=Christine Evers|author3=Lisa Starr|title=Biology: Today and Tomorrow With Physiology|url=http://books.google.com/books?id=dxC27ndpwe8C&pg=PA422|accessdate=7 June 2012|date=2 January 2009|publisher=Cengage Learning|isbn=978-0-495-56157-6|pages=422–}}</ref>
The human heart is enclosed within a protective double-walled sac known as the [[pericardium]]. The outer layer, called the fibrous pericardium, anchors the heart to surrounding structures and prevents over-expansion. Beneath this lies the serous pericardium, composed of two layers:
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* '''Parietal pericardium''' (outer serous layer)
It is enclosed in a double-walled sac called the [[pericardium]]. The pericardium's outer wall is called the parietal pericardium and the inner one the visceral pericardium. Between them there is some pericardial fluid which functions to permit the inner and outer walls to slide easily over one another with the heart movements. Outside the parietal pericardium is a fibrous layer called the [[fibrous pericardium]] which is attached to the mediastinal fascia.<ref>{{cite web|url=http://thorax.bmj.com/content/6/4/359.full.pdf|title=the anatomy and applied anatomy of the mediastinal fascia|accessdate=2013-02-27}}</ref> This sac protects the heart and anchors it to the surrounding structures.
* '''Visceral pericardium''' (inner serous layer or '''epicardium''', directly attached to the heart)
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Between these layers lies the pericardial cavity, which contains pericardial fluid, allowing frictionless heart movements.
The outer wall of the human heart is composed of three layers; the outer layer is called the [[epicardium]], or visceral pericardium since it is also the inner wall of the pericardium. The middle layer is called the [[myocardium]] and is composed of contractile cardiac muscle. The inner layer is called the [[endocardium]] and is in contact with the blood that the heart pumps.<ref name="Darpan2009">{{cite book|author=Pratiyogita Darpan|title=Competition Science Vision|url=http://books.google.com/books?id=yEU6mYWgUcwC&pg=PT55|accessdate=7 June 2012|date=April 2009|publisher=Pratiyogita Darpan|pages=55–}}</ref> Also, it merges with the inner lining ([[endothelium]]) of blood vessels and covers heart valves.<ref name="MedLookHeart">{{cite web|url=http://www.medicalook.com/human_anatomy/organs/Heart.html |title=Heart |accessdate=2010-05-03 |work=MedicaLook |publisher=Medicalook.com }}</ref>
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=== Layers of the heart wall ===
The human heart has four chambers, two [[Superior (anatomy)|superior]] atria and two [[Superior (anatomy)|inferior]] ventricles. The atria are the receiving chambers and the ventricles are the discharging chambers. During each [[cardiac cycle]], the atria contract first, forcing blood that has entered them into their respective ventricles, then the ventricles contract, forcing blood out of the heart. The pathway of the blood consists of a pulmonary circuit and a systemic circuit<ref>{{cite web|url=http://medical-dictionary.thefreedictionary.com/Pulmonary+circuit |title=Pulmonary circuit |accessdate=2011-05-10 |publisher=medical-dictionary.thefreedictionary.com }}</ref> which function simultaneously. Deoxygenated blood from the body flows via the [[vena cava]] into the right atrium, which pumps it through the [[tricuspid valve]] into the [[right ventricle]], whose subsequent contraction forces it out through the [[pulmonary valve]] into the [[pulmonary artery|pulmonary arteries]] leading to the [[lung]]s. Meanwhile, oxygenated blood returns from the lungs through the [[pulmonary vein]]s into the [[left atrium]], which pumps it through the [[mitral valve]] into the [[left ventricle]], whose subsequent strong contraction forces it out through the [[aortic valve]] to the [[aorta]] leading to the systemic circulation.<ref>{{cite book |title=Emergency Medical Responder: A Skills Approach |edition=3rd Canadian |location=Toronto |publisher=Pearson |year=2010 |page=131 |isbn=978-0-13-500485-2 }}</ref><ref>{{cite book |last=Marieb |first=Elaine Nicpon |title=Human Anatomy & Physiology |edition=6th |location=Upper Saddle River |publisher=Pearson Education |year=2003 |isbn=0-8053-5463-8 }}</ref>
The heart wall comprises three distinct layers:
===In fish=== <!--T:11-->
* '''[[Epicardium]]''' (outermost layer; same as the visceral pericardium)
Primitive fish have a four-chambered heart, but the chambers are arranged sequentially so that this primitive heart is quite unlike the four-chambered hearts of mammals and birds. The first chamber is the [[sinus venosus]], which collects deoxygenated blood, from the body, through the [[hepatic vein|hepatic]] and [[cardinal vein]]s. From here, blood flows into the [[Atrium (heart)|atrium]] and then to the powerful muscular [[ventricle (heart)|ventricle]] where the main pumping action will take place. The fourth and final chamber is the [[conus arteriosus]] which contains several valves and sends blood to the ''ventral aorta''. The ventral aorta delivers blood to the gills where it is oxygenated and flows, through the [[descending aorta|dorsal aorta]], into the rest of the body. (In [[tetrapod]]s, the ventral aorta has divided in two; one half forms the [[ascending aorta]], while the other forms the [[pulmonary artery]]).<ref name="VB" />
* '''[[Myocardium]]''' (middle, muscular layer responsible for contractions)
* '''[[Endocardium]]''' (innermost layer, lining heart chambers and valves, continuous with the blood vessel endothelium)
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=== Chambers and valves ===
In the adult fish, the four chambers are not arranged in a straight row but, instead form an S-shape with the latter two chambers lying above the former two. This relatively simpler pattern is found in [[cartilaginous fish]] and in the [[ray-finned fish]]. In [[teleost]]s, the conus arteriosus is very small and can more accurately be described as part of the aorta rather than of the heart proper. The conus arteriosus is not present in any [[amniote]]s, presumably having been absorbed into the ventricles over the course of evolution. Similarly, while the sinus venosus is present as a vestigial structure in some reptiles and birds, it is otherwise absorbed into the right atrium and is no longer distinguishable.<ref name="VB" />
The human heart contains four chambers:
===In double circulatory systems=== <!--T:13-->
* Two superior receiving chambers: the '''[[right atrium]]''' and '''[[left atrium]]'''
In [[amphibian]]s and most [[reptile]]s, a [[double circulatory system]] is used but the heart is not completely separated into two pumps. The development of the double system is necessitated by the presence of [[lung]]s which deliver oxygenated blood directly to the heart.
* Two inferior pumping chambers: the '''[[right ventricle]]''' and '''[[left ventricle]]'''
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Blood flow is controlled by four valves:
In living amphibians, the atrium is divided into two separate chambers by the presence of a muscular [[septum]] even though there is only one ventricle. The sinus venosus, which remains large in amphibians but connects only to the right atrium, receives blood from the [[venae cavae]], with the [[pulmonary vein]] by-passing it entirely to enter the left atrium.
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* '''[[Tricuspid valve]]''' (right atrioventricular valve, between right atrium and right ventricle)
In the heart of [[lungfish]], the septum extends part-way into the ventricle. This allows for some degree of separation between the de-oxygenated bloodstream destined for the lungs and the oxygenated stream that is delivered to the rest of the body. The absence of such a division in living amphibian species may be at least partly due to the amount of respiration that occurs through the skin in such species; thus, the blood returned to the heart through the vena cavae is, in fact, already partially oxygenated. As a result, there may be less need for a finer division between the two bloodstreams than in lungfish or other [[tetrapod]]s. Nonetheless, in at least some species of amphibian, the spongy nature of the ventricle seems to maintain more of a separation between the bloodstreams than appears the case at first glance. Furthermore, the conus arteriosus has lost its original valves and contains a spiral valve, instead, that divides it into two parallel parts, thus helping to keep the two bloodstreams separate.<ref name="VB" />
* '''[[Pulmonary valve]]''' (between right ventricle and pulmonary artery)
* '''[[Mitral valve]]''' (left atrioventricular or bicuspid valve, between left atrium and left ventricle)
* '''[[Aortic valve]]''' (between left ventricle and the aorta)
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== Blood circulation ==
The heart of most reptiles (except for crocodilians; ''see below'') has a similar structure to that of lungfish but, here, the septum is generally much larger. This divides the ventricle into two halves but, because the septum does not reach the whole length of the heart, there is a considerable gap near the openings to the pulmonary artery and the aorta. In practice, however, in the majority of reptilian species, there appears to be little, if any, mixing between the bloodstreams, so the aorta receives, essentially, only oxygenated blood.<ref name="VB" />
The heart supports two simultaneous circulation circuits:
==Invertebrate heart== <!--T:17-->
=== Pulmonary circulation ===
In [[invertebrate]]s that possess a circulatory system, the heart is typically a tube or small sac and pumps fluid that contains water and nutrients such as [[protein]]s, [[fat]]s, and [[sugar]]s. In [[insect]]s, the "heart" is often called the '''dorsal tube''' and insect "blood" is almost always not oxygenated since they usually [[Respiration (physiology)|respirate]] (breathe) directly from their body surfaces (internal and external) to air. However, the hearts of some other [[arthropod]]s (including [[spider]]s and [[crustacean]]s such as [[crab]]s and [[shrimp]]) and some other animals pump [[hemolymph]], which contains the [[copper]]-based protein [[hemocyanin]] as an oxygen transporter similar to the [[iron]]-based [[hemoglobin]] in [[red blood cell]]s found in [[vertebrate]]s.
[[Deoxygenated blood]] from systemic circulation enters the right atrium via the superior and inferior [[vena cavae]]. Blood then moves through the tricuspid valve into the right ventricle, which contracts to push blood through the pulmonary valve into the [[pulmonary arteries]], carrying blood to the lungs for oxygenation.
===The fully divided heart=== <!--T:18-->
=== Systemic circulation ===
Oxygen-rich blood from the lungs returns to the left atrium via the [[pulmonary veins]]. It then flows through the mitral valve into the left ventricle. Powerful contractions of the left ventricle pump blood through the aortic valve into the [[aorta]], supplying oxygenated blood to the entire body.
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== Cardiac cycle ==
[[Archosaur]]s ([[crocodilia]]ns and [[bird]]s) and [[mammal]]s show complete separation of the heart into two pumps for a total of four [[heart chamber]]s; it is thought that the four-chambered heart of archosaurs evolved independently from that of mammals. In crocodilians, there is a small opening, the [[foramen of Panizza]], at the base of the arterial trunks and there is some degree of mixing between the blood in each side of the heart, during a dive underwater;<ref name="nonimportantshunt1">{{cite journal |last1=Crigg |first1=Gordon |last2=Johansen |first2=Kjell|year=1987 |title=Cardiovascular Dynamics In Crocodylus Porosus Breathing Air And During Voluntary Aerobic Dives |journal=Journal of Comparative Physiology B |volume=157 |issue=3 |pages=381–392 |publisher=Springer-Verlag |doi=10.1007/BF00693365 |url=http://rd.springer.com/article/10.1007/BF00693365# |accessdate=July 3, 2012|archiveurl=http://espace.library.uq.edu.au/eserv/UQ:9269/_Crocodylus_poro.pdf |archivedate=July 3, 2012 }}</ref><ref name="nonimportantshunt2">{{cite journal |last1=Axelsson |first1=Michael |last2=Craig |first2=Franklin |last3=Löfman |first3=Carl |last4=Nilsson |first4=Stefan |last5=Crigg |first5=Gordon |year=1996 |title=Dynamic Anatomical Study Of Cardiac Shunting In Crocodiles Using High-Resolution Angioscopy |journal=The Journal of Experimental Biology |volume=199 |issue= |pages=359–365 |publisher=The Company of Biologists Limited |doi= |pmid=9317958 |url=http://jeb.biologists.org/content/199/2/359.full.pdf |accessdate=July 3, 2012}}</ref> thus, only in birds and mammals are the two streams of blood – those to the pulmonary and systemic circulations – permanently kept entirely separate by a physical barrier.<ref name="VB">{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 437–442|isbn= 0-03-910284-X}}</ref>
The rhythmic contraction and relaxation of heart chambers constitute the '''[[cardiac cycle]]''', comprising two phases:
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* '''[[Systole]]''' (contraction of ventricles, forcing blood into circulation)
In the human body, the heart is usually situated in the middle of the [[thorax]] with the largest part of the heart slightly offset to the left, although sometimes it is on the right (see [[dextrocardia]]), underneath the [[Human sternum|sternum]]. The heart is usually felt to be on the left side because the [[left heart]] (left ventricle) is stronger (it pumps to all body parts). The left [[lung]] is smaller than the right lung because the heart occupies more of the left hemithorax. The heart is fed by the [[coronary circulation]] and is enclosed by a sac known as the [[pericardium]]; it is also surrounded by the [[lung]]s. The pericardium comprises two parts: the fibrous pericardium, made of [[Dense connective tissue|dense fibrous connective tissue]], and a double membrane structure (parietal and visceral pericardium) containing a [[serous fluid]] to reduce friction during heart contractions. The heart is located in the [[mediastinum]], which is the central sub-division of the thoracic cavity. The mediastinum also contains other structures, such as the esophagus and trachea, and is flanked on either side by the right and left pulmonary cavities; these cavities house the [[lung]]s.<ref>{{cite book
* '''[[Diastole]]''' (relaxation phase, allowing chambers to refill with blood)
| last = Maton
| first = Anthea
| coauthors = Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright
| title = Human Biology and Health
| publisher = Prentice Hall
| year = 1993
| location = Englewood Cliffs, New Jersey
| isbn = 0-13-981176-1
| oclc = 32308337}}</ref>
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At rest, the typical adult heart beats about 72 times per minute, circulating approximately 4.7–5.7 liters of blood per minute. Over a lifespan averaging 66 years, the heart will beat roughly 2.5 billion times.
The ''apex'' is the blunt point situated in an inferior (pointing down and left) direction. A [[stethoscope]] can be placed directly over the apex so that the beats can be counted. It is located posterior to the 5th intercostal space just medial of the left mid-clavicular line. In normal adults, the mass of the heart is 250–350 grams (9–12 oz), or about twice the size of a clenched fist (it is about the size of a clenched fist in children), but an extremely diseased heart can be up to 1000 g (2 lb) in mass due to [[hypertrophy]]. It consists of four chambers, the two upper atria and the two lower ventricles.
==Functioning== <!--T:22-->
== Conduction system ==
[[File:Human-circulation.png|right|thumb]]
Heartbeats are coordinated by an electrical conduction system, comprising:
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* '''[[Sinoatrial node]] (SA node)''' – the heart’s natural pacemaker
In mammals, the function of the [[right heart|right side of the heart]] is to collect de-oxygenated blood, in the [[right atrium]], from the body (via superior and inferior vena cavae) and pump it, through the tricuspid valve, via the [[right ventricle]], into the lungs ([[pulmonary circulation]]) so that carbon dioxide can be [[Gas exchange|exchanged]] for oxygen. This happens through the passive process of [[diffusion]]. The left side (see [[left heart]]) collects oxygenated blood from the [[lung]]s into the [[left atrium]]. From the left atrium the blood moves to the [[left ventricle]], through the bicuspid valve (mitral valve), which pumps it out to the body (via the aorta). On both sides, the lower ventricles are thicker and stronger than the upper atria.
* '''[[Atrioventricular node]] (AV node)''' – transmits impulses from atria to ventricles
The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the [[systemic circulation]].
* '''[[Bundle of His]]''' and '''[[Purkinje fibers]]''' – distribute electrical signals throughout ventricles, ensuring coordinated contraction
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== Clinical significance ==
Starting in the right atrium, the blood flows through the [[tricuspid valve]] to the right ventricle. Here, it is pumped out the pulmonary [[semilunar valve]] and travels through the pulmonary [[artery]] to the lungs. From there, oxygenated blood flows back through the pulmonary [[vein]] to the left atrium. It then travels through the [[mitral valve]] to the left ventricle, from where it is pumped through the aortic semilunar valve to the [[aorta]]. The aorta forks and the blood is divided between major arteries which supply the upper and lower body. The blood travels in the arteries to the smaller arterioles and then, finally, to the tiny capillaries which feed each cell. The (relatively) deoxygenated blood then travels to the venules, which coalesce into veins, then to the inferior and superior venae cavae and finally back to the right atrium where the process began.
The study and treatment of heart diseases and disorders are central to '''[[cardiology]]'''. Common cardiac conditions include:
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* '''[[Coronary artery disease]]''' (blockage of heart arteries)
The heart is effectively a [[syncytium]], a meshwork of cardiac muscle cells interconnected by contiguous cytoplasmic bridges. This relates to electrical stimulation of one cell spreading to neighboring cells.
Diagnostic methods include '''[[echocardiography]]''', '''[[electrocardiography]] (ECG)''', '''[[cardiac MRI]]''', and '''[[cardiac catheterization]]'''.
Some cardiac cells are self-excitable, contracting without any signal from the nervous system, even if removed from the heart and placed in culture. Each of these cells have their own intrinsic contraction rhythm. A region of the human heart called the '''[[sinoatrial node|sinoatrial (SA) node]]''', or pacemaker, sets the rate and timing at which all cardiac muscle cells contract. The SA node generates electrical impulses, much like those produced by nerve cells. Because cardiac muscle cells are electrically coupled by inter-calculated disks between adjacent cells, impulses from the SA node spread rapidly through the walls of the artria, causing both artria to contract in unison. The impulses also pass to another region of specialized cardiac muscle tissue, a relay point called the '''[[atrioventricular node]]''', located in the wall between the right atrium and the right ventricle. Here, the impulses are delayed for about 0.1s before spreading to the walls of the ventricle. The delay ensures that the artria empty completely before the ventricles contract. Specialized muscle fibers called [[Purkinje fibers]] then conduct the signals to the apex of the heart along and throughout the ventricular walls. The Purkinje fibres form conducting pathways called bundle branches. This entire cycle, a single heart beat, lasts about 0.8 seconds. The impulses generated during the heart cycle produce electrical currents, which are conducted through body fluids to the skin, where they can be detected by electrodes and recorded as an [[electrocardiography|electrocardiogram]] (ECG or EKG).<ref>{{cite book |last=Campbell |last2=Reece |title=Biology |edition=7th |location=San Francisco |publisher=Pearson |year=2005 |isbn=0-8053-7171-0 |pages=873–874 }}</ref> The events related to the flow or [[blood pressure]] that occurs from the beginning of one [[heart sounds|heartbeat]] to the beginning of the next is called a [[cardiac cycle]].<ref name="Medical Physiology">{{cite book |last=Guyton |first=A. C. |last2=Hall |first2=J. E. |year=2006 |title=Textbook of Medical Physiology |edition=11th |location=Philadelphia |publisher=Elsevier Saunder |isbn=0-7216-0240-1 }}</ref>
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== Historical perspectives ==
The SA node is found in all [[amniote]]s but not in more primitive vertebrates. In these animals, the muscles of the heart are relatively continuous and the sinus venosus coordinates the beat which passes in a wave through the remaining chambers. Indeed, since the sinus venosus is incorporated into the right atrium in amniotes, it is likely [[homology (biology)|homologous]] with the SA node. In teleosts, with their vestigial sinus venosus, the main centre of coordination is, instead, in the atrium. The rate of heartbeat varies enormously between different species, ranging from around 20 beats per minute in [[codfish]] to around 600 in [[hummingbird]]s.<ref name="VB" />
Throughout history, the heart has held significant cultural and symbolic importance, representing emotions, vitality, and life itself. The modern understanding of cardiac anatomy and physiology advanced greatly with discoveries such as [[William Harvey]]’s elucidation of blood circulation in the 17th century.
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== Gallery ==
Cardiac arrest is the sudden cessation of normal heart rhythm which can include a number of pathologies such as [[tachycardia]], an extremely rapid heart beat which prevents the heart from effectively pumping blood, which is an irregular and ineffective heart rhythm, and [[asystole]], which is the cessation of heart rhythm entirely.
<gallery>
File:Heart anterior exterior view.png|Heart
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File:2011 Heart Valves.jpg|Heart
[[Cardiac tamponade]] is a condition in which the [[pericardium]] fills with excess fluid or blood, suppressing the heart's ability to beat properly. Tamponade is treated by [[pericardiocentesis]], the gentle insertion of the needle of a syringe into the [[pericardial sac]] (avoiding the heart itself) on an angle, usually from just below the [[Human sternum|sternum]], and gently withdrawing the tamponading fluids.
File:Diagram of the human heart (cropped).svg|Heart
File:Doctors stethoscope 1.jpg|Heart
File:2113ab Atherosclerosis.jpg|Heart
File:Heart corazón.svg|Heart
</gallery>
== See also ==
* [[Cardiac cycle]]
* [[Cardiology]]
* [[Circulatory system]]
* [[Electrocardiography]]
* [[Heart disease]]
* [[Vascular system]]
==Early development== <!--T:30-->
== External links ==
{{Main|Heart development}}
* [https://www.heart.org/ American Heart Association]
The [[mammalian]] heart is derived from embryonic [[mesoderm]] germ-layer cells that differentiate after [[gastrulation]] into [[mesothelium]], [[endothelium]], and [[myocardium]]. Mesothelial [[pericardium]] forms the outer lining of the heart. The inner lining of the heart, lymphatic and blood vessels, develop from endothelium. Heart muscle is termed myocardium.<ref>{{cite web|url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AnimalTissues.html |title=Animal Tissues |publisher=Users.rcn.com |date=2010-08-13 |accessdate=2010-10-17}}</ref>
* [https://www.bhf.org.uk/ British Heart Foundation]
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From [[splanchnopleuric]] mesoderm tissue, the [[cardiogenic plates]] develop cranially and laterally to the [[neural plate]]s. In the cardiogenic plates, two separate [[angiogenic]] cell clusters form on either side of the embryo. The cell clusters coalesce to form an [[endocardial heart tube]] continuous with a dorsal aorta and a vitteloumbilical vein. As embryonic tissue continues to fold, the two endocardial tubes are pushed into the thoracic cavity, begin to fuse together, and complete the fusing process at approximately 22 days.<ref>{{cite web|url=http://www.meddean.luc.edu/lumen/MedEd/GrossAnatomy/thorax0/heartdev/main_fra.html |title=Main Frame Heart Development|publisher=Meddean.luc.edu |accessdate=2010-10-17}}</ref>
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At 22 days after [[Fertilisation|conception]], the human heart begins beating at 70 to 80 beats per minute and accelerates linearly for the first month of beating. The human [[embryon]]ic heart begins beating at around 22 days after conception, or five weeks after the last normal [[menstrual cycle|menstrual period]] (LMP). The first day of the LMP is normally used to date the start of the gestation (pregnancy). The human heart begins beating at a rate near the mother’s, about 75–80 beats per minute (BPM). The embryonic heart rate (EHR) then accelerates linearly by approximately 100 BPM during the first month to peak at 165–185 BPM during the early 7th week after conception, (early 9th week after the LMP). This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, which is an increase of 100 BPM in the first month.<ref>{{cite web|author=DuBose, Miller , Moutos |url=http://www.obgyn.net/us/us.asp?page=/us/cotm/0001/ehr2000 |title=Embryonic Heart Rates Compared in Assisted and Non-Assisted Pregnancies |publisher=Obgyn.net |accessdate=2010-10-18}}</ref><ref>{{cite journal|author=DuBose TJ, Cunyus JA, and Johnson L|title= Embryonic Heart Rate and Age|journal= J Diagn Med Sonography|year=1990|volume= 6|doi=10.1177/875647939000600306 |pages=151–157|issue=3}}</ref><ref name="Fetal Sonography">DuBose, TJ (1996) ''Fetal Sonography'', pp. 263–274; Philadelphia: WB Saunders ISBN 0-7216-5432-0</ref>
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The regression formula which describes this linear acceleration before the embryo reaches 25mm in crown-rump length, or 9.2 LMP weeks, is: the Age in days = EHR(0.3)+6.
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After 9.1 weeks after the LMP, it decelerates to about 152 BPM (±25 BPM) during the 15th week post LMP. After the 15th week, the deceleration slows to an average rate of about 145 (±25 BPM) BPM, at term. There is no difference in female and male heart rates before birth.<ref>Terry J. DuBose [http://www.obgyn.net/english/pubs/features/dubose/ehr-age.htm Sex, Heart Rate and Age]</ref>
==History of discoveries== <!--T:35-->
The valves of the heart were discovered by a physician of the [[Hippocrates|Hippocratean school]] around the 4th century BC, although their function was not fully understood. On dissection, arteries are typically empty of blood because blood pools in the veins after death. Ancient anatomists subsequently assumed they were filled with air and served to transport it around the body.
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[[Philosophers]] distinguished veins from arteries, but thought the pulse was a property of arteries themselves. [[Erasistratos]] observed that arteries cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood which entered by very small vessels between veins and arteries. Thus he apparently postulated [[capillary|capillaries]], but with reversed flow of blood.
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The Greek physician [[Galen]] (2nd century AD) knew blood vessels carried blood and identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the [[liver]] from [[chyle]], while arterial blood gave vitality by containing pneuma (air) and originated in the heart. Blood flowed from both creating organs to all parts of the body, where it was consumed and there was no return of blood to the heart or liver. The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves.
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Galen believed the arterial blood was created by venous blood passing from the left ventricle to the right through 'pores' in the interventricular septum, while air passed from the lungs via the pulmonary artery to the left side of the heart. As the arterial blood was created, "sooty" vapors were created and passed to the lungs, also via the pulmonary artery, to be exhaled.
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[[Category:Organs]]
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File:Heart anterior exterior view.png|Heart
File:2011 Heart Valves.jpg|Heart
File:Diagram of the human heart (cropped).svg|Heart
The heart is a hollow, muscular organ that pumps blood throughout the body's blood vessels by repeated, rhythmic contractions. It is present in all animals possessing a circulatory system, notably the vertebrate species. The term "cardiac," meaning "related to the heart," is derived from the Greek καρδιά (kardia), meaning "heart". The medical specialty dedicated to the heart, its disorders, and abnormalities is known as cardiology.
The vertebrate heart is primarily composed of specialized cardiac muscle tissue, connective tissue, and a lining of endothelium. Cardiac muscle is a specialized, involuntary striated muscle, uniquely adapted to sustain continuous rhythmic contractions throughout an organism’s lifespan.
Cephalopods, such as octopuses and squids, possess three hearts—two "gill hearts" for pumping blood through the gills, and one systemic heart that circulates blood throughout the body. Vertebrates typically have a single heart located in the anterior portion of the body cavity, dorsal to the digestive organs.
In certain primitive vertebrates, like jawless fishes (hagfish), there is an additional heart-like structure located in the tail region, unique among vertebrates.
In humans, the heart is a muscular organ approximately the size of a closed fist, weighing around 250–300 grams (9–11 oz) in females and 300–350 grams (11–12 oz) in males. Located within the thoracic cavity, it sits anterior to the vertebral column and posterior to the sternum, slightly left of the body's midline.
The human heart is enclosed within a protective double-walled sac known as the pericardium. The outer layer, called the fibrous pericardium, anchors the heart to surrounding structures and prevents over-expansion. Beneath this lies the serous pericardium, composed of two layers:
Parietal pericardium (outer serous layer)
Visceral pericardium (inner serous layer or epicardium, directly attached to the heart)
Between these layers lies the pericardial cavity, which contains pericardial fluid, allowing frictionless heart movements.
Deoxygenated blood from systemic circulation enters the right atrium via the superior and inferior vena cavae. Blood then moves through the tricuspid valve into the right ventricle, which contracts to push blood through the pulmonary valve into the pulmonary arteries, carrying blood to the lungs for oxygenation.
Oxygen-rich blood from the lungs returns to the left atrium via the pulmonary veins. It then flows through the mitral valve into the left ventricle. Powerful contractions of the left ventricle pump blood through the aortic valve into the aorta, supplying oxygenated blood to the entire body.
The rhythmic contraction and relaxation of heart chambers constitute the cardiac cycle, comprising two phases:
Systole (contraction of ventricles, forcing blood into circulation)
Diastole (relaxation phase, allowing chambers to refill with blood)
At rest, the typical adult heart beats about 72 times per minute, circulating approximately 4.7–5.7 liters of blood per minute. Over a lifespan averaging 66 years, the heart will beat roughly 2.5 billion times.
Throughout history, the heart has held significant cultural and symbolic importance, representing emotions, vitality, and life itself. The modern understanding of cardiac anatomy and physiology advanced greatly with discoveries such as William Harvey’s elucidation of blood circulation in the 17th century.
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