The heart is a myogenic muscular Muscle is the contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to produce force and cause motion. Muscles can cause organ In biology and anatomy, an organ is a collection of tissues joined in structural unit to serve a common function found in all animals Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also with a circulatory system The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pH to maintain homeostasis (including all vertebrates Vertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony fish, sharks and rays, amphibians, reptiles,), that is responsible for pumping blood Blood is a specialized bodily fluid that delivers necessary substances to the body's cells – such as nutrients and oxygen – and transports waste products away from those same cells throughout the blood vessels The blood vessels are the part of the circulatory system that transport blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from by repeated, rhythmic contractions. The term cardiac (as in cardiology Cardiology is a medical specialty dealing with disorders of the heart. The field includes diagnosis and treatment of congenital heart defects, coronary artery disease, heart failure, valvular heart disease and electrophysiology. Physicians specializing in this field of medicine are called cardiologists. Cardiologists should not be confused with) means "related to the heart" and comes from the Greek Greek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical ancient Greek literature and the New Testament of καρδιά, kardia, for "heart."

The vertebrate heart is composed of cardiac muscle Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle. The cells that comprise cardiac muscle are called cardiomyocytes and are sometimes seen as an intermediate, which is an involuntary striated muscle tissue found only in this organ, and connective tissue Connective tissue is a form of fibrous tissue.. It is one of the four types of tissue in traditional classifications. The average human heart, beating at 72 beats per minute, will beat approximately 2.5 billion times during an average 66 year lifespan, and weighs approximately 250 to 300 grams (9 to 11 oz) in females and 300 to 350 grams (11 to 12 oz) in males.^[1]

In invertebrates An invertebrate is an animal without a backbone. The group includes 95% of all animal species — all animals except those in the chordate subphylum Vertebrata that possess a circulatory system, the heart is typically a tube or small sac and pumps fluid that contains water and nutrients such as proteins Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded, fats Fats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. Chemically, fats are generally triesters of glycerol and fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure and composition. Although the words "oils", "fats",, and sugars Sugar is an informal term for a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose characterized by a sweet flavor. In food, sugar almost exclusively refers to sucrose, which primarily comes from sugar cane and sugar beet. Other sugars are used in industrial food preparation, but are usually known by more specific. In insects Insects are a class within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse group of animals on the planet and include more than a million described species and represent more than half of all known living, the "heart" is often called the dorsal tube and insect "blood" is almost always not oxygenated since they usually respirate (breathe) directly from their body surfaces (internal and external) to air. However, the hearts of some other arthropods An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the Phylum Arthropoda (from Greek ἄρθρον arthron, "joint", and ποδός podos "foot", which together mean "jointed feet"), and include the insects, arachnids, crustaceans, and (including spiders Spiders are air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in and crustaceans Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crab with a leg span of up to 14 ft (4.3 m) and a mass such as crabs True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax. Other animals, such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice, are not true crabs and shrimp Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important food source for larger animals from fish to whales) and some other animals pump hemolymph Hemolymph or Haemolymph is a fluid in the circulatory system of some arthropods and is analogous to the fluids and cells making up both blood and interstitial fluid (including water, proteins, fats, sugars, hormones, etc.) in vertebrates such as birds and mammals. Additionally, some non-arthropods such as molluscs possess a hemolymphatic, which contains the copper Copper is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable, and a freshly exposed surface has a pinkish or peachy color. It is used as a thermal conductor, an electrical conductor, a building material, and a-based protein hemocyanin Hemocyanins are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule (O2). Oxygenation causes a color change between the colorless Cu(I) deoxygenated form and the blue Cu(II) oxygenated form. Hemocyanins carry oxygen in the hemolymph of most molluscs, and some arthropods, as an oxygen transporter similar to the iron Iron is the most common element in the earth as a whole, and the fourth most common in the Earth's crust. It is produced as a result of stellar fusion in high-mass stars, and it is the heaviest stable element produced by stellar fusion because the fusion of iron is the last nuclear fusion reaction that is exothermic. Iron is the most widely used-based hemoglobin Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and the tissues of some invertebrates. Hemoglobin in the blood is what transports oxygen from the lungs or gills to the rest of the body (i.e. the tissues) where it releases the oxygen for cell use in red blood cells Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen (O2) to the body tissues via the blood flow through the circulatory system. They take up oxygen in the lungs or gills and release it while squeezing through the body's capillaries found in vertebrates Vertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony fish, sharks and rays, amphibians, reptiles,.

Contents 1 Early development 2 Structure 2.1 In humans 2.2 In fish 2.3 In double circulatory systems 2.4 The fully divided heart 3 Functioning 4 History of discoveries 5 Healthy heart 6 See also 7 References 8 External links

Early development

Main article: Heart development The lateral plate mesoderm delaminates to form two layers: the dorsal somatic mesoderm and the ventral splanchnic (visceral) mesoderm. The heart precursor cells come from the two regions of the splanchnic mesoderm called the cardiogenic mesoderm. These cells can differentiate into endocardium which lines the heart chamber and valves and the

The mammalian Mammals are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain. Some mammals have sweat glands, but most do not heart is derived from embryonic mesoderm In humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to give rise to a number of tissues and structures including bone, muscle, connective tissue, and the middle layer of the skin. Some cells in mesodermal germ-layer cells that differentiate after gastrulation Gastrulation is a phase early in the development of most animal embryos, during which the morphology of the embryo is reorganized to form the three germ layers: ectoderm, mesoderm, and endoderm. The molecular mechanism and timing of gastrulation is different in different organisms. Gastrulation is followed by organogenesis, when individual organs into mesothelium The mesothelium is a membrane that forms the lining of several body cavities: the pleura , peritoneum (abdominal cavity including the mesentery) and pericardium (heart sac). Mesothelial tissue also surrounds the male internal reproductive organs (the tunica vaginalis testis) and covers the internal reproductive organs of women (the tunica serosa, endothelium The endothelium is the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary. These cells reduce, and myocardium Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle. The cells that comprise cardiac muscle are called cardiomyocytes and are sometimes seen as an intermediate. Mesothelial pericardium The pericardium is a double-walled sac that contains the heart and the roots of the great vessels 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.^[2]

From splanchnopleuric mesoderm In humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to give rise to a number of tissues and structures including bone, muscle, connective tissue, and the middle layer of the skin. Some cells in mesodermal tissue, the cardiogenic plate develops cranially and laterally to the neural plate In human embryology, formation of neural plate is the first step of neurulation. It is created by a flat thickening opposite to the primitive streak of the ectoderm. In the cardiogenic plate, two separate angiogenic cell clusters form on either side of the embryo. Each cell cluster coalesces to form an endocardial 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 21 days.^[3]

At 21 days after conception Fertilisation , is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external, the human heart begins beating at 70 to 80 beats per minute and accelerates linearly for the first month of beating.

The human embryonic An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination. In humans, it is called an embryo until about eight weeks after fertilization (i.e. ten weeks LMP), and from then it is instead called a fetus heart begins beating at around 21 days after conception, or five weeks after the last normal menstrual period (LMP). The first day of the LMP is normally used to date the start of the gestation (pregnancy). It is unknown how blood in the human embryo circulates for the first 21 days in the absence of a functioning heart. 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 approximately 100 BPM during the first month of beating, peaking at 165-185 BPM during the early 7th week, (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.^[4] 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. The regression formula, which describes this acceleration before the embryo reaches 25 mm in crown-rump length, or 9.2 LMP weeks, is: Age in days = EHR(0.3)+6. There is no difference in female and male heart rates before birth.^[5]

Structure

The structure of the heart varies among the different branches of the animal kingdom Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also. (See Circulatory system The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pH to maintain homeostasis.) Cephalopods A cephalopod (Greek plural Κεφαλόποδα ; "head-feet") is any member of the mollusc class Cephalopoda, characterized by bilateral body symmetry, a prominent head, and a modification of the mollusc foot, a muscular hydrostat, into the form of arms or tentacles. Teuthology, a branch of malacology, is the study of cephalopods. These 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 The pericardium is a double-walled sac that contains the heart and the roots of the great vessels, which is usually a distinct structure, but may be continuous with the peritoneum The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in higher vertebrates and some invertebrates (annelids, for instance). It is composed of a layer of mesothelium supported by a thin layer of connective tissue. The peritoneum both supports the in jawless and cartilaginous fish. Hagfishes Hagfish are marine craniates of the class Agnatha or Myxini, also known as Hyperotreti. Some researchers regard Myxini as not belonging to the subphylum Vertebrata. That is, they are the only living animals that have a skull but not a vertebral column, uniquely among vertebrates, also possess a second heart-like structure in the tail.^[6]

In humans

Main article: Human heart The human heart provides a continuous blood circulation through the cardiac cycle and is one of the most vital organs in the human body. It is divided into four chambers: the two upper chambers are called the left and right atria and two lower chambers are called the right and left ventricles. Normally the right ventricle pumps the same blood Structure diagram of the human heart. Blue components indicate de-oxygenated blood pathways and red components indicate oxygenated pathways.

The human heart is about the size of a fist and has a mass of between 250 and 350 grams. It is located anterior to the vertebral column and posterior to the sternum.

It is enclosed in a double-walled sac called the pericardium The pericardium is a double-walled sac that contains the heart and the roots of the great vessels. The superficial part of this sac is called the fibrous pericardium. This sac protects the heart, anchors its surrounding structures, and prevents overfilling of the heart with blood.

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 Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle. The cells that comprise cardiac muscle are called cardiomyocytes and are sometimes seen as an intermediate and is composed of muscle which contracts. The inner layer is called the endocardium The endocardium is the innermost layer of tissue that lines the chambers of the heart. Its cells are embryologically and biologically similar to the endothelial cells that line blood vessels and is in contact with the blood that the heart pumps. Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves.^[7]

The human heart has four chambers, two superior atria and two inferior ventricles. The atria are the receiving chambers and the ventricles are the discharging chambers. The right ventricle discharges into the lungs to oxygenate the blood. The left ventricle discharges its blood toward the rest of the body via the aorta.

The pathway of blood through the human heart consists of a pulmonary circuit and a systemic circuit. Blood flows through the heart in one direction, from the atria to the ventricles, and out of the great arteries, or the aorta for example. This is done by four valves which are the tricuspid valve, the mitral valve, the aortic valve, and the pulmonary valve.^[8]

In fish

Schematic of simplified fish heart

Primitive fish have a four-chambered heart; however, 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 de-oxygenated blood, from the body, through the hepatic and cardinal veins. From here, blood flows into the atrium and then to the powerful muscular 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 dorsal aorta, into the rest of the body. (In tetrapods, the ventral aorta has divided in two; one half forms the ascending aorta, while the other forms the pulmonary artery).^[6]

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 more primitive ray-finned fish. In teleosts, 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 amniotes, 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.^[6]

In double circulatory systems

In amphibians and most reptiles, 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 lungs which deliver oxygenated blood directly to the heart.

In living amphibians, the atrium is divided into two separate chambers by the presence of a muscular septum even though there is only a single ventricle. The sinus venosus, which remains large in amphibians but connects only to the right atrium, receives blood from the vena cavae, with the pulmonary vein by-passing it entirely to enter the left atrium.

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 tetrapods. 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.^[6]

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.^[6]

The fully divided heart

Human heart removed from a 64-year-old male. Surface anatomy of the human heart. The heart is demarcated by: -A point 9 cm to the left of the midsternal line (apex of the heart) -The seventh right sternocostal articulation -The upper border of the third right costal cartilage 1 cm from the right sternal line -The lower border of the second left costal cartilage 2.5 cm from the left lateral sternal line.^[9]

Archosaurs, (crocodilians, birds), and mammals show complete separation of the heart into two pumps for a total of four heart chambers; 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; thus, only in birds and mammals are the two streams of blood - those to the pulmonary and systemic circulations - kept entirely separate by a physical barrier.^[6]

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 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 lungs. The pericardium comprises two parts: the fibrous pericardium, made of 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 lungs.^[10]

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 g (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

Blood flow diagram of the human heart. Blue components indicate de-oxygenated blood pathways and red components indicate oxygenated pathways. Image showing the conduction system of the heart

In mammals, the function of the right side of the heart (see right 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 dropped off and oxygen picked up (gas exchange). This happens through the passive process of diffusion. The left side (see left heart) collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle, through the bicuspid 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. 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.

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 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.

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, 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-calated 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 electrocardiogram (ECG or EKG).^[11] The events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next can be referred to a cardiac cycle.^[12]

The SA node is found in all amniotes 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 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 hummingbirds.^[6]

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, fibrillation, which is an irregular and ineffective heart rhythm, and asystole, which is the cessation of heart rhythm entirely.

Cardiac tamponade is a condition in which the fibrous sac surrounding the heart 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 sternum, and gently withdrawing the tamponading fluids.

History of discoveries

A preserved human heart with a visible gunshot wound

The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BC. However, their function was not properly understood then. Because blood pools in the veins after death, arteries look empty. Ancient anatomists assumed they were filled with air and that they were for transport of air.

Philosophers distinguished veins from arteries but thought that the pulse was a property of arteries themselves. Erasistratos observed the arteries that were cut during life bleed. He described 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 capillaries but with reversed flow of blood.

The 2nd century AD, Greek physician Galenos (Galen) knew that 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.

Galen believed that the arterial blood was created by venous blood passing from the left ventricle to the right through 'pores' in the inter ventricular 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.

For more recent technological developments, see Cardiac surgery.

Healthy heart

Obesity, high blood pressure, and high cholesterol can increase the risk of developing heart disease. However, half the amount of heart attacks occur in people with normal cholesterol levels. Heart disease is a major cause of death (and the number one cause of death in the Western World).

Of course one must also consider other factors such as lifestyle, for instance the amount of exercise one undertakes and their diet, as well as their overall health (mental and social as well as physical).^{[13][14][15][16]}

See also

• Cardiac cycle
• Heart disease
• Human heart
• Electrocardiogram
• Electrical conduction system of the heart
• Physiology
• Trauma triad of death
• Langendorff Heart

References

1. ^ Kumar, Abbas, Fausto: Robbins and Cotran Pathologic Basis of Disease, 7th Ed. p. 556
2. ^ Animal Tissues
3. ^ Main Frame Heart Development>
4. ^ OBGYN.net "Embryonic Heart Rates Compared in Assisted and Non-Assisted Pregnancies"
5. ^ Terry J. DuBose Sex, Heart Rate and Age
6. ^ ^{a b c d e f g} Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 437–442. ISBN 0-03-910284-X.
7. ^ "Heart". MedicaLook. Medical-Look.com. http://www.medical-look.com/human_anatomy/organs/Heart.html. Retrieved 2010-05-03.
8. ^ Marieb, Elaine Nicpon. Human Anatomy & Physiology. 6th ed. Upper Saddle River: Pearson Education, 2003. Print
9. ^ Gray's Anatomy of the Human Body - 6. Surface Markings of the Thorax
10. ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey: Prentice Hall. ISBN 0-13-981176-1. OCLC 32308337.
11. ^ Campbell, Reece-Biology, 7th Ed. p.873,874
12. ^ Guyton, A.C. & Hall, J.E. (2006) Textbook of Medical Physiology (11th ed.) Philadelphia: Elsevier Saunder ISBN 0-7216-0240-1
13. ^ "Eating for a healthy heart". MedicineWeb. http://www.medicineweb.com/nutrition-/eating-for-a-healthy-heart. Retrieved 2009-03-31.
14. ^ Division of Vital Statistics; Arialdi M. Miniño, M.P.H., Melonie P. Heron, Ph.D., Sherry L. Murphy, B.S., Kenneth D. Kochanek, M.A. (2007-08-21). "Deaths: Final data for 2004" (PDF). National Vital Statistics Reports (United States: Center for Disease Control) 55 (19): 7. http://www.cdc.gov/nchs/data/nvsr/nvsr55/nvsr55_19.pdf. Retrieved 2007-12-30.
15. ^ White House News. "American Heart Month, 2007". http://georgewbush-whitehouse.archives.gov/news/releases/2007/02/20070201-2.html. Retrieved 2007-07-16.
16. ^ National Statistics Press Release 25 May 2006

External links

Look up heart in Wiktionary, the free dictionary.

Wikimedia Commons has media related to: Hearts

• Atlas of Human Cardiac Anatomy - Endoscopic views of beating hearts - Cardiac anatomy
• Heart contraction and blood flow (animation)
• Heart Disease
• eMedicine: Surgical anatomy of the heart
• Interactive 3D heart This realistic heart can be rotated, and all its components can be studied from any angle.
• Heart Information
• Oath of Awareness Heart disease awareness site

Human systems and organs TA 2-4: musculoskeletal Skeletal system Bone (Carpus · Collar bone (clavicle) · Thigh bone (femur) · Fibula · Humerus · Mandible · Metacarpus · Metatarsus · Ossicles · Patella · Phalanges · Radius · Skull (cranium) · Tarsus · Tibia · Ulna · Rib · Vertebra · Pelvis · Sternum) · Cartilage Joints Fibrous joint · Cartilaginous joint · Synovial joint Muscular system Muscle · Tendon · Diaphragm TA 5-11: splanchnic/ viscus mostly Thoracic Respiratory system URT (Nose, Nasopharynx, Larynx) · LRT (Trachea, Bronchus, Lung) mostly Abdominopelvic Digestive system+ adnexa Mouth (Salivary gland, Tongue) · upper GI (Oropharynx, Laryngopharynx, Esophagus, Stomach) · lower GI (Small intestine, Appendix, Colon, Rectum, Anus) · accessory (Liver, Biliary tract, Pancreas) GU: Urinary system Kidney · Ureter · Bladder · Urethra GU: Reproductive system Female (Uterus, Vulva, Ovary, Placenta) · Male (Scrotum, Penis, Prostate, Testicle, Seminal vesicle) Endocrine system Pituitary · Pineal · Thyroid · Parathyroid · Adrenal · Islets of Langerhans TA 12-16 Circulatory system Cardiovascular system peripheral (Artery, Vein, Lymph vessel) · Heart Lymphatic system primary (Bone marrow, Thymus) · secondary (Spleen, Lymph node) N/S Nervous system (Brain, Spinal cord, Nerve) · Sensory system (Ear, Eye) Integumentary system Skin · Subcutaneous tissue · Breast (Mammary gland) Non-TA Immune system general anatomy: , , , ,

Anatomy of torso, cardiovascular system: heart (TA A12.1, GA 5.524) General Surface base · apex · grooves (coronary/atrioventricular, interatrial, anterior interventricula, posterior interventricular) · surfaces (sternocostal, diaphragmatic) · borders (right, left) · Openings of smallest cardiac veins Internal atria (interatrial septum, musculi pectinati, sulcus terminalis) · ventricles (interventricular septum, trabeculae carneae, chordae tendineae, papillary muscle) · valves · cusps · Atrioventricular septum cardiac skeleton Intervenous tubercle Chambers Right heart (venae cavae, coronary sinus) → right atrium (atrial appendage, fossa ovalis, limbus of fossa ovalis, crista terminalis, valve of inferior vena cava, valve of coronary sinus) → tricuspid valve → right ventricle (conus arteriosus, moderator band/septomarginal trabecula) → pulmonary valve → (pulmonary artery and pulmonary circulation) Left heart (pulmonary veins) → left atrium (atrial appendage) → mitral valve → left ventricle → aortic valve (aortic sinus) → (aorta and systemic circulation) Layers Endocardium Heart valves Myocardium Conduction system: Cardiac pacemaker · SA node · AV node · bundle of His · Purkinje fibers Pericardial cavity Pericardial sinus Pericardium fibrous pericardium (Sternopericardiac ligaments) · serous pericardium (epicardium/visceral layer) · Fold of the left vena cava : HRT // //, / , drug (///)

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