document_title: Avatar: The Last Airbender

question_text: who was the first avatar ever in the last airbender

text: Avatar: The Last Airbender

text: Series overview

text: Setting

text: Avatar: The Last Airbender is set in a world—adjacent to a parallel spirit world—that is home to humans and hybrid animals. Human civilization is divided into four nations, named after the four classical elements: the Water Tribes, the Earth Kingdom, the Fire Nation, and the Air Nomads. Each nation has a distinct society in which select people, known as "benders" (waterbenders, earthbenders, firebenders and airbenders), have the ability to telekinetically manipulate and control their nation's element using martial arts.

text: Only the Avatar has the ability to bend all four elements. The Avatar, who may be male or female, is an international arbiter whose duty is to maintain harmony among the four nations, and act as a mediator between humans and spirits. When the Avatar dies, their spirit is reincarnated into the next of the four nations in what is known as the Avatar cycle: the Fire Nation, Air Nomads, Water Tribe and Earth Kingdom. An Avatar is required to master each bending art, beginning with the element of their home nation, and proceeding to learn the others in the order of the Avatar cycle, starting at the age of sixteen. Avatars also possess the ability to enter a condition known as the Avatar State, in which they gain the knowledge and abilities of all past Avatars. Although it is when they are most powerful, if an Avatar were ever to be killed while in the Avatar State, the reincarnation cycle would end and the avatar would never again be reborn.

text: Synopsis

text: More than a century before the events of the series begin, Fire Lord Sozin, ruler of the Fire Nation, planned a world war to expand his nation's territory and influence. He was prevented from carrying out his plans by Avatar Roku, who was born in the Fire Nation. After Roku's death, the Avatar was reincarnated as an Airbender named Aang. Because the Avatar was now a child, Sozin saw his chance and proceeded with his militant plans. At the age of twelve, Aang learned about his avatar status because of the threat of Sozin's war. Afraid of his new responsibilities and of separation from his mentor Gyatso, he fled his home on his flying bison, Appa. Aang was forced into the ocean by a storm; he entered the Avatar State and encased himself and Appa in suspended animation in an iceberg near the South Pole for one hundred years. Sozin, knowing the avatar's reincarnation cycle mandated an Air Nomad was the new Avatar, carried out a genocide against the Air Nomads during the passage of a once-a-century comet that increased the Firebenders' power, and continued his world conquest.

text: In the present day, as the Fire Nation continues its war against the other nations, Katara, a fourteen-year-old waterbender of the Southern Water Tribe, and her older brother, Sokka, find and revive Aang and Appa. Aang learns about the war and the siblings join him to reach the Northern Water Tribe at the North Pole so he and Katara can learn waterbending. Aang's return attracts the attention of Prince Zuko, the exiled son of the current Fire Lord Ozai, who pursues them, accompanied by his uncle Iroh. Aang is also pursued by Zhao, a Fire Nation admiral who intends to win Ozai's favor and rob Zuko of his redemption. En route to the North Pole, Aang learns about the genocide against his people when he visits the ruins of the Southern Air Temple. During the winter solstice, Aang meets the spirit of his predecessor, Avatar Roku, and comes to terms with his responsibilities. With the Northern Water Tribe, Aang and Katara learn advanced waterbending from Master Pakku; Sokka falls in love with the chief's daughter, Princess Yue. Zhao's navy lays siege to the tribe, seizing the mortal forms of the ocean and moon spirits—the source of waterbending—and causing a lunar eclipse. Zhao kills the moon spirit to deprive the waterbenders of their abilities but Aang joins with the ocean spirit to drive off the enemy fleet while Yue sacrifices her life to revive the moon spirit. When Ozai hears about his older brother Iroh's resistance to Zhao, he sends his daughter Azula to capture Iroh and Zuko.

text: After leaving the Northern Water Tribe, Katara continues teaching Aang waterbending while the group searches for an earthbending teacher. They meet Toph Beifong, a twelve-year-old, blind earthbending prodigy who wants independence from her upper-class family. Pursued by Azula, Zuko and Iroh lead new lives in the Earth Kingdom as wanderers and refugees, settling in the capital city Ba Sing Se. At a library guarded by the spirit Wan Shi Tong, Aang and his group learn an imminent solar eclipse could let them stop the Fire Nation before Sozin's Comet arrives. They journey to Ba Sing Se to inform the Earth King of this information. In the city, they find that the Earth King Kuei is a puppet ruler, manipulated by Long Feng, leader of the Dai Li secret police. After Aang's group exposes Long Feng's political machinations, Toph is captured but escapes by learning to bend metal. The Dai Li join Azula to instigate a coup d'état of Ba Sing Se, and Zuko, who has spent his time in Ba Sing Se trying to come to terms with his identity, sides with his sister Azula. During a face off in the catacombs underneath the ancient city, Azula nearly kills Aang, forcing the protagonists to retreat with help from Iroh and leaving the Earth Kingdom under the control of the Fire Nation.

text: Aang emerges from a coma to find his friends and allies disguised as soldiers on a Fire Nation ship, preparing to invade the Fire Nation capital during the solar eclipse. The invasion is at first successful but Aang and his friends cannot find Ozai and are forced to retreat. Zuko learns about his father's intention to destroy the Earth Kingdom during the return of Sozin's Comet; he begins to regret his decision, and abandons the Fire Nation to join Aang and teach him firebending. As the comet approaches, Aang, a pacifist, wrestles with the possibility that he will have to kill Ozai to end the war. As he, in seclusion, consults the spirits of his predecessors for advice, Katara and the others encounter Iroh, who is leading a secret society called the Order of the White Lotus. The Order liberates Ba Sing Se. Sokka, Toph, and the warrior Suki attack the Fire Nation's airships, while Zuko and Katara confront Azula to prevent her from being crowned as the new Fire Lord. As the comet arrives, Aang confronts Ozai but cannot get the upper hand until Ozai triggers Aang's connection to the Avatar State. Aang strips Ozai of his firebending powers. Zuko is crowned the new Fire Lord and arranges an armistice, establishing peace in the world.

text: Episodes

text: The series consists of sixty-one episodes. The first episode—an-hour-long premiere—aired on February 21, 2005, on Nickelodeon.[4] The series concluded with a two-hour television movie broadcast on July 19, 2008.[5] Each season of the series is known as a "book", in which each episode is referred to as a "chapter". Each book takes its name from one of the elements Aang, the protagonist, must master; Water, Earth, and Fire.[4] The show's first two seasons each consists of twenty episodes and the third season has twenty-one. The entire series has been released on DVD in regions 1, 2 and 4.[6]

text: SeasonBookEpisodesOriginally airedFirst airedLast aired1Book One: Water20February 21, 2005 (2005-02-21)December 2, 2005 (2005-12-02)2Book Two: Earth20March 17, 2006 (2006-03-17)December 1, 2006 (2006-12-01)3Book Three: Fire21September 21, 2007 (2007-09-21)July 19, 2008 (2008-07-19)

text: Development

text: Conception and production

text: Avatar: The Last Airbender was co-created and produced by Michael Dante DiMartino and Bryan Konietzko at Nickelodeon Animation Studios in Burbank, California. Its animation was mostly done by South Korean studios JM Animation, DR Movie, and MOI Animation. According to Konietzko, the series was conceived in early 2001 when he took an old sketch of a balding, middle-aged man and imagined the man as a child. He drew the character herding bison in the sky and showed the sketch to DiMartino, who was watching a documentary about explorers trapped at the South Pole. Konietzko described their early development of the concept; "There's an air guy along with these water people trapped in a snowy wasteland ... and maybe some fire people are pressing down on them".[7] The co-creators successfully pitched the idea to Nickelodeon vice-president and executive producer Eric Coleman two weeks later.[8]

text: The series was introduced to the public in a teaser reel at Comic-Con 2004,[9] and premiered on February 21, 2005.[10]

text: In an interview, Konietzko said: "Mike and I were really interested in other epic 'Legends & Lore' properties, like Harry Potter and Lord of the Rings, but we knew that we wanted to take a different approach to that type of genre. Our love for Japanese anime, Hong Kong action and kung fu cinema, yoga, and Eastern philosophies led us to the initial inspiration for Avatar: The Last Airbender."[11]

text: Influences

text: The series is notable for borrowing extensively from East Asian art and mythology for its universe. Its creators employed cultural consultants Edwin Zane and calligrapher Siu-Leung Lee to help determine its art direction and settings.[12][13] Its character designs are influenced by Chinese art and history, Hinduism, Taoism and Buddhism, and Yoga.[14][12] Jeremy Zuckerman and Benjamin Wynn composed the series' music and sound; they experimented with a wide range of instruments, including the guzheng, pipa, and duduk, to match the show's Asia-influenced setting.[15] The art style of the fictitious locations used in the series are based on real locations in Asia. Sites such as the Forbidden City and the Great Wall of China in Beijing were inspirations for the Earth Kingdom city of Ba Sing Se,[16] and Water Tribe locations were based off of Inuit and Sireniki cultures.[17] According to writer Aaron Ehasz, early Fire Nation designs were entirely based off of Japanese culture. In order to avoid accidentally making broad statements, they redesigned many settings and peoples to be more "broadly inspired".[17]

text: The fighting styles employed by the show's characters are derived from Chinese martial arts, for which the film-makers employed Sifu Kisu of the Harmonious Fist Chinese Athletic Association as a consultant.[18] Each fighting style is unique to the "benders" who use them or characters who are aligned to a certain element. For example, practitioners of "waterbending" use movements influenced by T'ai chi and focused on alignment, body structure, breath, and visualization. Hung Gar was the inspiration for practitioners of "earthbending", and was chosen for its firmly rooted stances and powerful strikes as a representation of the solidity of earth. Northern Shaolin, which uses strong arm and leg movements, was chosen to represent "firebending". Ba Gua, which uses dynamic circular movements and quick directional changes, was used for "airbending".[19] The Chu Gar Southern Praying Mantis style can be seen practiced by the earthbender Toph, who develops a unique fighting style as a result of her blindness.[20]  Asian cinema influenced the presentation of these martial-art bending moves.[7]

text: Themes

text: The series explored many concepts rarely touched on in children's television shows,[17] including issues relating to war, genocide, and imperialism, gender discrimination and female empowerment, marginalization and oppression, and the philosophical questions surrounding fate and destiny.

text: The show is set during a period where the entire world is engulfed in a imperialistic war initiated by the Fire Nation. While war is a constant backdrop, the show depicts these effects through the eyes of common people—the oppressed Earth Kingdom citizens as well as brainwashed Fire Nation schoolchildren—to show how war makes victims of everyone.[17] And while the Fire Nation is presented as the instigator of violence, the show also depicts the systemic inequality experienced by residents in the Earth Kingdom city of Ba Sing Se as well as the nefarious activities of the city's secret police. These situations show the corrupting nature of power and the nuances of good and evil.[17] The show introduces viewers to genocide early on when protagonist Aang visits his former home in the Southern Air Temple. He arrives to discover his people have been massacred, and allows him to display a range of emotions, from rage to loss.[21]

text: The character Zuko and his relationship with his father and Uncle Iroh is the series' main redemption arc and represents the show's message that destiny and fate are not binding or set by other people, but can be changed.[22] In season one, he struggles to conform to the destiny and path determined by his father,[17] but Iroh prods him, asking, "who are you, what do you want?"[23] The show also represents a diverse cast of characters in order to tackle the issue of marginalization. For example, in introducing a blind character like Toph and a paraplegic boy like Teo, the show depicted characters with vulnerabilities overcoming their physical and societal limitations.[17] This is also true when it comes to the show's female characters. For example, female protagonist Katara faces systemic sexism when she reaches the Northern Water Tribe to learn waterbending. In another instance, her brother, Sokka is initially dismissive of the all-female Kyoshi Warriors, but learns to respect and appreciate their skills.[17] According to Kirk Hamilton of Kotaku, these themes represent the show's message that it’s more important to be yourself than to hew to the roles society expects of you."[23]

text: Other media

text: Books

text: Several books based on the show have been published. Dark Horse Comics published an art book titled Avatar: The Last Airbender – The Art of the Animated Series on June 2, 2010, with 184 pages of original art from the series.[24] Several comic-book short stories were published in Nickelodeon Magazine, and Dark Horse published Avatar: The Last Airbender – The Lost Adventures—a collection of these and new comics—on June 15, 2011.[25]

text: Dark Horse published a graphic-novel series by Gene Yang that continues Aang's story after the Hundred Years' War. Avatar: The Last Airbender – The Promise, published in three volumes in 2012, explores the fate of the Fire Nation colonies that become The Legend of Korra's United Republic. A second set of three comic books, Avatar: The Last Airbender – The Search, focuses on Zuko and Azula, and the fate of their mother Ursa.[26] The third set, Avatar: The Last Airbender – The Rift, shifts the focus to Aang, the creation of Republic City, and Toph's relationship with her family.[27] The Rift was followed by Avatar: The Last Airbender – Smoke and Shadow about a resistance force in the Fire Nation against Firelord Zuko, who at the end of the original series assumed the throne.[28] The fifth graphic novel was Avatar: The Last Airbender – North and South, which follows the events of Smoke and Shadow and is about Katara and Sokka returning to the Water Tribe to see various changes to their homeland.[29] The next planned graphic novel is titled Imbalance and is planned to be released in October 2018. The series explores the emerging conflict between the benders and non-benders that becomes the center for the conflict in the first season of the sequel the Legend of Korra. Unlike the previous five books it is will not be written by Gene Yang.[30]

text: Video games

text: A video-game trilogy based on the series has been released. The Avatar: The Last Airbender video game was released on October 10, 2006,[31] and Avatar: The Last Airbender – The Burning Earth was released on October 16, 2007.[32] Avatar: The Last Airbender – Into the Inferno was released on October 13, 2008.[33] Avatar: Legends of the Arena, a massively multiplayer online role-playing game (MMORPG) for Microsoft Windows, was released on September 15, 2008, by Nickelodeon. Players can create their own character and interact with other players around the world.[34] Avatar: The Last Airbender was THQ's bestselling Nickelodeon game in 2006 and was one of Sony CEA's Greatest Hits.[35]

text: Film

text: The series' first season was the basis of the 2010 live-action film The Last Airbender, which was written and directed by M. Night Shyamalan. It was intended as the first of a trilogy of films, each of which would be based upon one of the three television seasons. The film's reception from critics and fans of the television series was overwhelmingly negative; it had a six-percent approval rating on Rotten Tomatoes and received five Razzies at the 31st Golden Raspberry Awards, including Worst Picture.[36][37][38] Although the film originally shared the title of the television series, the title The Last Airbender was used because producers feared it would be confused with James Cameron's film Avatar.[39] The Last Airbender stars Noah Ringer as Aang, Nicola Peltz as Katara, Jackson Rathbone as Sokka, Dev Patel as Zuko, and Shaun Toub as Iroh.[36]

text: Sequel

text: The Legend of Korra, a sequel series to Avatar: The Last Airbender, premiered on Nickelodeon on April 14, 2012.[40] It was written and produced by Michael Dante DiMartino and Bryan Konietzko, the creators and producers of the original series.[41] The show was initially titled Avatar: Legend of Korra, then The Last Airbender: Legend of Korra; its events occur  seventy years after the end of Avatar: The Last Airbender.[42] The series' protagonist is Korra, a 17-year-old woman from the Southern Water Tribe who is the incarnation of the Avatar after Aang's death.[40]

text: Reception

text: Ratings

text: The Avatar: The Last Airbender was the highest-rated animated television series in its demographic at its premiere;[43] an average of 3.1 million viewers watched each new episode.[43] It had 5.6 million viewers for its highest-rated episode and was a highly rated part of the Nicktoons lineup beyond its 6-to-11-year-old target demographic.[44][45] A one-hour special, The Secret of the Fire Nation, consisting of the episodes "The Serpent's Pass" and "The Drill", aired on September 15, 2006, and attracted 5.1 million viewers. According to the Nielsen Media Research, the special was the highest-rated cable-television program that week.[46] In 2007, Avatar: The Last Airbender was syndicated to more than 105 countries and was one of Nickelodeon's top-rated programs. The series ranked first on Nickelodeon in Germany, Indonesia, Malaysia, the Netherlands, Belgium, and Colombia.[47]

text: The four-part series finale, "Sozin's Comet", had the series' highest ratings. Its first airing averaged 5.6 million viewers, 95 percent more than Nickelodeon had in mid-July 2007.[48] During the week of July 14, it was the most-viewed program by the under-14 demographic.[49][50] The finale's popularity was reflected in online media; Rise of the Phoenix King, a Nick.com online game based on "Sozin's Comet", had almost 815,000 game plays in three days.[51] IGN ranked the series 35th on its list of top 100 animated TV shows.[52]

text: Critical response

text: Avatar: The Last Airbender received critical acclaim. Max Nicholson of IGN called it a "must-watch" and described it as "one of the greatest animated series of all time".[53] Nick Hartel of DVD Talk called the series a remarkable, "child friendly show" whose legacy "should endure for years to come".[22] Erik Amaya of Bleeding Cool described the series as "impressive in its sophistication" and "fantastic".[54] Henry Glasheen of SLUG Magazine called the series "adventurous and exciting", a "classic" and occasionally moving.[55] According to Brittany Lovely of Hypable, it tells "complex and beautiful" stories.[56] Joe Corey of Inside Pulse described the series as an anime-action hybrid.[57] Chris Mitchell of Popzara called it one of best shows to air on Nickelodeon, praising the series' background music and voice acting.[58] D. F. Smith of IGN recommended it to viewers who enjoy action-adventure cartoons.[59]

text:
Rob Keyes of Screen Rant called the series "one of the greatest cartoons ever made".[60] Mike Noyes of Inside Pulse recommended it to viewers who enjoy "great" adventure.[61] Gord Lacey of TVShowsonDVD.com called the series "one of the finest animated shows ever".[62] According to Todd Douglass, Jr. of DVD Talk, adults will enjoy the series as much as children do.[63] Joshua Miller of CHUD.com called it "phenomenal" and "one of the most well animated programs (children's or adult) American TV has ever had"; according to Miller, the series is heavily influenced by anime.[64] Tim Janson of Cinefantastique described it as "one of the most engaging animated shows produced".[65] Dennis Amith of J!ENT called the series "one of the best animated TV series shown in the US by American creators". Amith praised its sophisticated storylines, edginess, humor, and action.[66] Franco "Cricket" Te of Nerd Society described Avatar: The Last Airbender as "one of the best cartoon[s]" he had ever seen, recommending the series for its characters and plot.[67] Scott Thill of Wired called the series engaging and its setting, influenced by the Eastern world, "fantastic".[68]
Kirk Hamilton of Kotaku said the series should be part of the golden age of television, and recommended "the sophisticated kids show" to others.[23]

text: The show's writing and theme have been widely praised by critics. Michael S. Mammano of Den of Geek called the plot "smartly-written" and praised the animation.[69] Nicole Clark, writing for Vice News, wrote that the show's narrative depth was "its greatest asset", and praised the story's "emotional authenticity" and how it "expose[d] very young viewers to darker subject matter, like genocide and authoritarianism, while giving them a framework for understanding these issues."[17] Jenifer Rosenberg of ComicMix liked the program's emphasis on family, friends, community, and education.[70] According to Nick Hartel, the series touches on themes of "genocide and self-doubt" without frightening younger children; rogue characters are redeemable, sending an important message that people can change and are not bonded to "destiny".[22] Chris Mitchell called the plot "fantastic".[58] D. F. Smith compared the series' plot to Japanese action cartoons, calling its tone and dialogue "very American" and praising the humor leavening an epic, dramatic theme suitable for all ages.[59] Rob Keyes also praised the series' humor and affecting plot: "[It] will capture your hearts".[60]

text: According to Mike Noyes, the series amalgamates elements of "classic fantasy epics".[61] Todd Douglass, Jr. called the plot engaging, well-thought-out, and meaningful. The series' concept is "well-realized", with a consistent story. Douglass wrote that the characters "[have] a real sense of progression", and praised the writers for their humor, drama, and emotion.[63] Joshua Miller called the series surprisingly dark despite its "silly" theme; the plot is livelier than that of Lost and, similar to the latter show, emphasizes character development. According to Miller, its writing was "true adult levels of storytelling".[64] Tim Janson described the series as more than fantasy-and superhero-themed, seeing the characters as central and relateable.[65] "Cricket" Te praised the series' use of Buddhist philosophies and the diverse presentation of its themes of courage and life.[67] Kirk Hamiltion praises the series for expressing towards its audience to be themselves and for its quiet progressivism.[23]

text: Critics also praised Avatar: The Last Airbender's character development, art, animation, and choreography; Eric Amaya enjoyed the expressive animation that complements the writing. According to Amaya, the elements were influenced by Hayao Miyazaki.[54] Todd Douglass, Jr. called the character development interesting,[63] while Nicole Clark wrote that the show "managed to do what so few shows even today have: assemble a cast of characters that depicts the world as it is, with a range of identities and experiences."[17] Jenifer Rosenburg praised the series' portrayal of females as "strong, responsible, [and] intelligent".[70] According to Joshua Miller, the bender characters' use of bending for everyday activities brings "depth and believability" to the avatar world. Miller called the series' designs "rich and immersive", with each nation having its own, detailed look. He praised the action scenes as "well rendered", comparing the development of the avatar world to that of  The Lord of the Rings, and the fight choreography as "wonderful in its most minor details".[64] D. F. Smith enjoyed the series' painstaking backgrounds.[59] "Cricket" Te praised each episode's color palette and the choreography's combination of martial arts and magic.[67] Nick Hartel criticized the animation, although he found it an improvement over previous Nickelodeon shows.[22] Chris Mitchell called the animation fluid.[58] "Cricket" Te agreed, noting its manga influence.[67] According to Brittany Lovely, non-bender characters in battle are "overshadowed" by their bender counterparts.[56] Joe Corey called the animation's action and environments a "great achievement",[57] and Rob Keyes praised the series' fight choreography.[60] According to Kirk Hamilton, the action sequences in the series are amazing while being child-appropriate and exciting.[23]

text: Awards and nominations

text:
Year
Award
Category
Nominee
Status

2005

Pulcinella Awards

Best Action Adventure TV Series

Avatar: The Last Airbender

Won[71]

Best TV Series

Avatar: The Last Airbender

Won[71]

2006

33rd Annie Awards

Best Animated Television Production

Avatar: The Last Airbender

Nominated[72]

Storyboarding in an Animated Television Production

Lauren MacMullan for "The Deserter"

Won[72]

Writing for an Animated Television Production

Aaron Ehasz and John O'Bryan for "The Fortuneteller"

Nominated[72]

2007

Nickelodeon Australian Kids' Choice Awards 2007

Fave Toon

Avatar: The Last Airbender

Nominated[73]

34th Annie Awards

Character Animation in a Television Production

Yu Jae Myung for "The Blind Bandit"

Won[74]

Directing in an Animated Television Production

Giancarlo Volpe for "The Drill"

Won[74]

Genesis Awards

Outstanding Children's Programming

"Appa's Lost Days"

Won[75]

59th Primetime Emmy Awards

Outstanding Animated Program

"City of Walls and Secrets"

Nominated[76]

Outstanding Individual Achievement in Animation

Sang-Jin Kim for "Lake Laogai"

Won[77]

2008

2008 Kids' Choice Awards

Favorite Cartoon

Avatar: The Last Airbender

Won[78]

Annecy International Animated Film Festival

TV series

Joaquim Dos Santos for "The Day of Black Sun, Part 2: The Eclipse"

Nominated[79]

Peabody Awards

N/A

Avatar: The Last Airbender

Won[80]

13th Satellite Awards

Best Youth DVD

Book 3: Fire, Volume 4

Nominated[81]

2009

36th Annie Awards

Best Animated Television Production for Children

Avatar: The Last Airbender

Won[82]

Directing in an Animated Television Production

Joaquim Dos Santos for "Sozin's Comet, Part 3: Into the Inferno"

Won[82]

Golden Reel Awards

Best Sound Editing: Television Animation

"Sozin's Comet, Part 4: Avatar Aang"

Nominated[83]

Nickelodeon Australian Kids' Choice Awards 2009

Fave Toon

Avatar: The Last Airbender

Won[84]

2010

Nickelodeon Australian Kids' Choice Awards 2010

Top Toon

Avatar: The Last Airbender

Nominated[85]


document_title: Cardiac physiology

question_text: electrical impulses are conducted efficiently through cardia muscle via

root

text: Cardiac physiology

text: Blood flow[edit]

text: The heart functions as a pump and acts as a double pump in the cardiovascular system to provide a continuous circulation of blood throughout the body. This circulation includes the systemic circulation and the pulmonary circulation. Both circuits transport blood but they can also be seen in terms of the gases they carry. The pulmonary circulation collects oxygen from the lungs and delivers carbon dioxide for exhalation. The systemic circuit transports oxygen to the body and returns relatively deoxygenated blood and carbon dioxide to the pulmonary circuit.[1]

tag: p

text: Blood flows through the heart in one direction, from the atria to the ventricles, and out through the pulmonary artery into the pulmonary circulation, and the aorta into the systemic circulation. The pulmonary artery (also trunk) branches into the left and right pulmonary arteries to supply each lung. Blood is prevented from flowing backwards (regurgitation) by the tricuspid, bicuspid, aortic, and pulmonary valves.

text: The function of the right heart, is to collect de-oxygenated blood, in the right atrium, from the body via the superior vena cava, inferior vena cava and from the coronary sinus and pump it, through the tricuspid valve, via the right ventricle, through the semilunar pulmonary valve and into the pulmonary artery in the pulmonary circulation where carbon dioxide can be exchanged for oxygen in the lungs. This happens through the passive process of diffusion. In the left heart oxygenated blood is returned to the left atrium via the pulmonary vein. It is then pumped into the left ventricle through the bicuspid valve and into the aorta for systemic circulation. Eventually in the systemic capillaries exchange with the tissue fluid and cells of the body occurs; oxygen and nutrients are supplied to the cells for their metabolism and exchanged for carbon dioxide and waste products[1] In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.[1]

text: The ventricles are stronger and thicker than the atria, and the muscle wall surrounding the left ventricle is fatty than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation. Atria facilitate circulation primarily by allowing uninterrupted venous flow to the heart, preventing the inertia of interrupted venous flow that would otherwise occur at each ventricular systole.[2]

text: Cardiac muscle[edit]

text: Cardiac muscle tissue has autorhythmicity, the unique ability to initiate a cardiac action potential at a fixed rate - spreading the impulse rapidly from cell to cell to trigger the contraction of the entire heart. This autorhythmicity is still modulated by the endocrine and nervous systems.[1]

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

text: Structure of cardiac muscle[edit]

text: Cardiomyocytes, are considerably shorter and have smaller diameters than skeletal myocytes. Cardiac muscle (like skeletal muscle) is characterized by striations - the stripes of dark and light bands resulting from the organised arrangement of myofilaments and myofibrils in the sarcomere along the length of the cell. T (transverse) tubules are deep invaginations from the sarcolemma (cell membrane) that penetrate the cell, allowing the electrical impulses to reach the interior. In cardiac muscle the T-tubules are only found at the Z-lines.[1] When an action potential causes cells to contract, calcium is released from the sarcoplasmic reticulum of the cells as well as the T tubules. The calcium release triggers sliding of the actin and myosin fibrils leading to contraction.[3] A plentiful supply of mitochondria provide the energy for the contractions. Typically, cardiomyocytes have a single, central nucleus, but can also have two or more.[1]

text: Cardiac muscle cells branch freely and are connected by junctions known as intercalated discs which help the synchronized contraction of the muscle.[4] The sarcolemma (membrane) from adjacent cells bind together at the intercalated discs. They consist of desmosomes, specialized linking proteoglycans, tight junctions, and large numbers of gap junctions that allow the passage of ions between the cells and help to synchronize the contraction. Intercellular connective tissue also helps to strongly bind the cells together, in order to withstand the forces of contraction.[1]

text: Cardiac muscle undergoes aerobic respiration patterns, primarily metabolizing lipids and carbohydrates. Oxygen from the lungs attaches to haemoglobin and is also stored in the myoglobin, so that a plentiful supply of oxygen is available. Lipids, and glycogen are also stored within the sarcoplasm and these are broken down by mitochondria to release ATP. The cells undergo twitch-type contractions with long refractory periods followed by brief relaxation periods when the heart fills with blood for the next cycle.[1]

text: Electrical conduction[edit]

text: It is not very well known how the electric signal moves in the atria. It seems that it moves in a radial way, but Bachmann's bundle and coronary sinus muscle play a role in conduction between the two atria, which have a nearly simultaneous systole.[5][6][7] While in the ventricles, the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the myocardium.[8]

text: If embryonic heart cells are separated into a Petri dish and kept alive, each is capable of generating its own electrical impulse followed by contraction. When two independently beating embryonic cardiac muscle cells are placed together, the cell with the higher inherent rate sets the pace, and the impulse spreads from the faster to the slower cell to trigger a contraction. As more cells are joined together, the fastest cell continues to assume control of the rate. A fully developed adult heart maintains the capability of generating its own electrical impulse, triggered by the fastest cells, as part of the cardiac conduction system. The components of the cardiac conduction system include the atrial and ventricular syncytium, the sinoatrial node, the atrioventricular node, the bundle of His (atrioventricular bundle), the bundle branches, and the Purkinje cells.[1]

text: Sinoatrial (SA) node[edit]

text: Normal sinus rhythm is established by the sinoatrial (SA) node, the heart's pacemaker. The SA node is a specialized grouping of cardiomyocytes in the upper and back walls of the right atrium very close to the opening of the superior vena cava. The SA node has the highest rate of depolarization.[1]

text: This impulse spreads from its initiation in the SA node throughout the atria through specialized internodal pathways, to the atrial myocardial contractile cells and the atrioventricular node. The internodal pathways consist of three bands (anterior, middle, and posterior) that lead directly from the SA node to the next node in the conduction system, the atrioventricular node. The impulse takes approximately 50 ms (milliseconds) to travel between these two nodes. The relative importance of this pathway has been debated since the impulse would reach the atrioventricular node simply following the cell-by-cell pathway through the contractile cells of the myocardium in the atria. In addition, there is a specialized pathway called Bachmann's bundle or the interatrial band that conducts the impulse directly from the right atrium to the left atrium. Regardless of the pathway, as the impulse reaches the atrioventricular septum, the connective tissue of the cardiac skeleton prevents the impulse from spreading into the myocardial cells in the ventricles except at the atrioventricular node.[1] The electrical event, the wave of depolarization, is the trigger for muscular contraction. The wave of depolarization begins in the right atrium, and the impulse spreads across the superior portions of both atria and then down through the contractile cells. The contractile cells then begin contraction from the superior to the inferior portions of the atria, efficiently pumping blood into the ventricles.[1]

text: Atrioventricular (AV) node[edit]

text: The atrioventricular (AV) node is a second cluster of specialized myocardial conductive cells, located in the inferior portion of the right atrium within the atrioventricular septum. The septum prevents the impulse from spreading directly to the ventricles without passing through the AV node. There is a critical pause before the AV node depolarizes and transmits the impulse to the atrioventricular bundle. This delay in transmission is partially attributable to the small diameter of the cells of the node, which slow the impulse. Also, conduction between nodal cells is less efficient than between conducting cells. These factors mean that it takes the impulse approximately 100 ms to pass through the node. This pause is critical to heart function, as it allows the atrial cardiomyocytes to complete their contraction that pumps blood into the ventricles before the impulse is transmitted to the cells of the ventricle itself. With extreme stimulation by the SA node, the AV node can transmit impulses maximally at 220 per minute. This establishes the typical maximum heart rate in a healthy young individual. Damaged hearts or those stimulated by drugs can contract at higher rates, but at these rates, the heart can no longer effectively pump blood.[1]

text: Bundle of His, bundle branches, and Purkinje fibers[edit]

text: Arising from the AV node, the bundle of His, proceeds through the interventricular septum before dividing into two bundle branches, commonly called the left and right bundle branches. The left bundle branch has two fascicles. The left bundle branch supplies the left ventricle, and the right bundle branch the right ventricle. Since the left ventricle is much larger than the right, the left bundle branch is also considerably larger than the right. Portions of the right bundle branch are found in the moderator band and supply the right papillary muscles. Because of this connection, each papillary muscle receives the impulse at approximately the same time, so they begin to contract simultaneously just prior to the remainder of the myocardial contractile cells of the ventricles. This is believed to allow tension to develop on the chordae tendineae prior to right ventricular contraction. There is no corresponding moderator band on the left. Both bundle branches descend and reach the apex of the heart where they connect with the Purkinje fibers. This passage takes approximately 25 ms.[1]

text: The Purkinje fibers are additional myocardial conductive fibers that spread the impulse to the myocardial contractile cells in the ventricles. They extend throughout the myocardium from the apex of the heart toward the atrioventricular septum and the base of the heart. The Purkinje fibers have a fast inherent conduction rate, and the electrical impulse reaches all of the ventricular muscle cells in about 75 ms. Since the electrical stimulus begins at the apex, the contraction also begins at the apex and travels toward the base of the heart, similar to squeezing a tube of toothpaste from the bottom. This allows the blood to be pumped out of the ventricles and into the aorta and pulmonary trunk. The total time elapsed from the initiation of the impulse in the SA node until depolarization of the ventricles is approximately 225 ms.[1]

text: Membrane potentials and ion movement in cardiac conductive cells[edit]

text: Action potentials are considerably different between conductive and contractive cardiomyocytes. While sodium Na+ and potassium K+ ions play essential roles, calcium ions Ca2+ are also critical for both types of cell. Unlike skeletal muscles and neurons, cardiac conductive cells do not have a stable resting potential. Conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions that causes the membrane potential to rise slowly from an initial value of −60 mV up to about –40 mV. The resulting movement of sodium ions creates spontaneous depolarization (or prepotential depolarization).[1]

text: At this point, calcium channels open and Ca2+ enters the cell, further depolarizing it at a more rapid rate until it reaches a value of approximately +5 mV. At this point, the calcium ion channels close and potassium channels open, allowing outflux of K+ and resulting in repolarization. When the membrane potential reaches approximately −60 mV, the K+ channels close and Na+ channels open, and the prepotential phase begins again. This process gives the autorhythmicity to cardiac muscle.[1]

text: Membrane Potentials and ion movement in cardiac contractile cells[edit]

text: There is a distinctly different electrical pattern involving the contractile cells. In this case, there is a rapid depolarization, followed by a plateau phase and then repolarization. This phenomenon accounts for the long refractory periods required for the cardiac muscle cells to pump blood effectively before they are capable of firing for a second time. These cardiac myocytes normally do not initiate their own electrical potential, although they are capable of doing so, but rather wait for an impulse to reach them.[1]

text: Contractile cells demonstrate a much more stable resting phase than conductive cells at approximately −80 mV for cells in the atria and −90 mV for cells in the ventricles. Despite this initial difference, the other components of their action potentials are virtually identical. In both cases, when stimulated by an action potential, voltage-gated channels rapidly open, beginning the positive-feedback mechanism of depolarization. This rapid influx of positively charged ions raises the membrane potential to approximately +30 mV, at which point the sodium channels close. The rapid depolarization period typically lasts 3–5 ms. Depolarization is followed by the plateau phase, in which membrane potential declines relatively slowly. This is due in large part to the opening of the slow Ca2+ channels, allowing Ca2+ to enter the cell while few K+ channels are open, allowing K+ to exit the cell. The relatively long plateau phase lasts approximately 175 ms. Once the membrane potential reaches approximately zero, the Ca2+ channels close and K+ channels open, allowing K+ to exit the cell. The repolarization lasts approximately 75 ms. At this point, membrane potential drops until it reaches resting levels once more and the cycle repeats. The entire event lasts between 250 and 300 ms.[1]

text: The absolute refractory period for cardiac contractile muscle lasts approximately 200 ms, and the relative refractory period lasts approximately 50 ms, for a total of 250 ms. This extended period is critical, since the heart muscle must contract to pump blood effectively and the contraction must follow the electrical events. Without extended refractory periods, premature contractions would occur in the heart and would not be compatible with life.[1]

text: Calcium ions[edit]

text: Calcium ions play two critical roles in the physiology of cardiac muscle. Their influx through slow calcium channels accounts for the prolonged plateau phase and absolute refractory period. Calcium ions also combine with the regulatory protein troponin in the troponin complex. Both roles enabling the myocardium to function properly.[1]

text: Approximately 20 percent of the calcium required for contraction is supplied by the influx of Ca2+ during the plateau phase. The remaining Ca2+ for contraction is released from storage in the sarcoplasmic reticulum.[1]

text: Comparative rates of conduction system firing[edit]

text: The pattern of prepotential or spontaneous depolarization, followed by rapid depolarization and repolarization just described, are seen in the SA node and a few other conductive cells in the heart. Since the SA node is the pacemaker, it reaches threshold faster than any other component of the conduction system. It will initiate the impulses spreading to the other conducting cells. The SA node, without nervous or endocrine control, would initiate a heart impulse approximately 80–100 times per minute. Although each component of the conduction system is capable of generating its own impulse, the rate progressively slows from the SA node to the Purkinje fibers. Without the SA node, the AV node would generate a heart rate of 40–60 beats per minute. If the AV node were blocked, the atrioventricular bundle would fire at a rate of approximately 30–40 impulses per minute. The bundle branches would have an inherent rate of 20–30 impulses per minute, and the Purkinje fibers would fire at 15–20 impulses per minute. While a few exceptionally trained aerobic athletes demonstrate resting heart rates in the range of 30–40 beats per minute (the lowest recorded figure is 28 beats per minute for Miguel Indurain, a cyclist)–for most individuals, rates lower than 50 beats per minute would indicate a condition called bradycardia. Depending upon the specific individual, as rates fall much below this level, the heart would be unable to maintain adequate flow of blood to vital tissues, initially resulting in decreasing loss of function across the systems, unconsciousness, and ultimately death.[1]

text: Cardiac cycle[edit]

text: The period of time that begins with contraction of the atria and ends with ventricular relaxation is known as the cardiac cycle. The period of contraction that the heart undergoes while it pumps blood into circulation is called systole. The period of relaxation that occurs as the chambers fill with blood is called diastole. Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the body.[1]

text: Pressures and flow[edit]

text: Fluids, move from regions of high pressure to regions of lower pressure. Accordingly, when the heart chambers are relaxed (diastole), blood will flow into the atria from the higher pressure of the veins. As blood flows into the atria, the pressure will rise, so the blood will initially move passively from the atria into the ventricles. When the action potential triggers the muscles in the atria to contract (atrial systole), the pressure within the atria rises further, pumping blood into the ventricles. During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle.[1]

text: Phases of the cardiac cycle[edit]

text: At the beginning of the cardiac cycle, both the atria and ventricles are relaxed (diastole). Blood is flowing into the right atrium from the superior and inferior venae cavae and the coronary sinus. Blood flows into the left atrium from the four pulmonary veins. The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles. Approximately 70–80 percent of ventricular filling occurs by this method. The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left.[1]

text: Atrial systole and diastole[edit]

text: Contraction of the atria follows depolarization, represented by the P wave of the ECG. As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves. At the start of atrial systole, the ventricles are normally filled with approximately 70–80 percent of their capacity due to inflow during diastole. Atrial contraction, also referred to as the "atrial kick," contributes the remaining 20–30 percent of filling. Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.[1]

text: Ventricular systole[edit]

text: Ventricular systole follows the depolarization of the ventricles and is represented by the QRS complex in the ECG. It may be conveniently divided into two phases, lasting a total of 270 ms. At the end of atrial systole and just prior to atrial contraction, the ventricles contain approximately 130 mL blood in a resting adult in a standing position. This volume is known as the end diastolic volume (EDV) or preload.[1]

text: Initially, as the muscles in the ventricle contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar (pulmonary and aortic) valves and be ejected from the heart. However, blood pressure quickly rises above that of the atria that are now relaxed and in diastole. This increase in pressure causes blood to flow back toward the atria, closing the tricuspid and mitral valves. Since blood is not being ejected from the ventricles at this early stage, the volume of blood within the chamber remains constant. Consequently, this initial phase of ventricular systole is known as isovolumic contraction, also called isovolumetric contraction.[1]

text: In the second phase of ventricular systole, the ventricular ejection phase, the contraction of the ventricular muscle has raised the pressure within the ventricle to the point that it is greater than the pressures in the pulmonary trunk and the aorta. Blood is pumped from the heart, pushing open the pulmonary and aortic semilunar valves. Pressure generated by the left ventricle will be appreciably greater than the pressure generated by the right ventricle, since the existing pressure in the aorta will be so much higher. Nevertheless, both ventricles pump the same amount of blood. This quantity is referred to as stroke volume. Stroke volume will normally be in the range of 70–80 mL. Since ventricular systole began with an EDV of approximately 130 mL of blood, this means that there is still 50–60 mL of blood remaining in the ventricle following contraction. This volume of blood is known as the end systolic volume (ESV).[1]

text: Ventricular diastole[edit]

text: Ventricular relaxation, or diastole, follows repolarization of the ventricles and is represented by the T wave of the ECG. It too is divided into two distinct phases and lasts approximately 430 ms.[1]

text: During the early phase of ventricular diastole, as the ventricular muscle relaxes, pressure on the remaining blood within the ventricle begins to fall. When pressure within the ventricles drops below pressure in both the pulmonary trunk and aorta, blood flows back toward the heart, producing the dicrotic notch (small dip) seen in blood pressure tracings. The semilunar valves close to prevent backflow into the heart. Since the atrioventricular valves remain closed at this point, there is no change in the volume of blood in the ventricle, so the early phase of ventricular diastole is called the isovolumic ventricular relaxation phase, also called isovolumetric ventricular relaxation phase.[1]

text: In the second phase of ventricular diastole, called late ventricular diastole, as the ventricular muscle relaxes, pressure on the blood within the ventricles drops even further. Eventually, it drops below the pressure in the atria. When this occurs, blood flows from the atria into the ventricles, pushing open the tricuspid and mitral valves. As pressure drops within the ventricles, blood flows from the major veins into the relaxed atria and from there into the ventricles. Both chambers are in diastole, the atrioventricular valves are open, and the semilunar valves remain closed. The cardiac cycle is complete.[1]

text: Heart sounds[edit]

text: One of the simplest methods of assessing the heart's condition is to listen to it using a stethoscope.[1] In a healthy heart, there are only two audible heart sounds, called S1 and S2. The first heart sound S1, is the sound created by the closing of the atrioventricular valves during ventricular contraction and is normally described as "lub". The second heart sound, S2, is the sound of the semilunar valves closing during ventricular diastole and is described as "dub".[1] Each sound consists of two components, reflecting the slight difference in time as the two valves close.[9] S2 may split into two distinct sounds, either as a result of inspiration or different valvular or cardiac problems.[9] Additional heart sounds may also be present and these give rise to gallop rhythms. A third heart sound, S3 usually indicates an increase in ventricular blood volume. A fourth heart sound S4 is referred to as an atrial gallop and is produced by the sound of blood being forced into a stiff ventricle. The combined presence of S3 and S4 give a quadruple gallop.[1]

text: Heart murmurs are abnormal heart sounds which can be either pathological or benign and there are numerous kinds.[10] Murmurs are graded by volume, from 1) the quietest, to 6) the loudest, and evaluated by their relationship to the heart sounds and position in the cardiac cycle.[9] Phonocardiograms can record these sounds.[1] Murmurs can result from narrowing (stenosis), regurgitation or insufficiency of any of the main heart valves but they can also result from a number of other disorders, including atrial and ventricular septal defects.[9] One example of a murmur is Still's murmur, which presents a musical sound in children, has no symptoms and disappears in adolescence.[11]

text: A different type of sound, a pericardial friction rub can be heard in cases of pericarditis where the inflamed membranes can rub together.[12]

text: Heart rate[edit]

text: The resting heart rate of a newborn can be 120 beats per minute (bpm) and this gradually decreases until maturity and then gradually increases again with age. The adult resting heart rate ranges from 60-100 bpm. Exercise and fitness levels, age and basal metabolic rate can all affect the heart rate. An athlete's heart rate can be lower than 60bpm. During exercise the rate can be 150bpm with maximum rates reaching from 200 and 220 bpm.[1]

text: Cardiovascular centres[edit]

text: The normal sinus rhythm of the heart rate is generated by the SA node. It is also influenced by central factors through sympathetic and parasympathetic nerves[3]:116–22 of the two paired cardiovascular centres of the medulla oblongata. Activity is increased via sympathetic stimulation of the cardioaccelerator nerves, and inhibited via parasympathetic stimulation by the vagus nerve. During rest vagal stimulation normally predominates as, left unregulated, the SA node would initiate a sinus rhythm of approximately 100 bpm.[1]

text: Both sympathetic and parasympathetic stimuli flow through the paired cardiac plexus near the base of the heart. Without any nervous stimulation, the SA node would establish a sinus rhythm of approximately 100 bpm. Since resting rates are considerably less than this, it becomes evident that parasympathetic stimulation normally slows HR.[1] The cardioaccelerator center also sends additional fibers, forming the cardiac nerves via sympathetic ganglia (the cervical ganglia plus superior thoracic ganglia T1–T4) to both the SA and AV nodes, plus additional fibers 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. This shortens the repolarization period, thus speeding the rate of depolarization and contraction, which results in an increased heartrate. It opens chemical or ligand-gated sodium and calcium ion channels, allowing an influx of positively charged ions.[1] Norepinephrine binds to the beta–1 receptor. High blood pressure medications are used to block these receptors and so reduce the heart rate.[1]

text: The cardiovascular centres receive input from a series of visceral receptors with impulses traveling through visceral sensory fibers within the vagus and sympathetic nerves via the cardiac plexus. Among these receptors are various proprioreceptors, baroreceptors, and chemoreceptors, plus stimuli from the limbic system which normally enable the precise regulation of heart function, via cardiac reflexes. Increased physical activity results in increased rates of firing by various proprioreceptors located in muscles, joint capsules, and tendons. The cardiovascular centres monitor these increased rates of firing, suppressing parasympathetic stimulation or increasing sympathetic stimulation as needed in order to increase blood flow.[1]

text: Similarly, baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself. Rates of firing from the baroreceptors represent blood pressure, level of physical activity, and the relative distribution of blood. The cardiac centers monitor baroreceptor firing to maintain cardiac homeostasis, a mechanism called the baroreceptor reflex. With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation. As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation.[1]

text: There is a similar reflex, called the atrial reflex or Bainbridge reflex, associated with varying rates of blood flow to the atria. Increased venous return stretches the walls of the atria where specialized baroreceptors are located. However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase HR. The opposite is also true.[1]

text: Factors influencing heart rate[edit]

text: In addition to the autonomic nervous system, other factors can affect this. These include epinephrine, norepinephrine, and thyroid hormones; levels of various ions including calcium, potassium, and sodium; body temperature; hypoxia; and pH balance .[1]

text:
Table 1: Major factors increasing heart rate and force of contraction[1]

Factor
Effect

Cardioaccelerator nerves
Release of norepinephrine

Proprioreceptors
Increased rates of firing during exercise

Chemoreceptors
Decreased levels of O2; increased levels of H+, CO2, and lactic acid

Baroreceptors
Decreased rates of firing, indicating falling blood volume/pressure

Limbic system
Anticipation of physical exercise or strong emotions

Catecholamines
Increased epinephrine and norepinephrine

Thyroid hormones
Increased T3 and T4

Calcium
Increased Ca2+

Potassium
Decreased K+

Sodium
Decreased Na+

Body temperature
Increased body temperature

Nicotine and caffeine
Stimulants, increasing heart rate

Table 2: Factors decreasing heart rate and force of contraction[1]

Factor
Effect

Cardioinhibitor nerves (vagus)
Release of acetylcholine

Proprioreceptors
Decreased rates of firing following exercise

Chemoreceptors
Increased levels of O2; decreased levels of H+ and CO2

Baroreceptors
Increased rates of firing, indicating higher blood volume/pressure

Limbic system
Anticipation of relaxation

Catecholamines
Decreased epinephrine and norepinephrine

Thyroid hormones
Decreased T3 and T4

Calcium
Decreased Ca2+

Potassium
Increased K+

Sodium
Decreased Na+

Body temperature
Decrease in body temperature

tag: table

text: Factors that increase heart rate also trigger an increase in stroke volume. As with skeletal muscles the heart can increase in size and efficiency with exercise.[1] Thus endurance athletes such as marathon runners may have a heart that has hypertrophied by up to 40%.[3]:1063–64 The difference between maximum and minimum cardiac outputs is known as the cardiac reserve and this measures the residual capacity to pump blood.[1] Heart rates may reach up to 185–195 in exercise, depending on how fit a person is.[3]

text: Cardiac output[edit]

text: Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle (stroke volume, SV) in one minute. To calculate this, multiply stroke volume (SV), by heart rate (HR), in beats per minute.[1] It can be represented by the equation: CO = HR x SV[1]

text: SV is normally measured using an echocardiogram to record end diastolic volume (EDV) and end systolic volume (ESV), and calculating the difference: SV = EDV – ESV. SV can also be measured using a specialized catheter, but this is an invasive procedure and far more dangerous to the patient. A mean SV for a resting 70-kg (150-lb) individual would be approximately 70 mL. There are several important variables, including size of the heart, physical and mental condition of the individual, sex, contractility, duration of contraction, preload or EDV, and afterload or resistance. Normal range for SV would be 55–100 mL. An average resting HR would be approximately 75 bpm but could range from 60–100 in some individuals.[1] Using these numbers, (which refer to each ventricle, not both) the mean CO is 5.25 L/min, with a range of 4.0–8.0 L/min.[1]

text: SVs are also used to calculate ejection fraction, which is the portion of the blood that is pumped or ejected from the heart with each contraction. To calculate ejection fraction, SV is divided by EDV. Despite the name, the ejection fraction is normally expressed as a percentage. Ejection fractions range from approximately 55–70 percent, with a mean of 58 percent.[1]

text: Stroke volume[edit]

text: Many of the factors that regulate the heart rate also affect cardiac function by altering the stroke volume. While a number of variables are involved, stroke volume is dependent upon the difference between end diastolic volume and end systolic volume. The three primary factors involved are preload, afterload and contractility.[1]

text: Preload[edit]

text: Preload is another way of expressing EDV. Therefore, the greater the EDV, the greater the preload. A main factor is ventricular filling time. The faster the contractions are, the shorter the filling time and both the EDV and preload are lower.[1]

text: The relationship between ventricular stretch and contraction has been stated in the Frank-Starling mechanism which says that the force of contraction is directly proportional to the initial length of muscle fibre. So that the greater the stretch of the ventricle the greater the contraction. Any sympathetic stimulation to the venous system will increase venous return to the heart and ventricular filling.[1]

text: Afterload[edit]

text: The ventricles must develop a certain tension to pump blood against the resistance of the vascular system. This tension is called afterload. When the resistance is increased particularly due to stenotic valve damage the afterload must necessarily increase. A decrease in normal vascular resistance can also occur. Different cardiac responses operate to restore homeostasis of the pressure and blood flow.[1]

text: Contractility[edit]

text: The ability of the myocardium to contract, (its contractility), controls the stroke volume which determines the end systolic volume. The greater the contraction the greater the stroke volume and the smaller the end systolic volume. Positive or negative inotropic factors via sympathetic and parasympathetic stimulation respectively, can increase or decrease the force of contractions. Sympathetic stimulation triggers the release of norepinephrine from the cardiac nerves and also stimulates the adrenal cortex to secrete both epinephrine and norepinephrine. These secretions increase the heart rate, subsequent metabolic rate and contractility. Parasympathetic stimulation stimulates the release of acetylcholine (ACh) from the vagus nerve which decreases contractility, and stroke volume which increases end systolic volume.

text: Several synthetic drugs have been developed that can act either as a stimulant or inhibitor inotrope. The stimulant inotropes, such as Digoxin, cause higher concentrations of calcium ions which increase contractility. Excess calcium (hypercalcemia) is also a positive inotrope. Drugs that are negative inotropes include beta blockers and calcium channel blockers. Hypoxia, acidosis, hyperkalemia are also negative inotropic agents.

text:
Table 3: Cardiac response to decreasing blood flow and pressure due to decreasing cardiac output[1]

Baroreceptors (aorta, carotid arteries, venae cavae, and atria)
Chemoreceptors (both central nervous system and in proximity to baroreceptors)

Sensitive to
Decreasing stretch[1]
Decreasing O2 and increasing CO2, H+, and lactic acid[1]

Target
Parasympathetic stimulation suppressed[1]
Sympathetic stimulation increased[1]

Response of heart
Increasing heart rate and increasing stroke volume[1]
Increasing heart rate and increasing stroke volume[1]

Overall effect
Increasing blood flow and pressure due to increasing cardiac output; hemostasis restored[1]
Increasing blood flow and pressure due to increasing cardiac output; hemostasis restored[1]

Table 4: Cardiac response to increasing blood flow and pressure due to increasing cardiac output[1]

Baroreceptors (aorta, carotid arteries, venae cavae, and atria)
Chemoreceptors (both central nervous system and in proximity to baroreceptors)

Sensitive to
Increasing stretch[1]
Increasing O2 and decreasing CO2, H+, and lactic acid[1]

Target
Parasympathetic stimulation increased[1]
Sympathetic stimulation suppressed[1]

Response of heart
Decreasing heart rate and decreasing stroke volume[1]
Decreasing heart rate and decreasing stroke volume[1]

Overall effect
Decreasing blood flow and pressure due to decreasing cardiac output; hemostasis restored[1]
Decreasing blood flow and pressure due to decreasing cardiac output; hemostasis restored[1]


document_title: Oceanic crust

question_text: where is the youngest rock in the earths crust found

text: Oceanic crust

text: Composition[edit]

text: Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of ophiolites (sections of oceanic crust that are thrust onto and preserved on the continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles, dredging (especially from ridge crests and fracture zones) and drilling.[5] Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers. According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle.[6]

text:
Layer 1 is on an average 0.4 km thick. It consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge.[7] Near the continental margins sediment is terrigenous, meaning derived from the land, unlike deep sea sediments which are made of tiny shells of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous sediments transported by turbidity currents.[8]
Layer 2 could be divided into two parts: layer 2A – 0.5 km thick uppermost volcanic layer of glassy to finely crystalline basalt usually in the form of pillow basalt, and layer 2B – 1.5 km thick layer composed of diabase dikes.[9]
Layer 3 is formed by slow cooling of magma beneath the surface and consists of coarse grained gabbros and cumulate ultramafic rocks. It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness.[10]

tag: ul

text: Geochemistry[edit]

text: The most voluminous volcanic rocks of the ocean floor are the mid-oceanic ridge basalts, which are derived from low-potassium tholeiitic magmas. These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements. There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge hot spots such as surroundings of Galapagos Islands, the Azores and Iceland.[11]

text: Life cycle[edit]

text: Oceanic crust is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it. The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges.[12]

text: As the mantle rises it cools and melts, as the pressure decreases and it crosses the solidus. The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr−1 half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at lesser depth, thereby producing less melt and thinner crust. An example of this is the Gakkel Ridge under the Arctic Ocean. Thicker than average crust is found above plumes as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust. An example of this is Iceland which has crust of thickness ~20 km.[13]

text: The age of the oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it.[14] The oceanic lithosphere subducts at what are known as convergent boundaries. These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another. In the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old.[15] The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson cycle.

text: The oldest large scale oceanic crust is in the west Pacific and north-west Atlantic - both are about up to 180-200 million years old. However, parts of the eastern Mediterranean Sea are remnants of the much older Tethys ocean, at about 270 and up to 340 million years old.[16][17][18]

text: Magnetic anomalies[edit]

text: The oceanic crust displays a pattern of magnetic lines, parallel to the ocean ridges, frozen in the basalt. A symmetrical pattern of positive and negative magnetic lines emanates from the mid-ocean ridge.[19] New rock is formed by magma at the mid-ocean ridges, and the ocean floor spreads out from this point. When the magma cools to form rock, its magnetic polarity is aligned with the then-current positions of the magnetic poles of the Earth. New magma then forces the older cooled magma away from the ridge. This process results in parallel sections of oceanic crust of alternating magnetic polarity.