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Human heart
External view
Surface view of the heart
Latin	cor humanum
Gray's	subject #135 506
System	Circulatory
Artery	Right coronary artery, left coronary artery, anterior interventricular artery
Vein	Superior vena cava, inferior vena cava, right pulmonary veins, left pulmonary veins
MeSH	Heart
Dorlands/Elsevier	Heart

The human heart is a muscular organ that provides a continuous blood circulation through the cardiac cycle and is one of the most vital organs in the human body.^[1] The heart is divided into four main chambers: the two upper chambers are called the left and right Atrium and two lower chambers are called the right and left Ventricle. There is a thick wall of muscle separating the right side and the left side of the heart called the septum. Normally with each beat the right ventricle pumps the same amount of blood into the lungs that the left ventricle pumps out into the body. Physicians commonly refer to the right atrium and right ventricle together as the right heart and to the left atrium and ventricle as the left heart.^[2]

The electric energy that stimulates the heart occurs in the sinoatrial node, which produces a definite potential and then discharges, sending an impulse across the atria. In the atria the electrical signal moves from cell to cell^[3] while in the ventricles the signal is carried by specialized tissue called the Purkinje fibers^[4] which then transmit the electric charge to the myocardium.

At 21 days after conception, the human heart begins beating at 70 to 80 beats per minute and accelerates linearly for the first month of beating.

The human embryonic 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). 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 by approximately 100 BPM during the first month to peak at 165-185 BPM during the early 7th week afer conception, (early 9th week after the LMP). This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, which is an increase of 100 BPM in the first month.^[5][6][7] The regression formula, which describes this acceleration before the embryo reaches 25 mm in crown-rump length, or 9.2 LMP weeks, is: the Age in days = EHR(0.3)+6. .^[8]

After 9.1 weeks after the LMP, it decelerates to about 152 BPM (+/-25 BPM) during the 15th week post LMP. After the 15th week, the deceleration slows to an average rate of about 145 (+/-25 BPM) BPM, at term. There is no difference in female and male heart rates before birth. The human heart and its disorders (cardiopathies) are studied primarily by cardiology.

Real-time MRI of the human heart

Human-heart

Real-time MRI - Human Heart (short-axis view)

Structure[link]

Heart

anterior view, coronal section.

The blood supply of the heart is provided by the coronary circulation. Coronary arteries labeled in red text and other landmarks in blue text.

The human heart has a mass of between 250 and 350 grams and is about the size of a fist.^[9]

It is enclosed in a double-walled protective sac called the pericardium.^[10]The double membrane of pericardium consist of the pericardial fluid which nourishes the heart and prevents shocks. The superficial part of this sac is called the fibrous pericardium. The fibrous pericardial sac is itself lined with the outer layer of the serous pericardium (known as the parietal pericardium). This composite (fibrous-parietal-pericardial) sac protects the heart, anchors its surrounding structures, and prevents overfilling of the heart with blood. The inner layer also provides a smooth lubricated sliding surface within which the heart organ can move in response to its own contractions and to movement of adjacent structures such as the diaphragm and lungs.

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 (serous) pericardium. The middle layer of the heart is called the myocardium and is composed of muscle which contracts. The inner layer is called the endocardium 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.^[11]

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 pathways of blood through the human heart is part of the pulmonary and systemic circuits. These pathways include the tricuspid valve, the mitral valve, the aortic valve, and the pulmonary valve.^[12] The mitral and tricuspid valves are classified as the atrioventricular (AV) valves. This is because they are found between the atria and ventricles. The aortic and pulmonary semi-lunar valves separate the left and right ventricle from the pulmonary artery and the aorta respectively. These valves are attached to the chordae tendinae (literally the heartstrings), which anchors the valves to the papilla muscles of the heart.

The interatrioventricular septum separates the left atrium and ventricle from the right atrium and ventricle, dividing the heart into two functionally separate and anatomically distinct units.

Functioning[link]

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

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. Blood is prevented from flowing backwards by the tricuspid, bicuspid, aortic, and pulmonary valves.

The heart acts as a double pump. 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, 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 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. 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.^[13]

Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle. Here, it is pumped out of the pulmonary semilunar valve and travels through the pulmonary artery to the lungs. From there, 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 and to the rest of the body. The (relatively) deoxygenated blood finally returns to the heart through the inferior vena cava and superior vena cava, and enters the right atrium where the process began.

Lifestyle and heart health[link]

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

It is generally accepted that factors such as exercise, diet, and overall well-being, including both emotional and physiological components, affect heart health in humans.^{[14][15][16][17]}

See also[link]

• Electrocardiography
• Heart rate
• Blood pressure

References[link]

External links[link]

• Prenatal human heart development
• Anatomy of the Human Heart - Texas Heart Institute
• The Gross Physiology of the Cardiovascular System (2nd Ed., 2012)
• Dissection review showing vessels of the heart as well as internal and external features
• The Human Heart: An Online Exploration from The Franklin Institute
• Blood Flow Through the Human Heart

John Reed
Reed in 2007
Born	(1969-02-07)February 7, 1969 TriBeCa, New York City
Nationality	American
Alma mater	Columbia University
Occupation	novelist
Website
http://www.johnreed.org

John Reed (born February 7, 1969) is an American novelist. He is the author of four novels: A Still Small Voice (2000), Snowball's Chance (2002) with a preface by Alexander Cockburn, The Whole (2005), and All the World's a Grave: A New Play by William Shakespeare (2008). His fifth book, Tales of Woe (2010), is a collection of twenty-five stories, chronicling true stories of abject misery.

Biography[link]

Born in 1969 in New York City, Reed is the son of artist David Reed and Judy Rifka.^[1] He attended Hampshire College, and received a Masters in Fine Art in Creative Writing from Columbia University.^[2] He teaches at The New School.

Reed was an early contributor to, and subsequently an editor with, Open City, a New York literary journal published by Robert Bingham, who later founded the book series.

Works[link]

He is affiliated with the New York Press and The Brooklyn Rail. "Americans are extremely sophisticated in terms of narrative forms," said Reed in an interview. "We see it in commercials, we see it on TV, we see it in movies. But the narrative forms we're talking about are three acts, five acts, depending on how you want to look at it. They're all based on a Christian model of sin, suffering, redemption; which is not a large model."^[3]

A Still Small Voice[link]

A Still Small Voice (Delacorte 2000, Delta 2001), Reed’s first novel, is a historical novel based on the life of a girl growing up in Kentucky from 1850-1870.

Snowball's Chance[link]

Snowball's Chance (Roof Books 2002/2003), Reed’s second novel was a controversial send-up of George Orwell’s Animal Farm, and ended in a cataclysmic attack on the “Twin Mills” (reminiscent of the 9/11 attack on the World Trade Center). It became a bestseller in the field of books by independent literary publishers.^[4]

The Whole, or, Duh Whole[link]

Wikinews has related news: John Reed on Orwell, God, self-destruction and the future of writing

John Reed discusses politics after 9/11 and Reed's take on Orwell's neoconservativism.

Reed between BOMB magazine's senior editor Monica de la Torre and editor-in-chief Betsy Sussler.

The Whole, Reed’s third novel, parodied MTV and was released in 2005 by MTV Books (Simon & Schuster). The novel described a gigantic hole that appears in the middle of the country, which engulfs four states.

All the World's a Grave[link]

Reed, in a new work, returns to the overhaul of canonical English writers in All the World's a Grave, fall 2008, Penguin Books. The work, subtitled "A New Play by William Shakespeare", is a tragedy in five acts, a "mash-up" constructed of lines drawn from five Shakespeare tragedies and one Shakespeare history; Hamlet, King Lear, Macbeth, Othello, Romeo & Juliet and Henry V. In the "literary trick" (as described by Page Six of The New York Post) Shakespeare's lines are rearranged into a wholly new story, in which Prince Hamlet of Denmark goes to war to claim his bride and the daughter of King Lear, Juliet. Upon a triumphant return home, Hamlet discovers that his mother has murdered his father, and married Macbeth. Visited by his father's ghost, and goaded by the opportunistic Lieutenant Iago, Hamlet is driven mad by the erroneous belief that Juliet is having an affair with General Romeo.^[5]

References[link]

Wikiquote has a collection of quotations related to: John Reed (novelist)

External links[link]

• John Reed's website
• All the World's a Grave, book website.
• Kill the beavers!, The Daily Telegraph, November 22, 2002.
• John Reed: Saint George and the Damn Truth, Moby Lives, Nove,ber 10, 2003.
• John Reed: The Whole, an excerpt, Brooklyn Rail, January 2005.
• Dinitia Smith: A Pig Returns to the Farm, Thumbing His Snout at Orwell, The New York Times, November 25, 2002.
• Cathy Young: Blaming the Victim, Reason.com, December 3, 2002.
• Andrea Scrima: Review of Tales of Woe, Rumpus, November 16, 2010.
• Interview with Arthur Phillips on rewriting Shakespeare