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The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the ventricles of the
heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste ...

heart
. There are two atria in the human heart – the left atrium receives blood from the pulmonary (
lung The lungs are the primary organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma ...

lung
) circulation, and the right atrium receives blood from the
venae cavae The venae cavae (; from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power ...
(
venous circulation
venous circulation
). The atria receive blood while relaxed (
diastole Image:Heart diastole.png, upright=1.5, Heart performance during ventricular diastole: early diastole is a suction mechanism that draws blood 'down' from the left atrium (pink) and right atrium (blue) into each of the two ventricles. Then, in late ve ...
), then contract (
systole The systole ( ) is the part of the during which some chambers of the contract after refilling with blood. The term originates, via , from (''sustolē''), from (''sustéllein'' 'to contract'; from ''sun'' 'together' + ''stéllein'' 'to se ...
) to move blood to the ventricles. All animals with a
closed circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a group of organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many o ...
have at least one atrium. Humans have two atria. The atrium was formerly called the "auricle"; that term is still used to describe this chamber in some other animals, such as the ''
Mollusca Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...

Mollusca
''.


Structure

Human Humans (''Homo sapiens'') are the most abundant and widespread species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...

Human
s have a four-chambered heart consisting of the right atrium, left atrium, right ventricle, and left ventricle. The atria are the two upper chambers. The right atrium receives and holds deoxygenated blood from the
superior vena cava The superior vena cava (SVC) is the anatomical terms of location#Superior and inferior, superior of the two venae cavae, the great vein, venous trunks that return deoxygenated blood from the circulatory system, systemic circulation to the atrium ...
,
inferior vena cava The inferior vena cava is a large vein that carries the deoxygenated blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance th ...

inferior vena cava
,
anterior cardiac veins The anterior cardiac veins (or anterior veins of right ventricle) comprise a variable number of small vessels, usually between two and five, which collect blood from the front of the right ventricle A ventricle is one of two large chambers toward ...
and
smallest cardiac veins The smallest cardiac veins, also known as the Thebesian veins, are small valveless vein Veins are blood vessels in humans, and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the h ...
and the
coronary sinus The coronary sinus is a collection of veins joined together to form a large vessel that collects blood from the heart muscle (myocardium). It delivers less-oxygenated blood to the right atrium, as do the superior vena cava, superior and inferior v ...
, which it then sends down to the right ventricle (through the
tricuspid valve The tricuspid valve, or right atrioventricular valve, is on the right dorsal side of the mammalian heart, at the superior portion of the right ventricle A ventricle is one of two large chambers toward the bottom of the heart The heart i ...
), which in turn sends it to the
pulmonary artery A pulmonary artery is an artery An artery (plural arteries) () is a blood vessel that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are t ...

pulmonary artery
for
pulmonary circulation The mammalian heart is divided between the systemic and the pulmonary circulation, generally agreed upon as left and right sided circuits. The right circuit is the portion of the circulatory system The circulatory system, also called the ca ...
. The left atrium receives the oxygenated blood from the left and right
pulmonary veins The pulmonary veins are the vein Veins are blood vessels that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary vein, pulmonary and umbilical veins, both of which c ...
, which it pumps to the left ventricle (through the
mitral valve The mitral valve (), also known as the bicuspid valve or left atrioventricular valve, is one of the four heart valve A heart valve is a that allows in one direction through the chambers of the . Four valves are usually present in a mammalian h ...

mitral valve
) for pumping out through the
aorta The aorta ( ) is the main and largest in the , originating from the of the and extending down to the , where it into two smaller arteries (the ). The aorta distributes blood to all parts of the body through the . Structure Sections In a ...

aorta
for
systemic circulation The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a group of organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many o ...
. The right atrium and right ventricle are often referred to as the ''right heart''; similarly, the left atrium and left ventricle are often referred to as the ''left heart''. The atria do not have valves at their inlets, and as a result, a venous pulsation is normal and can be detected in the
jugular vein The jugular veins are vein Veins are blood vessels that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary vein, pulmonary and umbilical veins, both of which carry ...
as the
jugular venous pressure and marked jugular venous distention. External jugular vein marked by an arrow; however, JVP is not measured by looking at the external jugular vein even but is instead measured by pulsations of the skin from the internal jugular vein, which is not ...
. Internally, there are the rough
pectinate muscles The pectinate muscles (musculi pectinati) are parallel muscular ridges in the walls of the atriaAtria may refer to: *Atrium (heart) The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the Ventricle (heart) ...
and
crista terminalis The crista terminalis represents the junction between the sinus venosus and the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped ...
of His, which act as a boundary inside the atrium and the smooth-walled part of the right atrium, the ''sinus venarum'', which are derived from the
sinus venosus The sinus venosus is a large quadrangular cavity which precedes the atrium on the venous side of the chordate A chordate () is an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biol ...
. The sinus venarum is the adult remnant of the sinus venous and it surrounds the openings of the
venae cavae The venae cavae (; from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power ...
and the coronary sinus. Attached to the right atrium is the right atrial appendage – a pouch-like extension of the pectinate muscles. The
interatrial septum The interatrial septum is the wall of tissue that separates the right and left atria of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory system. ...
separates the right atrium from the left atrium; this is marked by a depression in the right atrium – the fossa ovalis. The atria are depolarised by
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
. High in the upper part of the left atrium is a muscular ear-shaped pouch – the left atrial appendage. This appears to "function as a decompression chamber during left ventricular systole and during other periods when left atrial pressure is high".


Conduction system

The sinoatrial (SA) node is located in the posterior aspect of the right atrium, next to the superior vena cava. This is a group of pacemaker cells which spontaneously depolarize to create an action potential. The cardiac action potential then spreads across both atria causing them to contract, forcing the blood they hold into their corresponding ventricles. The
atrioventricular node The atrioventricular node or AV node is a part of the electrical conduction system of the heart The electrical conduction system of the heart transmits signals generated usually by the sinoatrial node to cause contraction of the heart muscle. The ...
(AV node) is another node in the cardiac electrical conduction system. This is located between the atria and the ventricles.


Blood supply

The left atrium is supplied mainly by the left circumflex coronary artery, and its small branches. The
oblique vein of the left atrium The oblique vein of the left atrium (oblique vein of Marshall) is a small vessel which descends obliquely on the back of the left atrium and ends in the coronary sinus near its left extremity; it is continuous above with the ligament of the left ven ...
is partly responsible for venous drainage; it derives from the embryonic left superior vena cava.


Development

During
embryogenesis An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryogenesis
at about two weeks, a primitive atrium begins to be formed. It begins as one chamber, which over the following two weeks becomes divided by the
septum primum During heart development of a human embryo An embryo is the early stage of development of a multicellular organism. In general, in organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...
into the left atrium and the right atrium. The
interatrial septum The interatrial septum is the wall of tissue that separates the right and left atria of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory system. ...
has an opening in the right atrium, the foramen ovale, which provides access to the left atrium; this connects the two chambers, which is essential for fetal blood circulation. At birth, when the first breath is taken fetal blood flow is reversed to travel through the lungs. The foramen ovale is no longer needed and it closes to leave a depression (the fossa ovalis) in the atrial wall. In some cases, the foramen ovale fails to close. This abnormality is present in approximately 25% of the general population. This is known as a ''patent foramen ovale'', an
atrial septal defect Atrial septal defect (ASD) is a congenital heart defect A congenital heart defect (CHD), also known as a congenital heart anomaly and congenital heart disease, is a defect in the structure of the heart The heart is a cardiac muscle, mu ...

atrial septal defect
. It is mostly unproblematic, although it can be associated with paradoxical
embolization Embolization refers to the passage and lodging of an embolus within the circulatory system, bloodstream. It may be of natural origin (pathological), in which word sense, sense it is also called embolism, for example a pulmonary embolism; or it ...
and stroke. Within the fetal right atrium, blood from the inferior vena cava and the superior vena cava flow in separate streams to different locations in the heart; this has been reported to occur through the
Coandă effect The Coandă effect ( or ) is the tendency of a fluid jet to stay attached to a convex surface. It is named after Romanian inventor Henri Coandă, who described it as "the tendency of a jet of fluid emerging from an orifice to follow an adjace ...
.


Function

In human physiology, the atria facilitate circulation primarily by allowing uninterrupted venous flow to the heart during
ventricular systole and adjacent deflections. Re the cardiac cycle, ''atrial systole'' begins at the P wave; ''ventricular systole'' begins at the Q deflection of the QRS complex. The cardiac cycle is the performance of the human heart from the beginning of one he ...
. By being partially empty and distensible, atria prevent the interruption of to the heart that would occur during ventricular systole if the veins ended at the inlet valves of the heart. In normal physiologic states, the output of the heart is pulsatile, and the venous inflow to the heart is continuous and non-pulsatile. But without functioning atria, venous flow becomes pulsatile, and the overall circulation rate decreases significantly. Atria have four essential characteristics that cause them to promote continuous venous flow. (1) There are no atrial inlet valves to interrupt blood flow during atrial systole. (2) The atrial systole contractions are incomplete and thus do not contract to the extent that would block flow from the veins through the atria into the ventricles. During atrial systole, blood not only empties from the atria to the ventricles, but blood continues to flow uninterrupted from the
vein Veins are blood vessels The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A b ...

vein
s right through the atria into the ventricles. (3) The atrial contractions must be gentle enough so that the force of contraction does not exert significant back pressure that would impede venous flow. (4) The "let go" of the atria must be timed so that they relax before the start of ventricular contraction, to be able to accept venous flow without interruption. By preventing the inertia of interrupted venous flow that would otherwise occur at each ventricular systole, atria allow approximately 75% more cardiac output than would otherwise occur. The fact that atrial contraction is 15% of the amount of the succeeding ventricular ejection has led to a misplaced emphasis on their role in pumping up the ventricles (the so-called "atrial kick"), whereas the key benefit of atria is in preventing circulatory inertia and allowing uninterrupted venous flow to the heart. Also of importance in maintaining the blood flow are the presence of
atrial volume receptors Atrial volume receptors (also known as Veno-atrial stretch receptors) are low pressure baroreceptorBaroreceptors (or archaically, pressoreceptors) are sensors located in the carotid sinus (at the bifurcation of external and internal carotids) and i ...
. These are low-pressure
baroreceptorBaroreceptors (or archaically, pressoreceptors) are sensors located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch. They sense the blood pressure and relay the information to the brain, so that a ...
s in the atria, which send signals to the
hypothalamus The hypothalamus (from Ancient Greek wikt:ὑπό, ὑπό, "under", and wikt:θάλαμος, θάλαμος, "chamber") is a portion of the brain that contains a number of small Nucleus (neuroanatomy), nuclei with a variety of functions. One of ...

hypothalamus
when a drop in atrial pressure (which indicates a drop in blood volume) is detected. This triggers a release of
vasopressin Vasopressin, also called antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin, is a hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ...

vasopressin
.


Left atrial appendage

The left atrial appendage can be seen on a standard posteroanterior x-ray, where the lower level of the left hilum becomes concave. The left atrial appendage can serve as an approach for mitral valve surgery.


Disorders


Atrial septal defect

In an adult, an
atrial septal defect Atrial septal defect (ASD) is a congenital heart defect A congenital heart defect (CHD), also known as a congenital heart anomaly and congenital heart disease, is a defect in the structure of the heart The heart is a cardiac muscle, mu ...

atrial septal defect
results in the flow of blood in the reverse direction – from the left atrium to the right – which reduces cardiac output, potentially causing
cardiac failure Heart failure (HF), also known as congestive heart failure (CHF), (congestive) cardiac failure (CCF), and decompensatio cordis, is when the heart is unable to pump sufficiently to maintain blood flow to meet the body tissues' needs for metabo ...
, and in severe or untreated cases
cardiac arrest Cardiac arrest is a sudden loss of blood flow Hemodynamics American and British English spelling differences#ae and oe, or haemodynamics are the Fluid dynamics, dynamics of blood flow. The circulatory system is controlled by homeostasis, homeost ...
and sudden death.


Left atrial appendage thrombosis

In patients with
atrial fibrillation Atrial fibrillation (AF or A-fib) is an abnormal heart rhythm (arrhythmia) characterized by the rapid and irregular beating of the atrial chambers of the heart. It often begins as short periods of abnormal beating, which become longer or cont ...

atrial fibrillation
, mitral valve disease, and other conditions,
blood clots A thrombus, colloquially called a blood clot, is the final product of the blood coagulation Coagulation, also known as clotting, is the process by which blood Blood is a body fluid in humans and other animals that delivers necessary sub ...
have a tendency to form in the left atrial appendage. The clots may dislodge (forming
emboli An embolism is the lodging of an embolus, a blockage-causing piece of material, inside a blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport bloo ...
), which may lead to
ischemic Ischemia or ischaemia is a restriction in blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressi ...
damage to the brain, kidneys, or other organs supplied by the systemic circulation. In those with uncontrollable atrial fibrillation, left atrial appendage excision may be performed at the time of any open heart surgery to prevent future clot formation within the appendage.


Functional abnormalities

* Wolff-Parkinson-White syndrome *
Atrial flutter Atrial flutter (AFL) is a common abnormal heart rhythm that starts in the atrial chambers of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory ...
*
Atrial tachycardia Atrial tachycardia is a type of heart rhythm problem in which the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries ...
*
Sinus tachycardia Sinus tachycardia (also colloquially known as sinus tach or sinus tachy) is an elevated sinus rhythm characterized by an increase in the rate of electrical impulses arising from the sinoatrial node. In adults, sinus tachycardia is defined as a hea ...
*
Multifocal atrial tachycardia Multifocal (or multiform) atrial tachycardia (MAT) is an abnormal heart rhythm, specifically a type of supraventricular tachycardia, that is particularly common in older people and is associated with exacerbations of chronic obstructive pulmonary ...
– several types *
Premature atrial contraction Premature atrial contractions (PACs), also known as atrial premature complexes (APC) or atrial premature beats (APB), are a common cardiac dysrhythmia Arrhythmia, also known as cardiac arrhythmia or heart arrhythmia, is a group of conditions i ...


Other animals

Many other animals, including mammals, also have four-chambered hearts, which have a similar function. Some animals (amphibians and reptiles) have a three-chambered heart, in which the blood from each atrium is mixed in the single ventricle before being pumped to the aorta. In these animals, the left atrium still serves the purpose of collecting blood from the pulmonary veins. In some fish, the circulatory system is very simple: a two-chambered heart including one atrium and one ventricle. Among sharks, the heart consists of four chambers arranged serially (and therefore called a serial heart): blood flows into the most posterior chamber, the sinus venosus, and then to the atrium which moves it to the third chamber, the ventricle, before it reaches the conus anteriosus, which itself is connected to the ventral aorta. This is considered a primitive arrangement, and many vertebrates have condensed the atrium with the sinus venosus and the ventricle with the conus anteriosus. With the advent of lungs came a partitioning of the atrium into two parts divided by a septum. Among frogs, the oxygenated and deoxygenated blood are mixed in the ventricle before being pumped out to the body's organs; in turtles, the ventricle is almost entirely divided by a septum, but retains an opening through which some mixing of blood occurs. In birds, mammals, and some other reptiles (alligators in particular) the partitioning of both chambers is complete.


See also

*
Syncytium A syncytium or symplasm (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleu ...

Syncytium


References

{{DEFAULTSORT:Atrium (Heart) Cardiac anatomy de:Herz#Räume und Gefäße