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The atrium (; : atria) is one of the two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular mitral valve and tricuspid valve .
There are two atria in the human heart – the left atrium receives blood from the pulmonary circulation, and the right atrium receives blood from the venae cavae of the systemic circulation. During the cardiac cycle, the atria receive blood while relaxed in diastole, then contract in systole to move blood to the ventricles. Each atrium is roughly cube-shaped except for an ear-shaped projection called an atrial appendage, previously known as an auricle. All animals with a closed circulatory system have at least one atrium.
The atrium was formerly called the 'auricle'. That term is still used to describe this chamber in some other animals, such as the Mollusca. Auricles in this modern terminology are distinguished by having thicker muscular walls.
The right atrium and ventricle are often referred to together as the right heart, and the left atrium and ventricle as the left heart. As the atria do not have valves at their inlets, a venous pulsation is normal, and can be detected in the jugular vein as the jugular venous pressure. Internally, there are the rough pectinate muscles, and the crista terminalis 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 venarum is the adult remnant of the sinus venosus and it surrounds the openings of the venae cavae and the coronary sinus. Attached to each atrium is an 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. It can also be seen clearly using transesophageal echocardiography. The left atrial appendage can serve as an approach for mitral valve surgery. The body of the left atrial appendage is anterior to the left atrium and parallel to the left . The left pulmonary artery passes posterosuperiorly and is separated from the atrial appendage by the transverse sinus. In atrial fibrillation, the left atrial appendage fibrillates rather than contracts resulting in blood stasis that predisposes to the formation of thrombus. Because of consequent stroke risk, surgeons may choose to close it during open-heart surgery, using a left atrial appendage occlusion procedure.
The atrioventricular node (AV node) is another node in the cardiac conduction system. This is located between the atria and the ventricles.
The oblique vein of the left atrium is partly responsible for venous drainage; it derives from the embryonic left superior vena cava.
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. It is mostly unproblematic, although it can be associated with paradoxical embolization 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.
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 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. Anderson, RM. The Gross Physiology of the Cardiovascular System (2nd ed.) See "Chapter 1: Normal Physiology."
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. These are low-pressure in the atria, which send signals to the hypothalamus when a drop in atrial pressure (which indicates a drop in blood volume) is detected. This triggers a release of vasopressin.
In those with uncontrollable atrial fibrillation, left atrial appendage occlusion may be performed at the time of any open-heart surgery to prevent future clot formation within the appendage.
In most fish, the circulatory system is very simple: a two-chambered heart including one atrium and one ventricle. Among sharks, the heart consists of four parts arranged serially: blood flows into the most posterior part, the sinus venosus, and then to the atrium which moves it to the third part, 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 , the oxygenated and deoxygenated blood is mixed in the ventricle before being pumped out to the body's organs; in , 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.
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