Another way to group the causes of left atrial enlargement is to consider causes that result in volume overload in the atrium (septal defects, patent ductus arteriosus, and mitral regurgitation) and those that result in an increase in the left atrial pressure (chronic hypertension, mitral stenosis, and left ventricular hypertrophy). Independent from the cause, the end result is stretching of the atria. The progressive dilatation has been marked as an independent risk factor for increased mortality and morbidity as this phenomenon is associated with both atrial fibrillation and thromboembolic events (discussed earlier), as well as pulmonary hypertension. Most of the anterior surface of the left atrium is concealed behind the roots of the emerging great vessels. Furthermore, part of the transverse pericardial sinus (the space between the superior vena cava [posteriorly] and the great trunks of the great arteries [anteriorly]) passes in front of the left atrium as well. The left atrium also has an auricular appendage; however, it is more slender than its right counterpart and is also curved distally as it partially overlaps the trunk of the pulmonary artery. Structures surrounding the left atrium Under normal circumstances, shortly after birth, the increase in systemic pressure relative to that of the pulmonary pressure results in an overall increase in the pressures in the left side of the heart. Since the left atrial pressure becomes greater than that in the right atrium, it forces the overlapping membranes together. By the third month of extrauterine life, the septum secundum and the valve of the foramen ovale would have fused, leaving only the fossa ovalis as a remnant. Burns, Edward. "Right Atrial Enlargement - ECG Library LITFL". LITFL • Life In The Fast Lane Medical Blog, 2017, https://lifeinthefastlane.com/ecg-library/basics/right-atrial-enlargement/. Accessed 25 Apr 2018. Characteristics - Right auricle; locations for sinuatrial and atrioventricular nodes; three internal surfaces (venous, vestibular, auricular)

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Tatco , Vincent. "Right Atrial Enlargement | Radiology Reference Article | Radiopaedia.Org". Radiopaedia.Org, https://radiopaedia.org/articles/right-atrial-enlargement. Accessed 25 Apr 2018. Based on the embryological origins of the right atrium, the internal surface can be subdivided into the venous, vestibular, and auricular surfaces. They can be macroscopically distinguished from each other based on the fact that the auricular part has a trabeculated appearance (due to the overlapping pectinate muscles), the venous part is smooth, and the vestibular part is rigid. While the vestibular and auricular surfaces are derivatives of the primordial atrium proper, the venous compartment is the remnant of the sinus venosus. The latter fuses with the right atrium, thus merging the vena caval ostia with the posterior wall of the right atrium. At the midpoint of the fourth gestational week, internal differentiation begins to take place, resulting in the formation of primordial atria and ventricles. Entities such as bone morphogenetic proteins 2A and 4 (BMP-2A & BMP-4), transforming growth factor beta one and two (TGF- β1 & TGF- β2), and other inductive agents promote the differentiation of cardiac jelly (a specialized type of extracellular matrix) into the endocardial cushions. These cushions appear on the ventral and dorsal walls of the atrioventricular canal during the fifth gestational week. As the heart continues to develop, the endocardial cushions are populated by mesenchyme. Consequently, the opposing endocardial cushions begin to abut, and eventually fuse with each other. This leads to the formation of left and right atrioventricular canals; with the endocardial cushions both acting as a valve (to limit regurgitant streams from the ventricles to the atria) and to separate the atria from the ventricles.

Mulder, B. J. M., and E. E. van der Wall. "Size And Function Of The Atria". The International Journal Of Cardiovascular Imaging, vol 24, no. 7, 2008, pp. 713-716. Springer Nature, doi:10.1007/s10554-008-9323-3. Accessed 25 Apr 2018. The heart is at the center of this system, as it pumps blood through vascular channels towards the target tissue. Recall that the heart is a roughly pyramidal organ made up of two muscular pumps that are connected in-series – namely, the left and right heart. Each pump contains an upper chamber that functions as a receptacle for incoming blood, called the atrium, and a lower chamber that is responsible for pushing blood out of the heart called the ventricle. The heart is located in the mediastinum within a region known as the cardiac box; the boundaries of which include: Unfortunately, atrial fibrillation is associated with significant morbidity (syncopal episodes, palpitations, precipitation of underlying heart disease) and mortality (commonly caused by stroke). The poor atrial contraction leads to hemostasis within the atria. Hemostasis favors blood clot formation (recall Virchow’s triad) typically within the left atrial appendage; where these clots can cause a thromboembolic event.

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Guyton, Arthur C, and John E Hall. Textbook Of Medical Physiology. 11th ed., Elsevier Saunders, 2006.

If the endocardial cushions fail to fuse, then the ostium primum will remain patent since the septum primum has nothing to merge with. This is the most likely cause of endocardial cushion defects with ostium primum .

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Bouzas-Mosquera, A. et al. "Left Atrial Size And Risk For All-Cause Mortality And Ischemic Stroke". Canadian Medical Association Journal, vol 183, no. 10, 2011, pp. E657-E664. Joule Inc., doi:10.1503/cmaj.091688. Accessed 25 Apr 2018. As early as the third week of gestation, the cardiovascular system begins to develop. The primordial heart begins to take shape halfway through week three of gestation. Note that this coincides with the fact that the developing embryo is becoming more complexed and as such, can no longer be adequately supplied by simple diffusion of nutrients. At this time the heart is a continuous tube with primitive connections.Left atrial enlargement is more commonly encountered than right atrial enlargement, mainly because the causes of left atrial enlargement are more common. The causes can be divided into congenital (such as ventricular septal defects or patent ductus arteriosus) or acquired causes (such as left ventricular hypertrophy or mitral incompetence secondary to chronic hypertension, or mitral stenosis). Once ventricular contraction stops and the pressure within the atria overcomes the pressure within the ventricles, the atrioventricular valves open and the blood passes into the ventricles. This passive phase of ventricular filling accounts for roughly 80% of the ventricular volume at the beginning of systole. Keep in mind also, that while the atrioventricular valves are open, blood is still draining into the atria from their respective veins. The outer walls of the right atrium contribute to the convexity of the right pulmonary surface, the upper right part of the anatomical base, and the upper anterior surface of the heart. The dome of the atrium is pierced by the superior vena cava, while the posteroinferior part receives the inferior vena cava. A triangular, muscular sac known as the right auricle (right atrial appendage) extends anteriorly and to the left, partially covering the base of ascending aorta. Abnormal absorption of the septum primum where the incorrect part or too much of the septum was reabsorbed can give rise to a patent foramen ovale. An abnormally large foramen ovale can also persist due to the fact that it will not be adequately occluded by the remaining septum primum . In addition to the complications of atrial fibrillation and thromboembolism, patients may also experience left recurrent laryngeal nerve palsy ( Ortner syndrome) and difficulty swallowing in severe cases ( dysphagia megalatriensis) as a result of the mass effect of the enlarged left atrium. On the electrocardiogram, bifid P-waves (with at least 1 small box – 0.04 seconds between the two peaks) and a total increase in P-wave duration (about 0.11 seconds) are seen in lead II. Lead V1 will show a biphasic P-wave, with the negative portion (corresponding to left atrial depolarization) being at least 1 mm deeper. However, definitive diagnosis is best made with echocardiography, where the actual volume of the atrium can be measured. Right atrial enlargement

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Once the heart has recovered from the electrical refractory period (i.e. repolarization is complete), the sinuatrial node initiates the action potential required to generate atrial contraction. Both atria contract simultaneously and the remaining 20% of the ventricular volume is actively pumped into the ventricles. The autonomic nervous system works in tandem to regulate the activities of the sinuatrial node. The heart is said to be in sinus rhythm as long as there are coordinated atrial contractions, followed by normal ventricular contractions. This can be demonstrated on an electrocardiograph by a P-wave preceding each QRS-complex, with normal intervals. Atrial dilatation and ischaemic tissue facilitate the development of re-entrant circuits. The dilatation results in stretching of the electrical pathway, which slows down the propagation of an action potential through a particular loop. As a result, some of the tissues exit the normal post action potential refractory period (i.e. completing repolarization) and can, therefore, be prematurely depolarized by an ectopic beat. The myocardium heals by forming fibrous tissue, which is a poor conductor of electricity. Consequently, the action potential has to find an alternative (possibly longer) route to travel; which leads to a similar situation described above.

Revise the anatomy of the atria and the other parts of the heart with our heart diagrams, quizzes and labeled worksheets. Like the right atrium, the venous aspect of the inner left atrium is smooth and boasts the ostia of the four pulmonary veins in the cranial posterolateral aspect of the atrial wall. While four openings are usually seen in most cases, the left set of pulmonary veins may also emerge in a common conduit. The auricular surface is also highly trabeculated (as seen in the right atrium) as the left atrial auricle contains all the pectinate muscles found within the left atrium. Cadogan, Mike. "ECG - P Wave Morphology - LITFL ECG Library". LITFL • Life In The Fast Lane Medical Blog, 2017, https://lifeinthefastlane.com/ecg-library/basics/p-wave/. Accessed 25 Apr 2018. The left and right atria are separated by a fibromuscular wall known as the atrial (interatrial) septum, while the ventricles are separated by a similar structure, known as the ventricular (interventricular) septum. Additionally, each atrium is separated from the ventricle of the same side by the atrioventricular septum. However, unlike the interventricular and interatrial septa, the atrioventricular septum are fitted with valves (i.e. left and right atrioventricular valves) that allow blood to move from the upper to the lower chambers. These valves also promote a unidirectional flow of blood through the heart, as under normal circumstances, they prevent reflux of blood during ventricular contraction. The left atrium