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. 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. 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. Walker, Brian R et al. Davidson's Principles And Practice Of Medicine. 22nd ed., Elsevier Limited, 2014. 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:

Heart (right and left atrium): Anatomy and function | Kenhub

Like left atrial enlargement, right atrial enlargement will result from increased blood volume or pressure within the atrium. An increase in right atrial pressure can result from pumping against stenotic tricuspid or pulmonary valves, hypertrophied right ventricle, or transmitted pressure increases from pulmonary hypertension (secondary to chronic lung disease). An atrial septal defect with a left to right shunt can also result in an increase in transmitted pressure to the right atrium.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.

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Gaillard, Frank, and Rohit Sharma. "Left Atrial Enlargement | Radiology Reference Article | Radiopaedia.Org". Radiopaedia.Org, https://radiopaedia.org/articles/left-atrial-enlargement. Accessed 25 Apr 2018.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 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. Burns, Edward. "Left Atrial Enlargement - ECG Library LITFL". LITFL • Life In The Fast Lane Medical Blog, 2017, https://lifeinthefastlane.com/ecg-library/basics/left-atrial-enlargement/. Accessed 25 Apr 2018.