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Atria of the heart: want to learn more about it?

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Atria of the heart

Most species of animals rely on a well-organized circulatory system to move blood and nutrients around the body. The heart is a critical component of the human (and other animals’) circulatory system. While each aspect of the heart plays an important role in the circulatory system, the atria are particularly important as they help to fill the ventricles prior to ventricular contraction.

As such, the goal of this article is to discuss the embryology, anatomy, and blood supply of the atria of the heart. Furthermore, the physiological function, as well as pathological conditions affecting the atria will also be addressed.

Key facts about the atria of the heart
Embryology Week 3-8 of gestation
Left atrium

Receives oxygenated blood from the lungs via the pulmonary veins (4 ostia).

Characteristics - Thicker wall; left auricle (contains pectinate muscles)

Landmarks - T5 - T8 (supine), T6 - T9 (erect)

Right atrium

Receives deoxygenated blood from the systemic circulation via the superior and inferior vena cava

Characteristics - Right auricle; locations for sinoatrial and atrioventricular nodes; three internal surfaces (venous, vestibular, auricular)

Functions Reservoirs for blood and active pumps that help fill the ventricles
Clinical Atrial septal defects, sinoatrial node disorders, atrial fibrillation, atrial flutter, atrial enlargement

Basic anatomy of the heart

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:

  • an imaginary line passing through the jugular notch superiorly, 
  • a parallel line passing through the xiphoid process inferiorly,
  • bilaterally are two imaginary vertical lines, each passing through the respective left and right midclavicular lines (or nipple line in the non-pendulous breast).

Within this space, the heart is oriented obliquely, with the true cardiac apex pointing anteroinferiorly to the left. The surface that the heart rests on in vivo is not the true cardiac base, but rather it is the diaphragmatic surface. The anatomical base points dorsally and slightly upwards to the right. The right atrium and ventricle make up most of the sternocostal surface of the heart, while the diaphragmatic surface is made up by the right and left ventricles. The left atrium contributes to the anatomical base.

The heart contains three main layers of tissue. The innermost layer is known as the endocardium and the outermost layer is the epicardium. Between those two layers is a thick layer of specialized (i.e. cardiomyocytes) known as the myocardium. The thickness of the myocardium varies between regions of the heart. In the absence of underlying pathology, the ventricles are always thicker than the atria. Furthermore, the left chambers are thicker than the right counterparts, owing to the fact that the former has to pump blood against greater resistance when compared to the lower pulmonary resistance that the right chambers have to overcome. 

Although all blood – both nutrient-rich oxygenated and nutrient-poor deoxygenated – passes through the heart, the thickness of the heart walls reduces efficient transport of essential nutrients to the cardiac tissue. As a result, the heart has its own circulatory network known as the coronary arteries and veins. The coronary arteries are a paired set of vessels arising from the base of the ascending aorta. Each vessel arborizes to give numerous branches that supply all layers of the heart. There are several venous tributaries as well that eventually drain deoxygenated blood from the layers of the heart and return it mainly to the right atrium; however, some vessels have been shown to drain to the right ventricle, and less commonly to the left heart.

Learn more about the overall anatomy and histology of the heart using the following resources:

Anatomy of the atria

Much like the wide, open architectural atrium that functions as receiving sites for incoming guests, the cardiac atrium is a pair of chambers situated at the upper part of the heart that receives systemic and pulmonary blood. The right atrium receives deoxygenated blood from the systemic circulation via the superior and inferior vena cava. On the other hand, oxygenated blood leaving the lungs is carried to the left atrium via the pulmonary veins

In the anatomical position, the left atrium is concealed behind the right atrium, as the latter contributes to most of the upper part of the sternocostal surface of the heart. The interatrial groove (which is the surface marking for the atrial septum) serves as a landmark that separates the atria on the surface of the heart. It continues toward the posterior surface of the heart where it meets the atrioventricular and posterior interventricular grooves at the cardiac crux.

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

The left atrium is positioned slightly above and behind the right atrium. Although it is smaller in terms of the amount of blood it can hold, the left atrium has a thicker myocardial wall when compared to the right atrium. This is a result of the fact that the left atrium is exposed to higher pressures – and therefore does more work – than the right atrium. Like the right atrium, the cuboidal left atrial wall is made up of venous entities (in addition to auricular and vestibular parts as well). In this case, the four ostia of the pulmonary veins enter the posterior aspect of the left atrium. The vessels pierce either side of the posterior wall (which also contributes to the majority of the anatomical base of the heart) in pairs.

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

In the anatomical position, the left atrium is located between the 5th to 8th thoracic vertebrae if the individual is supine (lying flat) or the 6th to 9th vertebrae in someone who is standing erect. Also posteriorly related to the left atrium are the descending aorta, esophagus, and the previously described pulmonary veins.

Internal features of the left atrium

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.

The right atrium

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.

The basal surface of the right atrium also has a shallow depression known as the sulcus terminalis. It marks the point of fusion between the venous part of the right atrium (formed by the sulcus venosus) and the true right atrium. The sulcus terminalis also provides a surface marking for the crista terminalis (terminal crest), which serves as the origin for the pectinate muscles that extend perpendicularly.

Structures surrounding the right atrium

The lateral side of the right atrium is adjacent to the mediastinal surface of the right lung. However, the hilum of the right lung is slightly posterior to this aspect of the right atrium. Intervening between the right lung and ipsilateral atrium are the pericardium , pericardiophrenic vessels, right phrenic nerve, and pleura. Posterolaterally to the right of the right atrium is the right pulmonary veins, while the associated interatrial groove is located posteriorly and to the left of the right atrium. The anterior mediastinal aspect of the right lung is anteriorly related to the right atrium. The structures separating the two are the pleura and pericardium. 

The sinoatrial node, which is responsible for regulating the automaticity of the myocardium, is located in the posterior wall of the right atrium. More specifically, it is inferolateral to the opening of the superior vena cava, along the superior part of the crista terminalis. The secondary cardiac pacemaker – the atrioventricular node – is situated in the inferior aspect of the right atrium, within the triangle of Koch. Of note, the triangle of Koch is limited by the coronary sinus, the septal cusp of the tricuspid valve, and the tendon of Todaro.

Internal features of the right atrium

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. 

While numerous minor venous tributaries drain directly into the right atrium, the principal afferents to the right atrium are the venae cavae and the coronary sinus. On occasion, the right marginal and anterior cardiac veins may open directly into the right atrium as well. The inferior vena cava and coronary sinus are the only two vessels draining into the right atrium that have valvular mechanisms to prevent venous reflux.

Chambers and great vessels of the heart in a cadaver

The Eustachian valve is the valve of the inferior vena cava, while the Thebesian valve is the valve of the coronary sinus. The valve of the inferior vena cava is anteriorly related to the opening of the vein. It travels along the right border of the vessel and traverses the sinus septum (partition between the oval fossa [fossa ovale] and the coronary sinus) inferiorly; after which it merges with the valve of the coronary sinus. The thin, semi-circular Thebesian valve continues from its attachment with the Eustachian valve, toward the round tendon of Todaro. Of note, the tendon of Todaro is attached proximally at the sinus septum and inserts into the central fibrous body (strongest part of the cardiac skeleton).

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Functions of the atria

Both atria carry out three distinct functions in an ordered sequence. While the ventricles are contracting and the atrioventricular valves are closed, blood still continuously flows from the venae cavae on the right, and the pulmonary veins on the left, to fill the atria. During this phase, the atria act as reservoirs that store blood temporarily. 

Revise the anatomy of the atria and the other parts of the heart with our heart diagrams, quizzes and labeled worksheets.

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. 

Once the heart has recovered from the electrical refractory period (i.e. repolarization is complete), the sinoatrial 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.

Embryology of the atria

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.

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.

Concurrently, the primitive interatrial septa also begin to form during the 4th gestational week. Initially, a thin moon-shaped membranous structure begins to form from the roof of the primordial atria to the endocardial cushion. This early wall is known as the septum primum (i.e. primary septum); it partially separates the atria into the left and right components. The free crescentic edge of the septum primum, along with the superior border of the endocardial cushion, provides superior and inferior (respectively) boundaries for the foramen primum (i.e. the first foramen). It acts as a physiological shunt that allows oxygenated blood to pass into the left atrium. Over time, the septum primum continues to proliferate and fuse with the endocardial cushions, thus obliterating the foramen primum.

Embryonic atrial heart valves

During the time that the lower part of the septum primum begins to fuse, fenestrations begin to appear in the middle of the membrane. These fenestrations, which originated following several apoptotic signals to the septum primum, coalesce to form the foramen secundum (i.e. the second foramen). This opening maintains the physiological right to left shunt that was created with the foramen primum. Between the 5th and 6th gestational weeks, a thicker crescentic muscular membrane, known as the septum secundum (i.e. the second septum) originates in the roof of the right atrium and grows caudally where it gradually covers the foramen secundum. Although the septum secundum completely covers the foramen secundum, the inferodorsal aspect of the membrane remains incomplete. The septal defect that remains is oval in shape and is therefore referred to as the foramen ovale (i.e. the oval foramen). Simultaneously, the cranial part of the septum primum undergoes apoptotic degeneration, resulting in the obliteration of the foramen secundum. The foramen ovale now functions as the pathway for the physiological right to left shunt. The remaining caudal part of the septum primum acts as a valve to prevent premature reversal of the shunt; it is therefore referred to as the valve of the foramen ovale

Prior to the development of the sinoatrial node, the automaticity of the heart is regulated by the primordial atrial myocardium. During the 5th gestational week, the sinoatrial node begins to develop within the walls of the right sinus venosus. As the sinus venosus becomes incorporated into the wall of the right atrium, the sinoatrial node also migrates to its final location near the orifice of the superior vena cava.

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.

Atria of the heart: want to learn more about it?

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