The heart consists of four chambers: the two atria and the two ventricles.
Blood returning to the heart enters the atria, and is then pumped into the ventricles. From the left ventricle, blood passes into the aorta and enters the systemic circulation. From the right, it enters the pulmonary circulation via the pulmonary arteries.
In this article we shall look at the anatomy of the atria and the ventricles, and we will consider their clinical correlations.
The right atrium receives deoxygenated blood from the superior and inferior vena cavae, and from the coronary veins. It pumps this blood through the right atrioventricular orifice (guarded by the tricuspid valve) into the right ventricle.
In the anatomical position, the right atrium forms the right border of the heart. Extending from the antero-medial portion of the chamber is the right auricle (right atrial appendage) – a muscular pouch that acts to increase the capacity of the atrium.
The interior surface of the right atrium can be divided into two parts, each with a distinct embryological origin. These two parts are separated by a muscular ridge called the crista terminalis:
- Sinus venarum – located posterior to the crista terminalis. This part receives blood from the superior and inferior vena cavae. It has smooth walls and is derived from the embryonic sinus venosus.
- Atrium proper – located anterior to the crista terminalis, and includes the right auricle. It is derived from the primitive atrium, and has rough, muscular walls formed by pectinate muscles.
The coronary sinus receives blood from the coronary veins. It opens into the right atrium between the inferior vena cava orifice and the right atrioventricular orifice.
The interatrial septum is a solid muscular wall that separates the right and left atria.
The septal wall in the right atrium is marked by a small oval-shaped depression called the fossa ovalis. This is the remnant of the foramen ovale in the foetal heart, which allows right to left shunting of blood to bypass the lungs. It closes once the newborn takes its first breath.
Clinical Relevance: Atrial Septal Defect
An atrial septal defect is an abnormal opening in the interatrial septum, persistent after birth. The most common site is the foramen ovale, and this is known as a patent foramen ovale.
In the adult, left atrial pressure is usually greater than that of the right atrium, so blood is shunted through the opening from left to right. In large septal defects, this can cause right ventricular overload, leading to pulmonary hypertension, right ventricular hypertrophy and ultimately right heart failure.
Definitive treatment is closure of the defect by surgical or transcatheter closure.
The left atrium receives oxygenated blood from the four pulmonary veins, and pumps it through the left atrioventricular orifice (guarded by the mitral valve) into the left ventricle.
In the anatomical position, the left atrium forms the posterior border (base) of the heart. The left auricle extends from the superior aspect of the chamber, overlapping the root of the pulmonary trunk.
The interior surface of the left atrium can be divided into two parts, each with a distinct embryological origin:
- Inflow portion – receives blood from the pulmonary veins. Its internal surface is smooth and it is derived from the pulmonary veins themselves.
- Outflow portion – located anteriorly, and includes the left auricle. It is lined by pectinate muscles, and is derived from the embryonic atrium.
The left and right ventricles of the heart receive blood from the atria and pump it into the outflow vessels; the aorta and the pulmonary artery respectively.
The right ventricle receives deoxygenated blood from the right atrium, and pumps it through the pulmonary orifice (guarded by the pulmonary valve), into the pulmonary artery.
It is triangular in shape, and forms the majority of the anterior border of the heart. The right ventricle can be divided into an inflow and outflow portion, which are separated by a muscular ridge known as the supraventricular crest.
The interior of the inflow part of the right ventricle is covered by a series of irregular muscular elevations, called trabeculae carnae. They give the ventricle a ‘sponge-like’ appearance, and can be grouped into three main types:
- Ridges – attached along their entire length on one side to form ridges along the interior surface of the ventricle.
- Bridges – attached to the ventricle at both ends, but free in the middle. The most important example of this type is the moderator band, which spans between the interventricular septum and the anterior wall of the right ventricle. It has an important conductive function, containing the right bundle branches.
- Pillars (papillary muscles) – anchored by their base to the ventricles. Their apices are attached to fibrous cords (chordae tendineae), which are in turn attached to the three tricuspid valve cusps. By contracting, the papillary muscles ‘pull’ on the chordae tendineae to prevent prolapse of the valve leaflets during ventricular systole.
Outflow Portion (Conus arteriosus)
The outflow portion (leading to the pulmonary artery) is located in the superior aspect of the ventricle. It is derived from the embryonic bulbus cordis. It is visibly different from the rest of the right ventricle, with smooth walls and no trabeculae carneae.
The interventricular septum separates the two ventricles, and is composed of a superior membranous part and an inferior muscular part.
The muscular part forms the majority of the septum and is the same thickness as the left ventricular wall. The membranous part is thinner, and part of the fibrous skeleton of the heart.
The left ventricle receives oxygenated blood from the left atrium, and pumps it through the aortic orifice (guarded by the aortic valve) into the aorta.
In the anatomical position, the left ventricle forms the apex of the heart, as well as the left and diaphragmatic borders. Much like the right ventricle, it can be divided into an inflow portion and an outflow portion.
The walls of the inflow portion of the left ventricle are lined by trabeculae carneae, as described with the right ventricle. There are two papillary muscles present which attach to the cusps of the mitral valve.
The outflow part of the left ventricle is known as the aortic vestibule. It is smooth-walled with no trabeculae carneae, and is a derivative of the embryonic bulbus cordis.
Clinical Relevance: Tetralogy of Fallot
Tetralogy of Fallot is a cyanotic congenital heart disease, comprising four abnormalities as a result of a single development defect. The four abnormalities are:
- Ventricular septal defect
- Overriding aorta (this is where the aorta is positioned directly over the VSD)
- Pulmonary valve stenosis
- Right ventricular hypertrophy
Stenosis of the pulmonary valve increase the force needed to pump blood through it, resulting in right ventricular hypertrophy. Eventually, the pressure in the right ventricle becomes higher than that of the left – and blood then shunts from right to left through the ventricular septal defect. The overriding aorta lies over the ventricular septal defect, resulting in deoxygenated blood passing into the aorta.
It is usually treated surgically in the first few months of life or in severe cases, soon after birth.