The heart consists of four chambers – two atria and two ventricles:
- Bloodreturning to the heartentersthe 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 ventricle, blood enters the pulmonary circulation via the pulmonary arteries.
In this article we shall look at the anatomy of the chambers of the heart – their location, internal structure and clinical correlations.
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Atria
Right Atrium
The right atriumreceives deoxygenated blood from the superior and inferiorvena cavae, and from thecoronary veins. It pumps this blood through the right atrioventricular orifice (guarded by thetricuspid valve) into theright 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 theright 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 separatedby a muscular ridge called thecrista terminalis:
- Sinus venarum – located posteriorto thecrista 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, andhas 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.
Interatrial Septum
Theinteratrial 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 thefossaovalis. This is the remnant of theforamen ovale in the fetal heart, which allows right to left shunting of blood to bypass the lungs. It closes once the newborn takes its first breath.
By TeachMeSeries Ltd (2024)
Fig 1
The right atrium and interatrial septum. The atrium proper is only partially visible on this illustration.
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 greaterthan that of theright atrium, so blood is shunted through the openingfrom left to right. In large septal defects, this can cause right ventricular overload, leading to pulmonary hypertension, right ventricular hypertrophy and ultimatelyright heart failure.
Definitive treatment is closure of the defect by surgical or transcatheter closure.
Left Atrium
The left atrium receives oxygenated blood from the fourpulmonary veins, andpumps it through the left atrioventricular orifice (guarded by themitral valve) into the left ventricle.
In the anatomical position, the left atrium forms theposterior 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 leftatrium 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 bypectinate muscles, and is derived from the embryonic atrium.
Ventricles
The left and right ventriclesof the heartreceive blood from the atria and pump it into the outflow vessels; the aorta and the pulmonary arteryrespectively.
Right Ventricle
The right ventricle receives deoxygenated blood from the right atrium, and pumps it through the pulmonary orifice (guarded by the pulmonary valve), into thepulmonary artery.
Itis triangular in shape, and forms the majority of theanterior 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 thesupraventricular crest.
Inflow Portion
The interior of the inflow part of the right ventricle is covered by a series of irregular muscular elevations, calledtrabeculae 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 freein the middle. The most important example of this type is the moderator band, which spansbetween 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)– anchoredby their base to the ventricles. Their apices are attached tofibrous cords (chordae tendineae),which are in turn attached to the threetricuspid valve cusps. By contracting, the papillary muscles ‘pull’ on the chordae tendineae to prevent prolapseof 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. Itis derived from the embryonicbulbuscordis. It is visibly different from the rest of the right ventricle, with smooth walls and no trabeculae carneae.
By OpenStax College [CC BY 3.0], via Wikimedia Commons
Fig 2
Frontal section of the heart, showing the attachment of the papillary muscles to the tricuspid and mitral valves.
Interventricular Septum
The interventricular septum separates the two ventricles, andis composed of a superiormembranouspart and an inferiormuscular part.
The muscular part forms the majority of the septum andis the same thickness as the left ventricular wall. The membranous part is thinner, and part of thefibrous skeleton of the heart.
Left Ventricle
The left ventricle receives oxygenated blood from the left atrium, and pumps it through the aortic orifice (guarded by the aortic valve)into theaorta.
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.
Inflow 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.
Outflow Portion
The outflow part of the left ventricle is known as theaortic vestibule. It is smooth-walled with no trabeculae carneae, and is a derivative of the embryonic bulbus cordis.
By TeachMeSeries Ltd (2024)
Fig 3
The papillary muscles and inflow portion of the left ventricle.
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 valveincreasethe force needed to pump blood through it, resulting in right ventricularhypertrophy. 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 theaorta.
It is usually treated surgically in the first few months of life or in severe cases, soon after birth.
By TeachMeSeries Ltd (2024)
Fig 4
The four structural defects in Tetralogy of Fallot.
