Ventricles of the heart

Review of cardiac anatomy

The heart is a mediastinal structure that has the most important role in the circulatory system. In the anatomical position, the heart is obliquely positioned, with its anatomical base (formed by the left atrium) pointing posterolaterally to the right and the apex of heart directed anteroinferiorly to the left. The right cardiac chambers account for most of the anterior or sternocostal surface of the heart. However, the walls of both ventricles constitute the inferior or diaphragmatic surface. 

An overview of the heart and its surfaces is given in the following resource:

The outer surface of the heart is marked by depressions known as grooves. They act as landmarks for the interatrial and interventricular septa inside the heart. The left and right pulmonary surfaces are made up by the left ventricle and right atrium, respectively. Of note, the left pulmonary surface rests in the cardiac notch of the left lung. The heart is made up of four muscular chambers that work synergistically to propel blood throughout the body. The heart is divided into two sides by the interatrial and interventricular septa. Each half of the heart has two chambers: an atrium, and a lower chamber called the ventricle. The function of the atria is to collect and pump blood into the ventricles. The function of the ventricles is to pump blood into systemic and pulmonary circulations. Details regarding the direction of the flow of blood can be found in the cardiac cycle article.

The atria are separated from the ventricles by the atrioventricular septa. This is a fibroelastic structure that not only prevents inappropriate blood flow from the atrium to the ventricle, but it also prevents unwanted electrical conduction from the atrial myocardium to the ventricles. In the absence of this structure, there would be backflow of blood and electrical activity across the myocardium. 

However, blood is able to flow from each atrium into the ipsilateral ventricle via the respective atrioventricular valves, the tricuspid valve on the right and mitral valve on the left. Blood also leaves each ventricle via their respective ventricular outflow tracts that are guarded by the semilunar valves. On the left-hand side, this outflow tract is the aorta, which is guarded by the aortic valve. From here, blood flows into systemic circulation. On the right-hand side, the outflow tract is the pulmonary artery, which is guarded by the pulmonary valve. From here, blood flows into pulmonary circulation.

The following resources provide you with clarifications and details about the heart valves:

Test your knowledge on the heart valves with this quiz.

Right ventricle

Left ventricle

Interventricular septum

An important structure that separates the two ventricles is the interventricular septum. Anatomically it is divided into two parts: a thick, muscular part and a relatively thin membranous part. The muscular part of the interventricular septum separates most of the left and right ventricles from each other. However, the membranous part of the interventricular septum – which is positioned posteriorly and superiorly within the left ventricle – separates the right chambers from the subaortic area. There is an additional division system used to segregate parts of the septum. With echocardiography, the interventricular septum can be divided into five parts. These are listed in no particular order:

• Basal anteroseptal
• Basal inferoseptal
• Midanteroseptal
• Midinferoseptal
• Apical septal

In addition to separating the ventricles, the interventricular septum also functions as a conduit for part of the conducting system of the heart. The atrioventricular bundle of His, which arises from the atrioventricular node, arborizes within the septum to reach its target points. 

The interventricular septum is quite dynamic during each cardiac cycle. It contracts with the ventricles during systole such that it shortens longitudinally (from the base to the apex) and becomes thicker. This is because cardiomyocytes become shorter longitudinally, thinner circumferentially, and wider radially during muscle contraction. Additionally, the septum allows for complex motions of the heart during each contraction. It acts as a support structure against which the free wall of the right ventricle contracts against during systole. 

Ventricular function

The principal role of the ventricles is to pump blood out of the heart and into either systemic or pulmonary circulation. During diastole (relaxation) the ventricle is in the phase of passive filling where blood passes through the atria and into the ventricles. The atrioventricular valves are particularly important here because if they are not functioning (like in valvular stenosis), ventricular filling is impaired. When the atria depolarize and contract toward the end of diastole, they force any residual blood from the atria into their respective ventricles. As the ventricles fill, they dilate until they become depolarized and contract. Blood not only flows toward the semilunar valves but also toward the atrioventricular valves as well. The force of blood will cause competent atrioventricular valves to close (preventing reflux of blood into the atria) but will cause the semilunar valves to open. 

Overwhelmed by the anatomical details of the ventricles and heart? Simplify your learning, cement your knowledge, and test yourself with our diagrams, quizzes & worksheets of the heart!

While the electrical activity throughout the heart is important for each cardiac cycle to occur in a timely manner, the pressure gradient across the valves and within the chambers are equally as important. The pressure dynamics within the ventricles are important to force the semilunar valves open, and to keep the atrioventricular valves closed. In the early phase of systole (contraction), intraventricular pressure (that is, the pressure within the ventricle) rapidly rises in order to overcome the pressure on the other side of the respective valves. However, until the intraventricular pressure is sufficient to open the semilunar valves, no blood will leave the ventricle. This state is known as isovolumetric contraction because even though there is ventricular contraction, there is no loss of blood from the ventricles. The ventricles enter the ejection phase when intraventricular pressure exceeds the pressure within the outflow tract: approximately 8 mmHg on the right side, and 85 mmHg on the left. The valves will open and blood will be ejected from the respective ventricle into systemic or pulmonary circulation. 

Do you want to master the cardiac cycle, understand how it relates to the conducting system of the heart and how to examine the heart? Then take a look below.

Embryology

The heart is one of the first organs to begin development. A heartbeat can be detected on ultrasound as early as the 6th gestational week. Early development and activity are necessary so that blood can be adequately circulated throughout the developing embryo. 

Towards the end of the 3rd gestational week, the primitive heart tube begins pumping blood throughout the primitive embryo. It is a mesodermal derivative that appears like a tubular structure but subsequently undergoes significant morphological changes. The process starts with the appearance of the cardiogenic area (region of the growing heart near the head end (cephalic pole)) of the developing embryo. Subsequently, two strands of cardiogenic cords (mesenchymal derivatives with cardiac potential) arise within the cardiogenic area. As the embryo continues to grow, the cardiogenic tubes migrate centrally and fuse, giving rise to the endocardial tubes (18th - 22nd day of development). 

The endocardial tubes are characterized by a truncus arteriosus (gives rise to the pulmonary trunk and aorta) in the cephalic aspect. This is followed by the bulbus cordis, primitive ventricle, primitive atrium (future atria), and the sinus venosus (future coronary sinus and oblique vein). By the end of the 4th gestational week, the heart has begun its intricate process of cardiac origami–where the heart undergoes significant changes and folding. The folding of the endocardial tubes results in the bulbus cordis assuming a ventral relationship with respect to the ventricle. Eventually, both the bulbus cordis (which will form the right ventricle) and primitive ventricle (forms the left ventricle) will be inferiorly related to the atria. Also important to note that the truncus arteriosus gives rise to the aorta and pulmonary artery–the outflow tracts of the left and right ventricles, respectively. 

The bulbus cordis and primitive ventricle are in communication with each other and form a common ventricle. There is a median ridge on the floor of the common ventricle near the apical aspect of the chamber. This is known as the muscular interventricular septum which is subsequently populated by myocytes (muscle cells) originating from precursor ventricles. Myoblasts (precursors to myocytes) within the developing septum contribute to the growth of the interventricular septum as well. Endocardial cushions (which are located superior to the cushions) are also quite important in the formation of the interventricular septum. They are aggregates of modified cardiac tissue that contribute to the formation of the cardiac valves. This septal proliferation continues with a crescent-shaped defect intervening between the endocardial cushion and the interventricular septum. This ventricular septal defect persists up until the 7th gestational week when the bulbar ridge and the endocardial cushions meet. The point of fusion of the two membranes gives rise to the membranous part of the interventricular septum. 

The trabecular patterns of the luminal surface of the ventricles develop after heart folding has begun. Initially, the lumen is covered with a two-layered epithelial lining that subsequently develops into sheets of myocardial elevations. The elevations extend into the lumen of the ventricle and concentrate near the inlets and apical areas, but recede near the outlet. The trabeculae carneae continue to acquire more cellular layers as they continue to mature.