Ventricle

A ventricle is one of the two lower chambers of the heart, responsible for pumping blood either to the lungs (right ventricle) or to the rest of the body (left ventricle). The heart has two ventricles: the right ventricle, which pumps deoxygenated blood to the lungs through the pulmonary artery, and the left ventricle, which pumps oxygenated blood into the systemic circulation via the aorta. The ventricles are muscular chambers with thick walls that generate the force needed for the heart to circulate blood effectively.

Location

The ventricles are located in the lower part of the heart. The right ventricle is positioned on the right side of the heart, beneath the right atrium, and extends towards the front of the heart. The left ventricle is situated on the left side, beneath the left atrium, and is located more posteriorly compared to the right ventricle. The left ventricle has a thicker wall compared to the right ventricle due to the higher force required to pump blood throughout the body. Both ventricles are enclosed within the pericardium and separated from each other by the interventricular septum.

Structure and Anatomy

Overview

The heart’s ventricles are two large, muscular chambers that form the lower part of the heart. The right ventricle and left ventricle are separated by the interventricular septum and are responsible for pumping blood to the pulmonary and systemic circulations, respectively. The structure of the ventricles is designed to generate the necessary force for blood propulsion, with distinct differences in their thickness and internal features, reflecting the different pressures they must overcome.

Right Ventricle

• Location: The right ventricle is located in the anterior and right side of the heart, just below the right atrium. It forms most of the anterior surface of the heart and is situated towards the front in comparison to the left ventricle.
• Chamber Shape and Structure:
  • The right ventricle has a crescent-shaped or “U”-shaped cross-section, due to its thinner wall and the close proximity of the interventricular septum.
  • Its walls are generally thinner (about 3-5 mm) compared to the left ventricle because the right ventricle only needs to generate enough pressure to pump blood to the lungs, which is a low-resistance circuit.
• Internal Features:
• • Trabeculae Carneae: The inner surface of the right ventricle is irregular due to the presence of muscular ridges called trabeculae carneae. These ridges prevent the walls of the ventricle from sticking together during contraction.
  • Papillary Muscles: There are typically three papillary muscles in the right ventricle: anterior, posterior, and septal. These muscles attach to the tricuspid valve (the valve between the right atrium and right ventricle) via chordae tendineae, which help prevent the valve leaflets from inverting into the atrium during ventricular contraction.
  • Moderator Band: A unique feature of the right ventricle is the moderator band (or septomarginal trabecula), a muscular band that stretches from the interventricular septum to the anterior papillary muscle. This band carries part of the right bundle branch of the conduction system, facilitating the electrical signal to the right ventricle for coordinated contraction.
• Outflow Tract (Infundibulum): The superior part of the right ventricle, just before it transitions into the pulmonary trunk, is called the infundibulum or conus arteriosus. This smooth-walled outflow tract ensures smooth blood flow from the ventricle into the pulmonary circulation.

Left Ventricle

• Location: The left ventricle is positioned on the posterior and left side of the heart, beneath the left atrium. It forms a significant portion of the apex (the pointed lower part of the heart) and the posterior surface of the heart.
• Chamber Shape and Structure:
  • The left ventricle has a conical or circular cross-section, reflecting its more powerful structure, which allows it to generate the high pressure needed to pump blood through the systemic circulation.
  • The walls of the left ventricle are much thicker than those of the right ventricle, typically measuring between 8-15 mm. This thickness is necessary to overcome the higher resistance in the systemic arteries.
• Internal Features:
• • Trabeculae Carneae: Like the right ventricle, the left ventricle also has trabeculae carneae, though these ridges are more finely woven and denser than in the right ventricle.
  • Papillary Muscles: The left ventricle contains two papillary muscles, anterior and posterior. These muscles are larger and stronger than those in the right ventricle, and they attach to the mitral valve (the valve between the left atrium and left ventricle) via chordae tendineae. The papillary muscles and chordae tendineae prevent the mitral valve from inverting during ventricular contraction.
• Outflow Tract (Aortic Vestibule): The smooth-walled outflow portion of the left ventricle, just before it connects to the aorta, is called the aortic vestibule. This area ensures smooth blood flow from the left ventricle into the aorta. It lies just below the aortic valve, which regulates the passage of blood into the aorta.

Interventricular Septum

The interventricular septum is a thick, muscular wall that separates the right and left ventricles. It plays a vital role in the contraction of both ventricles and the transmission of electrical impulses through the heart. The septum has two main components:

• Muscular Portion:The muscular portion makes up most of the interventricular septum and is responsible for providing the necessary strength for the simultaneous contraction of both ventricles.
• Membranous Portion:The membranous portion is a smaller, thin section located near the aortic and pulmonary valves. This area is important for its role in the conduction of electrical impulses through the bundle of His, which helps synchronize the contraction of the ventricles.

Valves Associated with the Ventricles

Each ventricle is associated with an atrioventricular valve and a semilunar valve, which control the flow of blood into and out of the ventricles:

• Right Ventricle Valves:
  • Tricuspid Valve: Located between the right atrium and right ventricle, the tricuspid valve allows blood to flow from the atrium to the ventricle during diastole and prevents backflow during systole. It is supported by the chordae tendineae and papillary muscles.
  • Pulmonary Valve: The pulmonary valve lies at the junction between the right ventricle and the pulmonary trunk. It is a semilunar valve with three cusps that prevent backflow of blood into the right ventricle after contraction.
• Left Ventricle Valves:
• • Mitral Valve: Located between the left atrium and left ventricle, the mitral valve (also known as the bicuspid valve) allows blood to flow from the left atrium to the left ventricle and prevents backflow during systole. It also relies on chordae tendineae and papillary muscles for support.
  • Aortic Valve: This semilunar valve is located between the left ventricle and the aorta. It prevents the backflow of blood into the left ventricle after contraction, ensuring one-way flow into the systemic circulation.

Coronary Arteries and Blood Supply

The ventricles are heavily dependent on a rich blood supply from the coronary arteries due to the immense energy demands of the heart muscle:

• Right Ventricle Blood Supply:The right ventricle is primarily supplied by the right coronary artery (RCA), particularly its acute marginal branches, which supply oxygenated blood to the myocardium of the right ventricle.
• Left Ventricle Blood Supply:The left ventricle receives most of its blood supply from the left coronary artery (LCA), specifically the left anterior descending artery (LAD) and circumflex artery (LCx). The LAD supplies the anterior wall and the interventricular septum, while the LCx provides blood to the lateral and posterior walls of the left ventricle.

Myocardial Thickness

The thickness of the myocardium varies significantly between the ventricles due to their respective roles in the circulatory system:

• Right Ventricle:The myocardium of the right ventricle is relatively thin (3-5 mm), as it only needs to generate enough pressure to pump blood to the lungs, where the resistance in the pulmonary circulation is low.
• Left Ventricle:The left ventricle has a much thicker myocardium (8-15 mm), as it must generate high pressure to pump blood throughout the entire systemic circulation, overcoming the higher resistance in the systemic arteries.

Function

Pumping Blood to the Pulmonary and Systemic Circulations

The primary function of the ventricles is to pump blood into the circulatory systems. Each ventricle serves a distinct function within the heart’s dual circulation system:

• Right Ventricle:The right ventricle is responsible for pumping deoxygenated blood into the pulmonary circulation. When the right ventricle contracts, it pushes blood into the pulmonary trunk, which then branches into the right and left pulmonary arteries, directing the blood to the lungs for oxygenation. This process, known as pulmonary circulation, is crucial for reoxygenating the blood that has returned from the systemic circulation.
• Left Ventricle:The left ventricle is responsible for pumping oxygenated blood into the systemic circulation, which delivers oxygen-rich blood to all tissues and organs of the body. When the left ventricle contracts, it pushes blood into the aorta, the body’s largest artery, which then branches into smaller arteries that carry oxygenated blood to the entire body. The left ventricle must generate significantly higher pressure than the right ventricle, as it must pump blood through the entire body, overcoming the higher resistance of systemic blood vessels.

Generation of High Pressure for Blood Ejection

Both ventricles are tasked with generating the force needed to eject blood from the heart, but the left ventricle is particularly important for generating high pressure due to the demands of the systemic circulation:

• Right Ventricle:The right ventricle generates relatively low pressure because the pulmonary circulation is a low-resistance circuit. The thin walls of the right ventricle (3-5 mm) are sufficient to generate the force necessary to pump blood to the lungs, which are located close to the heart and do not require high pressure for blood to flow through the pulmonary arteries.
• Left Ventricle:The left ventricle must generate much higher pressure, as it pumps blood into the systemic circulation, which involves moving blood through a vast network of arteries and capillaries across the entire body. To achieve this, the left ventricle has a thick myocardium (8-15 mm) that contracts forcefully, generating sufficient pressure to open the aortic valve and propel blood into the aorta.

Coordination of Blood Flow with Heart Valves

The ventricles work in close coordination with the heart’s atrioventricular (AV) valves and semilunar valves to ensure the proper direction of blood flow:

• Atrioventricular (AV) Valves:The tricuspid valve (in the right ventricle) and the mitral valve (in the left ventricle) are responsible for ensuring one-way blood flow from the atria to the ventricles during diastole (the heart’s relaxation phase). These valves prevent the backflow of blood into the atria when the ventricles contract.
• Semilunar Valves:The pulmonary valve (in the right ventricle) and the aortic valve (in the left ventricle) open during systole (ventricular contraction), allowing blood to be ejected from the ventricles into the pulmonary trunk and aorta, respectively. Once the ventricles relax, these valves close to prevent the backflow of blood into the ventricles.

The coordination of the ventricles and their associated valves ensures that blood flows efficiently and in one direction, with each contraction propelling blood forward into either the pulmonary or systemic circulation.

Contraction and Relaxation for Blood Circulation

The ventricles undergo a rhythmic cycle of contraction (systole) and relaxation (diastole) that drives the circulation of blood throughout the body:

• Ventricular Systole:During systole, the ventricles contract to generate the force needed to push blood into the pulmonary and systemic circulations. The contraction of the ventricular myocardium, particularly in the left ventricle, reduces the volume of the ventricles and increases the pressure inside, forcing the semilunar valves to open and eject blood.
• Ventricular Diastole:During diastole, the ventricles relax, allowing blood to fill the ventricles from the atria. The relaxation phase is essential for the refilling of the ventricles, ensuring that they are ready for the next contraction. The closing of the AV valves prevents blood from flowing back into the atria, and the elastic recoil of the ventricular walls aids in drawing blood into the chambers.

Maintaining Cardiac Output

The ventricles are essential for maintaining cardiac output, which is the volume of blood pumped by the heart per minute. Cardiac output is a critical parameter for ensuring that sufficient oxygenated blood reaches the tissues of the body. The ventricles adjust their force of contraction and heart rate based on the body’s needs:

• Right Ventricle Contribution:The right ventricle maintains pulmonary blood flow, ensuring that deoxygenated blood reaches the lungs to be reoxygenated. This function is crucial for maintaining the overall balance of blood flow between the pulmonary and systemic circuits.
• Left Ventricle Contribution:The left ventricle plays a primary role in maintaining systemic blood flow. During periods of increased physical activity, emotional stress, or other factors, the left ventricle increases both the rate and force of contraction to meet the body’s increased oxygen demands. This ensures that tissues receive an adequate supply of oxygenated blood under various conditions.

Synchronization with the Atria for Efficient Filling and Ejection

The ventricles work in concert with the atria to ensure efficient filling and ejection of blood:

• Atrial Systole:During atrial systole, the atria contract to fill the ventricles with blood. The ventricles must remain relaxed during this phase to allow for maximum filling before they contract.
• Ventricular Systole:After the atria contract and the ventricles are filled, the ventricles contract to eject the blood into the circulation. This coordination between the atria and ventricles ensures that blood moves smoothly through the heart with minimal resistance and that the ventricles are filled adequately before contraction.

Pressure Regulation in Circulatory Systems

The ventricles play an essential role in regulating blood pressure within the pulmonary and systemic circulations:

• Right Ventricle:The right ventricle maintains a low-pressure system in the pulmonary circulation. This low pressure is necessary to protect the delicate capillaries in the lungs from damage due to high pressure while still allowing efficient gas exchange.
• Left Ventricle:The left ventricle generates the high pressure needed for the systemic circulation. This high pressure is essential for moving blood through the vast network of arteries, arterioles, and capillaries in the body, overcoming the resistance in these vessels and ensuring that all tissues receive oxygenated blood.

Clinical Significance

The ventricles play a crucial role in heart function, and any disorders affecting them can lead to serious cardiovascular conditions. Ventricular dysfunction is often linked to heart failure, where the ventricles are unable to pump blood efficiently. This can occur in the form of systolic heart failure (inability of the ventricles to contract effectively) or diastolic heart failure (impaired filling of the ventricles).

A common condition affecting the ventricles is ventricular hypertrophy, where the myocardium thickens due to high blood pressure or heart disease, particularly in the left ventricle. This can lead to increased stiffness and reduced ability to pump blood effectively. Myocardial infarction (heart attack) can damage ventricular tissue, particularly in the left ventricle, which can severely impair heart function and lead to heart failure or arrhythmias.

Ventricular arrhythmias, such as ventricular fibrillation or ventricular tachycardia, are life-threatening conditions where abnormal electrical activity in the ventricles causes irregular heartbeats, potentially leading to sudden cardiac death if untreated.