Heart

The heart is a muscular organ that pumps blood throughout the body via the circulatory system. It is approximately the size of a fist and consists of four chambers: two atria and two ventricles. The heart is composed of specialized muscle tissue, known as myocardium, and is encased within a double-layered protective sac called the pericardium. The heart works in a rhythmic cycle of contraction and relaxation, ensuring continuous circulation of blood.

Location

The heart is located in the mediastinum, the central compartment of the thoracic cavity. It lies slightly to the left of the midline, behind the sternum, and between the lungs. The heart is situated on the diaphragm at its base, with its apex pointing downward and to the left. It is enclosed by the pericardium and surrounded by major blood vessels like the aorta, pulmonary arteries, and veins. The heart’s exact position varies slightly from person to person, but its general location is central within the chest, protected by the ribcage.

Structure and Anatomy

Overview

The heart is a four-chambered muscular organ with a complex structure that allows it to efficiently pump blood throughout the body. It is divided into two halves—the right and left sides—each responsible for different aspects of circulation. The heart consists of four chambers: the right atrium, right ventricle, left atrium, and left ventricle. These chambers are separated by valves and muscular septa, which direct blood flow and maintain efficient circulation. The heart wall is composed of three layers: the epicardium, myocardium, and endocardium.

External Anatomy of the Heart

• Apex and Base:
  • The apex of the heart is the pointed lower tip that is oriented downward and to the left. It is formed by the tip of the left ventricle and is located at the level of the 5th intercostal space, typically slightly to the left of the midline.
  • The base of the heart is its broader upper part and is primarily formed by the atria, especially the left atrium. It faces posteriorly and is situated at the level of the 2nd intercostal space. The base is where the great vessels – the aorta, pulmonary trunk, and superior vena cava – enter or exit the heart.
• Surfaces of the Heart:
  • Sternocostal (Anterior) Surface: This surface is formed mostly by the right ventricle and a small portion of the left ventricle. The right atrium also contributes to this surface, while the heart rests on the sternum and the costal cartilages of the ribs.
  • Diaphragmatic (Inferior) Surface: This surface is formed by both ventricles, primarily the left ventricle, and is in contact with the diaphragm.
  • Pulmonary (Left) Surface: This surface is mainly composed of the left ventricle and is in contact with the left lung.
• Borders of the Heart:
• • Right Border: Formed by the right atrium.
  • Left Border: Formed by the left ventricle and part of the left atrium.
  • Inferior Border: Formed by the right ventricle and part of the left ventricle.
  • Superior Border: Formed by the right and left atria and the great vessels.

Chambers of the Heart

• Right Atrium:
  • The right atrium is a thin-walled chamber located in the upper right side of the heart. It receives deoxygenated blood from the body through the superior vena cava (from the upper body) and inferior vena cava (from the lower body), and from the heart itself via the coronary sinus.
  • The right atrium contains the auricle, a small ear-shaped muscular pouch, and the fossa ovalis, a remnant of the fetal foramen ovale. Blood flows from the right atrium into the right ventricle through the tricuspid valve.
• Right Ventricle:
  • The right ventricle is located below the right atrium and pumps blood into the pulmonary circulation. It has relatively thin walls compared to the left ventricle and is crescent-shaped in cross-section.
  • The right ventricle’s internal surface is irregular due to the presence of trabeculae carneae and papillary muscles, which anchor the tricuspid valve via the chordae tendineae. Blood flows from the right ventricle through the pulmonary valve into the pulmonary trunk and into the lungs.
• Left Atrium:
  • The left atrium is located posteriorly, forming much of the base of the heart. It receives oxygenated blood from the lungs through the four pulmonary veins (two from each lung).
  • Like the right atrium, the left atrium also has a muscular auricle. Blood flows from the left atrium into the left ventricle through the mitral valve.
• Left Ventricle:
• • The left ventricle is the thickest chamber of the heart due to its role in pumping oxygenated blood into the systemic circulation. It has a conical shape and forms the apex of the heart.
  • The internal structure of the left ventricle is similar to the right ventricle, with trabeculae carneae, papillary muscles, and chordae tendineae. Blood flows from the left ventricle through the aortic valve into the aorta, which distributes oxygenated blood to the body.

Valves of the Heart

The heart has four valves that ensure one-way blood flow between the chambers and into the great vessels:

• Tricuspid Valve:Located between the right atrium and right ventricle, the tricuspid valve has three leaflets or cusps. It prevents backflow of blood into the right atrium during ventricular contraction.
• Pulmonary Valve:The pulmonary valve is located between the right ventricle and the pulmonary trunk. It has three semilunar cusps that prevent backflow of blood into the right ventricle after contraction.
• Mitral Valve (Bicuspid Valve):Located between the left atrium and left ventricle, the mitral valve has two cusps. It prevents backflow of blood into the left atrium during ventricular contraction.
• Aortic Valve:The aortic valve is located between the left ventricle and the aorta. It has three semilunar cusps and prevents backflow of blood into the left ventricle after contraction.

Heart Wall Layers

• Epicardium:The epicardium is the outermost layer of the heart and is also considered the visceral layer of the serous pericardium. It is a thin, transparent layer that protects the heart and produces serous fluid to reduce friction during heart movements.
• Myocardium:The myocardium is the thick, muscular middle layer of the heart wall and is composed of cardiomyocytes (heart muscle cells). The myocardium is responsible for the contractile force of the heart. It is thickest in the left ventricle, which must generate enough force to pump blood throughout the entire body.
• Endocardium:The endocardium is the innermost layer of the heart and lines the interior of the heart chambers and valves. It is made of a thin layer of endothelial cells and connective tissue, providing a smooth surface for blood flow and protecting the heart tissue from direct contact with blood.

Septa of the Heart

• Interatrial Septum:The interatrial septum separates the right and left atria. It contains the fossa ovalis, a depression that is a remnant of the fetal foramen ovale, which allowed blood to bypass the lungs in fetal circulation.
• Interventricular Septum:The interventricular septum separates the right and left ventricles. It is composed of a thick muscular portion and a thinner membranous portion near the base of the heart. The interventricular septum plays a critical role in supporting the contraction of the ventricles and maintaining the integrity of the heart chambers.

Conducting System of the Heart

The heart’s electrical conduction system controls the timing and coordination of the heart’s contractions:

• Sinoatrial (SA) Node:The SA node is located in the right atrium near the opening of the superior vena cava. It acts as the natural pacemaker of the heart, initiating electrical impulses that spread across the atria, causing them to contract.
• Atrioventricular (AV) Node:The AV node is located at the junction of the atria and ventricles. It delays the electrical impulse slightly, allowing the ventricles to fill with blood before they contract.
• Bundle of His:The bundle of His is a pathway of specialized muscle fibers that carries electrical impulses from the AV node to the ventricles. It runs along the interventricular septum and divides into the right and left bundle branches.
• Purkinje Fibers:The Purkinje fibers are specialized conducting fibers that spread throughout the ventricular myocardium, ensuring coordinated contraction of the ventricles.

Blood Supply to the Heart

• Coronary Arteries:
  • The heart receives its blood supply through the coronary arteries, which branch from the ascending aorta. There are two main coronary arteries:
    • Right Coronary Artery (RCA): Supplies blood to the right atrium, right ventricle, and portions of the left ventricle and interventricular septum.
    • Left Coronary Artery (LCA): Divides into the left anterior descending artery (LAD), which supplies the anterior walls of the left ventricle and septum, and the circumflex artery (LCx), which supplies the lateral and posterior walls of the left ventricle.
• Cardiac Veins:Deoxygenated blood from the myocardium is drained by the cardiac veins, which empty into the coronary sinus, a large vein that returns the blood to the right atrium.

Function

The heart functions as a vital pump responsible for circulating blood throughout the body. Its primary role is to maintain the flow of oxygenated and deoxygenated blood to sustain bodily functions. The heart achieves this through a coordinated system of chambers, valves, and electrical impulses, which work together to regulate blood flow, pressure, and oxygen delivery.

Pumping Blood to the Pulmonary and Systemic Circulations

The heart is divided into two sides—right and left—each responsible for a specific circulation pathway:

• Right Side: Pulmonary Circulation:The right atrium receives deoxygenated blood from the body through the superior and inferior vena cava. This blood flows into the right ventricle, which pumps it through the pulmonary valve into the pulmonary trunk and pulmonary arteries, transporting the blood to the lungs. In the lungs, carbon dioxide is exchanged for oxygen in the alveoli.
• Left Side: Systemic Circulation:The left atrium receives oxygenated blood from the lungs via the pulmonary veins. This blood flows into the left ventricle, which contracts powerfully, pumping blood through the aortic valve into the aorta and systemic circulation. The oxygenated blood is distributed to the tissues and organs of the body through a network of arteries, delivering oxygen and nutrients essential for cellular functions.

Maintaining Unidirectional Blood Flow

The heart ensures unidirectional blood flow through a series of valves that prevent backflow and maintain efficient circulation:

Atrioventricular (AV) Valves:

• Tricuspid Valve: Between the right atrium and right ventricle, this valve prevents backflow of blood into the right atrium during ventricular contraction.
• Mitral Valve: Located between the left atrium and left ventricle, the mitral valve ensures blood does not flow backward into the left atrium when the left ventricle contracts.

Semilunar Valves:

• Pulmonary Valve: Situated between the right ventricle and pulmonary trunk, this valve prevents the return of blood into the right ventricle after it has been ejected to the lungs.
• Aortic Valve: Positioned between the left ventricle and the aorta, the aortic valve ensures that blood flows only into the aorta and does not return to the ventricle after contraction.

These valves coordinate the movement of blood between chambers and major arteries, ensuring a one-way flow throughout the heart and circulatory system.

Generation of Blood Pressure for Ejection

The heart generates the pressure required to propel blood through both the pulmonary and systemic circulations:

• Right Ventricle: The right ventricle generates low pressure, sufficient to pump deoxygenated blood to the lungs, where resistance is low. Since the lungs are nearby and the pulmonary arteries are short, less force is required for blood circulation through the pulmonary circuit.
• Left Ventricle: The left ventricle generates high pressure to pump oxygenated blood throughout the entire body. The systemic circulation involves greater resistance due to the extensive network of arteries, capillaries, and veins that blood must travel through to reach tissues far from the heart. The thicker myocardium of the left ventricle provides the necessary force for this task.

Synchronizing the Cardiac Cycle

The heart works in a rhythmic cycle of contraction and relaxation, known as the cardiac cycle, which ensures continuous blood flow. This cycle is divided into two main phases:

• Systole (Contraction):During systole, the ventricles contract, increasing pressure within the chambers. This pressure forces blood out of the heart into the pulmonary arteries and aorta. The atrioventricular valves (tricuspid and mitral) close to prevent backflow, and the semilunar valves (pulmonary and aortic) open to allow blood to flow into the arteries.
• Diastole (Relaxation):During diastole, the ventricles relax, reducing pressure and allowing them to refill with blood from the atria. The atrioventricular valves open to facilitate this filling, while the semilunar valves close to prevent blood from flowing back into the heart. Diastole is essential for proper chamber filling and ensuring sufficient blood volume is pumped out during the next contraction.

Maintaining Cardiac Output

Cardiac output refers to the volume of blood the heart pumps per minute and is a key indicator of the heart’s function. It is determined by the heart rate (beats per minute) and stroke volume (the amount of blood pumped per beat). The heart adjusts both heart rate and stroke volume to meet the body’s demands:

• Heart Rate (Chronotropy):The heart can increase or decrease its rate of contraction in response to factors such as exercise, stress, or rest. The sinoatrial (SA) node acts as the natural pacemaker of the heart, controlling the rate at which the heart beats by generating electrical impulses that spread through the heart muscle.
• Stroke Volume (Inotropy):Stroke volume depends on the contractility of the heart muscle, the volume of blood filling the ventricles (preload), and the resistance the ventricles must overcome to eject blood (afterload). The heart can adjust its stroke volume by increasing the force of contraction or altering the volume of blood it pumps, depending on the body’s needs.

Regulation of Oxygen and Nutrient Delivery

The heart ensures that oxygen and nutrients are delivered to tissues efficiently by maintaining blood pressure and flow. Oxygenated blood from the left ventricle is distributed throughout the body via arteries and capillaries, supplying cells with essential nutrients. The continuous pumping action of the heart ensures that oxygen-depleted blood is returned to the right side of the heart, from where it is sent to the lungs for reoxygenation. This cycle ensures that all tissues and organs receive a constant supply of oxygen and nutrients for proper metabolic functioning.

Coordination with the Conduction System

The heart’s electrical conduction system coordinates the contraction and relaxation of the atria and ventricles, ensuring efficient blood flow. The sequence of electrical impulses begins in the sinoatrial (SA) node, spreads through the atria, and reaches the atrioventricular (AV) node. From there, the impulse travels down the bundle of His and through the Purkinje fibers, causing the ventricles to contract:

• SA Node: The pacemaker of the heart, the SA node initiates electrical signals that regulate the heart rate.
• AV Node: Acts as a relay station, delaying the impulse slightly to allow the atria to finish contracting before the ventricles contract.
• Bundle of His and Purkinje Fibers: Conduct electrical signals to the ventricles, ensuring a coordinated and forceful contraction that efficiently pumps blood out of the heart.

Regulation of Blood Pressure

The heart plays a critical role in maintaining blood pressure, which is necessary for blood to reach all parts of the body:

• Systolic Pressure: The force exerted by blood against the walls of the arteries during ventricular contraction (systole). This pressure is responsible for pushing blood through the arteries.
• Diastolic Pressure: The pressure in the arteries when the ventricles are relaxed (diastole). Diastolic pressure ensures that blood continues to flow between heartbeats and helps maintain consistent circulation.

The heart’s ability to adjust its output in response to the body’s changing demands—such as during exercise, stress, or rest—allows for the regulation of blood pressure and ensures that tissues receive an adequate supply of blood.

Maintaining Homeostasis

The heart is integral to maintaining homeostasis by ensuring that oxygen, nutrients, hormones, and waste products are transported efficiently throughout the body. By continuously circulating blood, the heart helps regulate body temperature, pH levels, and the distribution of essential electrolytes. It also plays a role in filtering waste products through organs like the kidneys and liver, which remove toxins from the bloodstream.

Clinical Significance

The heart is central to maintaining life, and its proper function is essential for sustaining circulation and oxygen delivery throughout the body. Any abnormalities in the heart’s structure or function can lead to serious cardiovascular conditions. Common heart-related disorders include:

• Coronary Artery Disease (CAD): Narrowing or blockage of coronary arteries due to plaque buildup, which can lead to myocardial infarction (heart attack) when the blood supply to the heart muscle is compromised.
• Heart Failure: A condition where the heart cannot pump blood efficiently, leading to fluid retention, fatigue, and reduced oxygen delivery to tissues. It can affect the left, right, or both sides of the heart.
• Arrhythmias: Abnormal heart rhythms due to dysfunction in the heart’s electrical conduction system, leading to bradycardia (slow heart rate), tachycardia (fast heart rate), or irregular rhythms such as atrial fibrillation.
• Valvular Heart Disease: Disorders of the heart valves, such as aortic stenosis, mitral regurgitation, or tricuspid insufficiency, can impede normal blood flow and increase the heart’s workload.