Review Sheet 30 Anatomy Of The Heart

The human heart, a marvel of biological engineering, tirelessly pumps life-sustaining blood throughout the body. Understanding its intricate anatomy is crucial for healthcare professionals, students, and anyone curious about the wonders of the human body. This review sheet delves into the key anatomical features of the heart, providing a comprehensive overview to aid in comprehension and retention.

External Anatomy: The Heart's Outer Landscape

The heart, roughly the size of a clenched fist, is located in the mediastinum, the space between the lungs in the thoracic cavity. Its pointed apex rests on the diaphragm, while its broader base lies beneath the second rib. The heart isn't perfectly vertical; it's rotated slightly, with the right side more anterior than the left.

• Pericardium: The heart is enclosed within a double-layered sac called the pericardium.
  • The fibrous pericardium, the outer layer, is tough and inelastic, protecting the heart and anchoring it to surrounding structures.
  • The serous pericardium, the inner layer, is further divided into two layers: the parietal layer (fused to the fibrous pericardium) and the visceral layer (also known as the epicardium), which adheres directly to the heart's surface.
  • The pericardial cavity, a space between the parietal and visceral layers, contains a small amount of serous fluid that lubricates the heart, reducing friction during contractions.
• Surface Features:
  • Auricles (Atrial Appendages): These wrinkled, pouch-like structures on the anterior surface of the atria slightly increase their volume.
  • Sulci (Grooves): These grooves on the heart's surface mark the boundaries of the chambers and contain coronary blood vessels and fat.
    • The coronary sulcus (atrioventricular groove) encircles the heart and separates the atria from the ventricles.
    • The anterior interventricular sulcus and posterior interventricular sulcus separate the right and left ventricles.

Internal Anatomy: Chambers, Valves, and Vessels

The heart comprises four chambers: two atria (receiving chambers) and two ventricles (pumping chambers). Valves ensure unidirectional blood flow, and major vessels connect the heart to the systemic and pulmonary circulations.

Atria: The Receiving Chambers

The atria are thin-walled chambers that receive blood returning to the heart.

• Right Atrium: Receives deoxygenated blood from three major sources:
  • Superior Vena Cava (SVC): Returns blood from the head, neck, upper limbs, and thorax.
  • Inferior Vena Cava (IVC): Returns blood from the trunk, viscera, and lower limbs.
  • Coronary Sinus: Collects blood draining from the heart muscle itself.
• Left Atrium: Receives oxygenated blood from the lungs via four pulmonary veins (two from each lung).
• Internal Structures:
  • Pectinate Muscles: Ridges of muscle tissue lining the anterior walls of the atria.
  • Interatrial Septum: A wall separating the right and left atria.
    • Fossa Ovalis: A shallow depression in the interatrial septum, a remnant of the foramen ovale, an opening present in the fetal heart that allows blood to bypass the non-functional fetal lungs.

Ventricles: The Pumping Chambers

The ventricles are thick-walled chambers that pump blood out of the heart.

• Right Ventricle: Pumps deoxygenated blood to the lungs via the pulmonary trunk.
• Left Ventricle: Pumps oxygenated blood to the entire body via the aorta. The left ventricle is significantly thicker than the right ventricle, reflecting its greater workload in pumping blood against higher systemic pressure.
• Internal Structures:
  • Trabeculae Carneae: Irregular ridges of muscle lining the ventricular walls.
  • Papillary Muscles: Cone-shaped muscles projecting from the ventricular walls. These muscles attach to the chordae tendineae.
  • Chordae Tendineae: Tendon-like cords connecting the papillary muscles to the cusps of the atrioventricular valves. These cords prevent the valve cusps from everting (bulging back) into the atria during ventricular contraction.
  • Interventricular Septum: A thick wall separating the right and left ventricles.

Valves: Ensuring Unidirectional Blood Flow

The heart has four valves that ensure blood flows in only one direction: two atrioventricular (AV) valves and two semilunar (SL) valves.

• Atrioventricular (AV) Valves: Located between the atria and ventricles, these valves prevent backflow of blood into the atria when the ventricles contract.
  • Tricuspid Valve: Located between the right atrium and right ventricle. It has three cusps (flaps).
  • Bicuspid (Mitral) Valve: Located between the left atrium and left ventricle. It has two cusps.
• Semilunar (SL) Valves: Located at the base of the pulmonary trunk and the aorta, these valves prevent backflow of blood into the ventricles when they relax.
  • Pulmonary Valve: Located between the right ventricle and the pulmonary trunk. It has three semilunar cusps.
  • Aortic Valve: Located between the left ventricle and the aorta. It has three semilunar cusps.

Major Vessels: Highways of Blood Flow

Several major blood vessels are connected to the heart, facilitating the flow of blood to and from the lungs and the rest of the body.

• Aorta: The largest artery in the body, arising from the left ventricle. It carries oxygenated blood to the systemic circulation.
  • Ascending Aorta: The initial portion of the aorta.
  • Aortic Arch: Curves over the heart, giving rise to major arteries that supply the head, neck, and upper limbs.
  • Descending Aorta: Continues down through the thorax and abdomen, supplying blood to the trunk and lower limbs.
• Pulmonary Trunk: Arises from the right ventricle and carries deoxygenated blood to the lungs.
  • It quickly divides into the right and left pulmonary arteries, each carrying blood to one lung.
• Pulmonary Veins: Carry oxygenated blood from the lungs to the left atrium. There are typically four pulmonary veins, two from each lung.
• Venae Cavae (Superior and Inferior): The superior vena cava returns deoxygenated blood from the upper body to the right atrium, while the inferior vena cava returns deoxygenated blood from the lower body to the right atrium.
• Coronary Sinus: Returns deoxygenated blood from the heart muscle itself to the right atrium.

Cardiac Muscle and Heart Wall Structure

The heart wall is composed of three layers: the epicardium, myocardium, and endocardium.

• Epicardium (Visceral Pericardium): The outermost layer, composed of a single layer of epithelial cells and a thin layer of connective tissue.
• Myocardium: The middle and thickest layer, composed of cardiac muscle tissue. This layer is responsible for the heart's pumping action. Cardiac muscle cells are:
  • Striated: Like skeletal muscle, cardiac muscle exhibits striations due to the arrangement of actin and myosin filaments.
  • Involuntary: Unlike skeletal muscle, cardiac muscle contraction is not under conscious control.
  • Intercalated Discs: Specialized junctions that connect cardiac muscle cells, containing gap junctions and desmosomes. Gap junctions allow for rapid spread of electrical impulses, enabling coordinated contraction of the heart. Desmosomes provide strong adhesion between cells, preventing separation during contraction.
• Endocardium: The innermost layer, lining the heart chambers and covering the valves. It is composed of a thin layer of epithelial cells and a thin layer of connective tissue. It is continuous with the endothelium of the blood vessels.

The Heart's Conduction System: The Electrical Orchestra

The heart has a unique intrinsic conduction system that ensures coordinated and efficient contraction. This system consists of specialized cardiac muscle cells that generate and conduct electrical impulses.

• Sinoatrial (SA) Node: Located in the right atrium, near the entrance of the superior vena cava. It is the heart's primary pacemaker, generating impulses at a rate of 60-100 beats per minute. These impulses initiate each heartbeat.
• Atrioventricular (AV) Node: Located in the interatrial septum, near the tricuspid valve. It receives impulses from the SA node. The AV node delays the impulse slightly, allowing the atria to contract completely before the ventricles begin to contract.
• Atrioventricular (AV) Bundle (Bundle of His): A bundle of specialized fibers that originates from the AV node and extends down the interventricular septum. It is the only electrical connection between the atria and ventricles.
• Right and Left Bundle Branches: The AV bundle divides into right and left bundle branches, which run along the interventricular septum towards the apex of the heart.
• Purkinje Fibers: A network of fibers that extend from the bundle branches and penetrate the ventricular myocardium. They transmit the impulse rapidly throughout the ventricles, causing them to contract almost simultaneously.

Coronary Circulation: Nourishing the Heart Muscle

The heart, like any other organ, requires its own blood supply. This is provided by the coronary circulation, which consists of the coronary arteries and cardiac veins.

• Coronary Arteries: Branch off the aorta just above the aortic valve and supply oxygenated blood to the heart muscle.
  • Right Coronary Artery (RCA): Supplies the right atrium, right ventricle, and part of the left ventricle.
  • Left Coronary Artery (LCA): Divides into the left anterior descending (LAD) artery and the circumflex artery.
    • Left Anterior Descending (LAD) Artery: Supplies the anterior walls of both ventricles and the interventricular septum.
    • Circumflex Artery: Supplies the left atrium and the posterior wall of the left ventricle.
• Cardiac Veins: Drain deoxygenated blood from the heart muscle and return it to the right atrium via the coronary sinus.
  • Great Cardiac Vein: Runs alongside the LAD artery.
  • Middle Cardiac Vein: Runs alongside the posterior interventricular artery.
  • Small Cardiac Vein: Runs alongside the right marginal artery.

Clinical Significance: When the Heart's Anatomy Matters

Understanding the anatomy of the heart is crucial for diagnosing and treating various cardiovascular conditions.

• Myocardial Infarction (Heart Attack): Occurs when a coronary artery becomes blocked, depriving the heart muscle of oxygen. The most common cause is atherosclerosis, a buildup of plaque in the arteries. The location of the blockage determines the area of heart muscle that is damaged. For example, a blockage in the LAD artery can lead to significant damage to the anterior wall of the left ventricle, potentially causing severe heart failure.
• Valve Disorders: Valves may become stenotic (narrowed), restricting blood flow, or incompetent (leaky), allowing backflow. These disorders can lead to heart failure if the heart is forced to work harder to compensate. Valve disorders can often be treated with medication or surgery, including valve replacement or repair.
• Congenital Heart Defects: Structural abnormalities present at birth, such as a septal defect (a hole in the interatrial or interventricular septum) or transposition of the great arteries (the aorta and pulmonary artery are switched). These defects can disrupt normal blood flow and oxygenation. Many congenital heart defects can be corrected with surgery.
• Arrhythmias: Irregular heart rhythms caused by disruptions in the heart's electrical conduction system. Arrhythmias can range from mild to life-threatening. They can be treated with medication, pacemakers, or implantable cardioverter-defibrillators (ICDs).
• Cardiomyopathy: Disease of the heart muscle that can weaken the heart's ability to pump blood. Cardiomyopathy can be caused by genetic factors, infections, or other conditions.

Common Questions About Heart Anatomy

• What is the purpose of the pericardium? The pericardium protects the heart, anchors it to surrounding structures, and reduces friction during contractions.
• Why is the left ventricle thicker than the right ventricle? The left ventricle pumps blood to the entire body against higher systemic pressure, requiring more muscular strength.
• What is the function of the chordae tendineae and papillary muscles? They prevent the AV valves from everting into the atria during ventricular contraction.
• What is the sinoatrial (SA) node? The heart's primary pacemaker, generating electrical impulses that initiate each heartbeat.
• What happens if a coronary artery is blocked? It can lead to a myocardial infarction (heart attack), depriving the heart muscle of oxygen and causing damage.
• What is the coronary sinus? A large vein on the posterior surface of the heart that collects blood draining from the heart muscle itself and returns it to the right atrium.
• What are the pectinate muscles? Muscular ridges located on the anterior wall of the right and left atria.
• What are the trabeculae carneae? Irregular muscular ridges located on the inner surfaces of the ventricles.
• How does the heart receive its own blood supply? Via the coronary arteries, which branch off of the aorta.
• What is the importance of intercalated discs in cardiac muscle? They contain gap junctions that allow for rapid spread of electrical impulses, enabling coordinated contraction, and desmosomes that provide strong adhesion between cells.

Conclusion: A Symphony of Structure and Function

The anatomy of the heart is a complex and fascinating subject. Understanding its external and internal structures, chambers, valves, vessels, conduction system, and coronary circulation is essential for comprehending its function and for diagnosing and treating cardiovascular diseases. This review sheet has provided a comprehensive overview of the heart's anatomy, serving as a valuable resource for students, healthcare professionals, and anyone interested in learning more about this vital organ. The heart truly is a marvel of engineering, a testament to the intricate beauty and remarkable functionality of the human body.