ANA124: Heart I - Lecture Review

ANA124: Heart I - Comprehensive Study Notes

The Mediastinum

  • The mediastinum is the central compartment of the thoracic cavity, located between the lungs.

  • Image Credit: The HIVE, UBC (hive.med.ubc.ca, clinicalanatomy.ca)

  • Boundaries and Contents of the Mediastinum:

    • Superior Mediastinum:

      • Contains the superior aspect of the Esophagus.

      • Houses the Trachea.

      • Includes the Superior Vena Cava (SVC).

      • Contains the arch of the Aorta.

    • Inferior Mediastinum:

      • This large division is further subdivided into three regions:

        • Anterior Mediastinum:

          • Primarily contains the Thymus gland.

        • Middle Mediastinum:

          • This is the most significant part of the inferior mediastinum and is centrally located in the chest between the two lungs.

          • Its main contents include:

            • The Heart itself.

            • The Great Vessels that enter and leave the heart.

            • The Pericardium, which is the protective sac covering the heart.

            • The Phrenic Nerves, which are typically found adherent to the pericardium.

        • Posterior Mediastinum:

          • Contains the inferior aspect of the descending aorta.

          • Houses the Inferior Vena Cava (IVC).

          • Includes the inferior portion of the Esophagus.

Layers of the Heart

  • The heart wall consists of three distinct layers, surrounded by a protective sac.

  • A. Pericardium:

    • The covering that encases the entire heart and the roots of the great vessels.

    • It is composed of 33 main layers:

      • Fibrous Pericardium:

        • The tough, inelastic, and protective outer layer.

        • Often referred to as the pericardial sac.

        • Its fibrous nature anchors the heart within the mediastinum and prevents its overfilling with blood.

        • At the superior aspect, it fuses with the adventitia of the great vessels.

      • Serous Pericardium:

        • A thinner, more delicate double-layered membrane located deep to the fibrous pericardium.

        • Parietal Layer of Serous Pericardium: The outer part of the serous pericardium, lining the inner surface of the fibrous pericardium.

        • Visceral Layer of Serous Pericardium (Epicardium): The inner part of the serous pericardium, which is directly fused to the surface of the heart muscle.

      • Pericardial Cavity:

        • A potential space located between the parietal and visceral layers of the serous pericardium.

        • Contains a small amount of serous fluid that lubricates the moving surfaces, allowing the heart to beat with minimal friction.

  • B. Epicardium:

    • This is synonymous with the visceral layer of the serous pericardium.

    • It is a thin, elastic layer that provides additional protection against friction during cardiac contractions.

    • Significantly, it contains the coronary vessels (arteries and veins) that nourish the heart muscle itself.

  • C. Myocardium:

    • The thickest and most crucial layer of the heart wall.

    • Composed of specialized cardiac muscle tissue.

    • It is the primary contractile tissue of the heart, responsible for generating the force needed to pump blood.

  • D. Endocardium:

    • The smooth, innermost layer that lines the chambers of the heart (atria and ventricles).

    • It also covers the heart valves.

    • Its smooth surface minimizes friction as blood flows through the heart and prevents blood clot formation.

Heart Location

  • The heart is positioned obliquely within the middle mediastinum.

  • Shape: The overall shape of the heart is described as trapezoidal.

  • Location on the Chest Wall:

    • The heart's borders can be approximated to the 5th5^{th} intercostal space on both the left and right sides of the sternum.

    • Apex: The inferior-most, pointed part of the heart, which typically points to the left of the sternum.

    • Base: The broadest part of the heart, formed primarily by the left atrium, which is oriented posteriorly.

  • Oblique Axis: An imaginary line that extends through the apex, the interventricular (IV) septum, and the base (specifically the left atrium) of the heart, reflecting its angled position in the thorax.

Heart Anatomy: Anterior View Features

  • A. Right Atrium:

    • Receives deoxygenated blood from the systemic circulation.

    • Auricle: A small, ear-shaped muscular appendage on the anterior surface, which helps to increase the capacity of the atrium and regulate blood flow into the ventricle.

    • Superior Vena Cava (SVC): A large vein that drains deoxygenated blood from the head, neck, upper limbs, and thorax into the right atrium.

  • B. Right Ventricle:

    • Pumps deoxygenated blood to the pulmonary circulation.

    • Pulmonary Trunk: A large artery emerging from the right ventricle, carrying deoxygenated blood to the lungs. It quickly bifurcates into the right and left pulmonary arteries.

  • C. Left Atrium:

    • Receives oxygenated blood from the pulmonary circulation.

    • Auricle: Similar in function to the right auricle, it aids in regulating blood flow to the left ventricle.

  • D. Left Ventricle:

    • Pumps oxygenated blood to the entire systemic circulation.

    • Aorta: The largest artery in the body, originating from the left ventricle. It arches superiorly and then descends to supply blood to the rest of the body.

      • Branches of the Aortic Arch (visible proximally):

        • Brachiocephalic Trunk: The first branch, supplying blood to the right head, neck, and right arm.

        • Left Common Carotid Artery: The second branch, supplying blood to the left head and neck.

        • Left Subclavian Artery: The third branch, supplying blood to the left arm, and parts of the head and thorax.

  • Cardiac Sulci (Grooves):

    • Coronary Sulcus (Atrioventricular Groove): A prominent groove that encircles the heart, separating the atria superiorly from the ventricles inferiorly. It contains several coronary arteries and cardiac veins.

    • Anterior Interventricular Sulcus (LAD Groove):

      • A visible groove on the anterior surface of the heart, marking the boundary between the left and right ventricles.

      • It houses the Left Anterior Descending (LAD) coronary artery (also known as the "widowmaker" due to its critical supply) and the Great Cardiac Vein.

Heart Anatomy: Posterior View Features

  • Right Atrium:

    • Superior Vena Cava (SVC): Collects deoxygenated blood from the upper half of the body (head, neck, arms, and chest) and delivers it to the right atrium.

    • Inferior Vena Cava (IVC): Collects deoxygenated blood from the lower half of the body and returns it to the right atrium.

  • Right Ventricle:

    • Right & Left Pulmonary Arteries: These branches from the pulmonary trunk enter the lungs. The right pulmonary artery supplies the right lung, and the left pulmonary artery supplies the left lung, carrying deoxygenated blood for oxygenation.

  • Left Atrium:

    • Right & Left Pulmonary Veins: Typically, two from each lung (total of four), these veins carry freshly oxygenated blood from the lungs back to the left atrium.

  • Left Ventricle:

    • Arch of Aorta Branches (mentioned for origin/supply):

      • Brachiocephalic Trunk: Supplies the head, neck, and left arm (note: the transcript here lists 'Left arm' for brachiocephalic, which is usually right arm, but maintaining fidelity to the transcript).

      • Left Common Carotid Artery.

      • Left Subclavian Artery.

  • Cardiac Sulci (Grooves):

    • Coronary Sulcus: Continues posteriorly, separating the atria from the ventricles.

    • Posterior Interventricular Sulcus:

      • A groove on the posterior surface that separates the left ventricle from the right ventricle.

      • It contains the Posterior Interventricular Artery (also known as the Posterior Descending Artery) and the Middle Cardiac Vein.