Heart Anatomy and Mediastinum

Overview of the Mediastinum

• The mediastinum is a space within the thorax containing the heart and surrounding structures.

• Divided into subdivisions that help categorize the organs and tissues.

Subdivisions of the Mediastinum

• Superior Mediastinum: No subdivisions; contains major structures including trachea, esophagus, and major blood vessels (arch of aorta, brachiocephalic trunk).

• Inferior Mediastinum: Divided into three subdivisions:

  • Anterior Mediastinum: Narrow space between the sternum and heart; contains a small amount of connective tissue.

  • Middle Mediastinum: Contains heart and surrounding major blood vessels.

  • Posterior Mediastinum: Space behind heart, includes structures like the esophagus and descending aorta.

Boundaries of the Mediastinum

• Superior Boundaries: Rib number one (above).

• Inferior Boundaries: Diaphragm (below).

• Anterior Boundary: Sternum.

• Posterior Boundary: Vertebral column.

Major Structures in the Mediastinum

Superior Mediastinum Contains:

• Trachea: Air passage from larynx.

• Esophagus: Food passage located posterior to trachea.

• Arch of aorta: Major blood vessel giving rise to three branches:

  • Brachiocephalic trunk: Supplies blood to right arm and right side of the head.

  • Left common carotid artery: Supplies blood to left side of head.

  • Left subclavian artery: Supplies blood to left arm.

• Brachiocephalic veins: Drain blood from arms and head.

• Phrenic nerve: Innervates the diaphragm; arises from the cervical plexus.

• Vagus nerve (Cranial nerve X): Innervates various thoracic and abdominal structures.

• Thymus gland: Located primarily in the anterior mediastinum.

Middle Mediastinum

• Contains the heart encased within pericardial membranes.

• Major blood vessels entering and exiting the heart.

Structures in the Posterior Mediastinum

• Esophagus: Continues from superior mediastinum.

• Descending aorta: Continuation of aorta after it arches over the left main bronchus.

• Thoracic duct: Major lymphatic vessel draining lymph into the venous system.

Features of the Heart

• The heart is surrounded by three layers of connective tissue important for its normal function:

  • Epicardium (outer layer),

  • Myocardium (muscle layer),

  • Endocardium (inner layer).

• The heart is the first organ to start functioning in an embryo (approximately 4 weeks).

• It is the last organ to stop working, remaining functional until death.

Internal Structure and Chamber Functions

• Right Atrium: Receives deoxygenated blood from the body via the superior and inferior vena cavae.

  • Contains fossa ovalis, a remnant of a fetal connection between atria.

• Right Ventricle: Pumps deoxygenated blood to the lungs through the pulmonary arteries.

• Left Atrium: Receives oxygenated blood from the lungs via pulmonary veins.

• Left Ventricle: Pumps oxygenated blood to the body through the aorta.

Heart Valves

• Atrioventricular (AV) Valves:

  • Tricuspid valve (right AV valve).

  • Bicuspid (Mitral) valve (left AV valve).

• Semilunar Valves:

  • Pulmonary semilunar valve (leads to pulmonary trunk).

  • Aortic semilunar valve (leads to aorta).

Papillary Muscles and Chordae Tendineae

• Papillary muscles: Prevent valve inversion during ventricular contraction using chordae tendineae.

• Trabeculae carneae: Muscular ridges in ventricle walls.

Blood Flow Pathway Through the Heart

• Deoxygenated blood enters right atrium from the body via the superior and inferior vena cavae → right ventricle → pulmonary artery → lungs for oxygenation → oxygenated blood returns to left atrium via pulmonary veins → left ventricle → aorta → body.

Anatomic Location of the Heart

• The heart is located in the middle mediastinum of the thoracic cavity.

• Surface markings:

  • Upper Right Angle: 3rd intercostal space, 1 inch from midline.

  • Upper Left Angle: 2nd intercostal space, 1 inch from midline.

  • Lower Right Angle: 6th intercostal space, 1 inch from midline.

  • Lower Left Angle: 5th intercostal space, 3-4 inches from midline.

Surfaces of the Heart

• Anterior surface: Faces lungs and chest wall.

• Inferior (diaphragmatic) surface: Sits on diaphragm.

• Posterior surface (base): Opposite the apex and faces the posterior thoracic wall.

  • Contains coronary sinus, draining venous blood from heart into the right atrium.

• Coronary sulcus: Groove separating atria and ventricles, encircling the heart.

• Anterior interventricular sulcus: Separates the left and right ventricles on the anterior surface.

• Posterior interventricular sulcus: Separates ventricles on the inferior surface.

Clinical Connections

• Conditions affecting the heart, such as mitral valve issues leading to dysphagia due to esophageal compression, exemplify the importance of understanding heart anatomy in clinical practice.

Coverings of the Heart

• Layers of Connective Tissue Around the Heart

  • The heart is surrounded by multiple layers of connective tissue.

  • During embryonic development, an empty space is initially occupied by a large balloon-like structure that accommodates the developing heart.

  • This balloon represents the mediastinum.

  • The heart develops into this space, pushing into the balloon without being contained inside it.

• Serous Membranes

  • Two Layers Surrounding the Heart:

  • Parietal Serous Layer (Serous Pericardium):

    • The outer layer that does not contact the heart directly.

  • Visceral Serous Layer (Epicardium):

    • The inner layer that is in direct contact with the heart.

  • Function: To reduce friction between the heart and surrounding structures.

• Pericardial Cavity

  • The space between the parietal and visceral layers filled with a small amount of serous fluid, which lubricates the two layers during heart movement.

• Fibrous Pericardium

  • An additional connective tissue layer on the outside, providing mechanical protection to the heart.

  • It is composed of dense, irregular connective tissue, similar to dura mater in the brain, providing stability and protection.

• Clinical Significance: Pericarditis

  • Inflammation of the serous membranes surrounding the heart, referred to as periocarditis, can cause the membranes to produce excess fluid, leading to pressure on the heart (cardiac tamponade).

  • This can be life-threatening and requires immediate medical attention, typically involving the extraction of fluid to relieve pressure.

Cardiac Conductive System

• Composition

  • Consists of modified myocardial cells (not neurons) capable of generating electrical impulses and contracting.

• Key Components

  • Sinoatrial Node (SA Node):

  • Located in the right atrium, it acts as the heart's natural pacemaker, initiating impulses that cause atrial contraction.

  • Atrioventricular Node (AV Node):

  • Receives impulses from the SA Node, located at the junction of the right atrium and ventricle.

  • Atrioventricular Bundle (Bundle of His):

  • Transmits impulses from the AV Node down through the interventricular septum, splitting into right and left bundle branches.

  • Purkinje Fibers:

  • Terminal branches that run throughout the ventricles, ensuring coordinated contractions.

• Autonomic Regulation

  • The cardiac plexus, comprised of sympathetic fibers (T1-T4 spinal cord segments) and parasympathetic fibers (from the vagus nerve), regulates heart rate based on physical activity levels.

• Referred Pain

  • Pain from the heart can be referred to the shoulder due to shared nerve pathways between the heart and upper body regions, causing confusion in pain perception.

  • Referred pain patterns may differ by sex, with women often reporting pain in the right shoulder and men in the left.

Arterial Supply to the Heart

• Coronary Arteries

  • The heart receives oxygenated blood through the right and left coronary arteries, which branch off from the ascending aorta.

  • Right Coronary Artery:

  • Supplies the right ventricle and posterior part of the heart.

  • Continuation enters the posterior interventricular sulcus as the posterior interventricular artery.

  • Left Coronary Artery:

  • Shorter artery that divides into the circumflex artery (supplying the left ventricle) and the anterior interventricular artery (left anterior descending, or LAD).

  • LAD is critical and known as the “widow maker” if blocked.

• Coronary Dominance

  • Dominance can either be right (70% of the population) or left based on which coronary artery supplies the posterior interventricular groove.

Venous Drainage of the Heart

• Coronary Sinus

  • Collects deoxygenated blood from the heart via three major veins:

  • Small Cardiac Vein: Located along the right margin.

  • Middle Cardiac Vein: Sits in the posterior interventricular groove.

  • Great Cardiac Vein: Located in the anterior interventricular groove.

  • The coronary sinus drains into the right atrium, along with blood from the superior and inferior venae cavae.

Clinical Implications of Atherosclerosis

• Atherosclerosis

  • A condition characterized by the narrowing of arteries due to the deposition of fat and calcium, which can occur as early as infancy.

• Diagnostic Procedures

  • Angiograms help visualize arterial blockages and narrowing.

• Treatment Options

  • Angioplasty: A minimally invasive procedure to widen narrowed arteries using a balloon catheter.

  • Stent Placement: Insertion of a metal mesh to keep the artery open.