Thorax.2 Mediastinum subdivisions, Anterior middle mediastinum

Mediastinum Overview

  • Central portion located between two pleural sacs.

  • Limited by mediastinal pleura on each side.

  • Boundaries:

    • In front: Sternum

    • Behind: T1-T12 vertebrae

    • Above: Thoracic inlet

    • Below: Diaphragm

Mediastinum Divisions

  • Superior Mediastinum

  • Inferior Mediastinum: Divided into three subdivisions:

    • Anterior Mediastinum

    • Middle Mediastinum

    • Posterior Mediastinum

Anterior Mediastinum

  • Boundaries:

    • In front: Body of the sternum

    • Behind: Pericardium

    • Above: Imaginary horizontal plane from the sternal angle to lower border of T4

    • Below: Diaphragm

    • On each side: Mediastinal pleura

  • Contents:

    • Superior and inferior sterno-pericardial ligaments

    • Thymus gland (primary lymphoid organ, located behind manubrium)

    • Loose areolar tissue

    • Retrosternal lymph nodes

Middle Mediastinum

  • Widest subdivision of the mediastinum.

  • Contents:

    • Heart enclosed within the pericardium

    • Phrenic and deep cardiac nodes

    • Nerves: plexus of nerves

    • Lymph nodes

    • Arteries: Ascending aorta, pulmonary trunk, bifurcation into right and left pulmonary arteries

    • Veins: Lower part of superior vena cava, arch of azygos vein, and 4 pulmonary veins

    • Tubes: Bifurcation of trachea, right and left bronchi

Pericardium

  • Structure:

    • A conical fibroserous sac enclosing the heart and roots of great vessels.

    • Composed of two layers:

      • Fibrous Pericardium:

        • Fibrous tissue; maintains thoracic positioning of the heart.

        • Relations: apex continuous with tunica adventitia, base blends with diaphragm central tendon, anteriorly attached to sternum by sterno-pericardial ligaments.

      • Serous Pericardium:

        • Closed sac lined by mesothelium, consists of visceral and parietal layers, covering the heart and great vessels.

        • Pericardial cavity: Between the two layers, filled with pericardial fluid, allowing movement.

Clinical Consideration

  • Pericarditis: Inflammation causing chest pain, affecting the serous pericardium.

Nerve Supply of Pericardium

  • Fibrous pericardium and parietal layer: Supplied by phrenic nerve; pain-sensitive.

  • Visceral layer: Supplied by sympathetic fibers; insensitive to pain.

Blood Flow through the Heart

  • Pathway:

    • Deoxygenated blood through superior/inferior vena cava to the right atrium.

    • To right ventricle, then to lungs via pulmonary arteries.

    • Oxygenated blood returns through pulmonary veins to left atrium.

    • To left ventricle, then to body through aorta.

Heart External Features

  • Chambers: 4 (2 atria, 2 ventricles)

  • Surfaces:

    • Sternocostal (anterior)

    • Diaphragmatic (inferior)

    • Left surface

  • Borders:

    • Superior, Inferior, Right, Left

  • Apex: Formed by left ventricle; located in left 5th intercostal space.

  • Base: Formed by two atria; openings of pulmonary veins and vena cava visible.

Heart Valves

  • Atrioventricular Valves (AV): Maintain one-way blood flow

    • Bicuspid (Mitral) valve: Between left atrium and ventricle

    • Tricuspid valve: Between right atrium and ventricle

  • Semilunar Valves:

    • Aortic valve (left ventricle to aorta)

    • Pulmonary valve (right ventricle to pulmonary trunk)

Blood Supply and Innervation of the Heart

  • **Blood Supply: }

    • Coronary Arteries: Right and Left coronary arteries.

    • Drainage: Cardiac veins & coronary sinus.

  • Innervation: Sympathetic and parasympathetic nerves forming the cardiac plexus.

    • Parasympathetic: Decreases heart rate.

    • Sympathetic: Increases heart rate.