Mediastinum Anatomy

Cardiac Arterial supply overview

• Coronary Arteries 

  • Right vs left coronary arteries

  • The first branches off aorta

  • F: supply myocardium/epicardium

Right Coronary Artery

(origin, course, supplies, anastomosis, artery branches)

• Origin - base of aorta

• Course - follows coronary (AV) sulcus

• Supplies: 

  • Right atrium

  • Most of right ventricle/part of left ventricle

  • Posterior ⅓ of interventricular septum

  • SA node

  • AV node 

• Anastamoses

  • Right PDA/RCA with left circumflex (posterior)

  • Right marginal and Right PDA with LAD/branches (inferior) 

• Artery branches

  • SA nodal artery

  • Conus artery

  • Atrial artery

  • Ventricular arteries

  • Right marginal artery

  • Posterior interventricular (descending) artery 

  • AV nodal artery 

Right marginal Artery

(origin, course, supplies, anastomosis)

• Origin - RCA

• Course - passes right → left along inferior border of heart toward apex 

• Supplies: 

  • Myocardium of the right lateral side of the heart 

• Anastomosis

  • Interventricular branches of LAD 

Posterior interventricular (descending) artery

(origin, course, supplies, anastomoses)

• Origin - RCA (in 67%)

• Course: runs in the posterior interventricular groove toward apex 

• Supplies: posterior ventricular walls

• Anastomoses: anterior interventricular branch of LCA/LAD 

Left coronary artery

(origin, course, supplies, anastomoses)

• Origin: base of aorta 

• Course: short, runs between left auricle and pulmonary trunk

  • Quickly bifurcates into: anterior interventricular (LAD) artery (branches) and circumflex artery (left marginal arteries) 

• Supplies: 

  • Most of left atrium/ventricle

  • ⅔ of interventricular septum

  • Atrioventricular bundles

  • May supply the AV node

• Anastomoses 

  • Same as RCA 

Left anterior descending artery (LAD)

(origin, course, branch, supplies, anastomoses)

• Anterior intraventricular artery

• Origin - LCA

• Course - passes along anterior interventricular groove to apex of heart

• Branches - diagonal 

• Supplies

  • Anterior walls of right/left ventricles 

  • Anterior ⅔ of interventricular septum

• Anastomoses 

  • Right marginal inferiorly

  • Right PDA posteriorly 

Circumflex artery

(origin, course, branch, supplies, anastomoses)

• Origin - LCA

• Course - passes to left in AV sulcus toward posterior surface of heart 

• Branches - marginal 

• Supplies 

  • Left atrium

  • Posterior wall of left ventricle 

• Anastomoses

  • Posterior IV/descending artery posteriorly 

Cardiac veous drainage overview

Overview 

• Venous blood is collected from capillary blood by venous veins whose paths follow those of the coronary arteries

  • Great cardiac vein (left anterior) 

  • Posterior vein of left ventricle (left posterior) 

  • Middle cardiac vein (posterior middle)

  • Small cardiac vein (left anterior) 

• These are direct tributaries to the coronary sinus

• Coronary sinus empties blood into right atrium 

Some venous blood is emptied directly into the right atrium via anterior cardiac veins

Great cardiac vein

• Begins at anterior apex 

• Ascends in the anterior IV groove

• Runs parallel to anterior IV (LAD) branch and the circumflex artery of LCA 

Posterior vein of left ventricle

• Begins at inferior end of left lateral boarder

• Runs up left lateral border

• Runs parallel to left marginal artery

Middle cardiac vein

(begins, ascends, parallel)

• Begins at posterior apex

• Ascends in posterior IV groove 

• Runs parallel to posterior IV (descending) branch of RCA 

Small cardiac vein

• Begins middle of inferior border of heart

• Runs around the right side of the heart and up 

  • Superolateral on the anterior surface

  • Superomedial on posterior surface

• Runs parallel to right marginal branch of RCA 

Cardiac plexus

(origin, branches, course)

• Origin 

  • Cervical and cardiac branches of vagal nerve (parasympathetic) 

    • Sets HR to 60-100

  • Sympathetic trunk/chain 

• Branches

  • Superficial cardiac plexus 

  • Deep cardiac plexus 

• Course 

  • Diffuse network of branches

  • Arch of aorta

  • Heart

  • Coronary arteries

  • SA node 

Lymphatic drainage

(lymphatic plexuses, passage, right vs left side drainage)

• Three lymphatic plexuses (one found in each layer) 

  • Subepicardial

  • Myocardial

  • Subendocardial

• Vessels from these plexuses follow coronary arteries

• Path: 

  • Lymph capillaries in heart tissue

  • Lymph vessel

  • Lymph nodes

    • Anterior mediastinal

    • Tracheobronchial 

    • Paratracheal

  • Right lymphatic duct or thoracic duct 

    • Right side of heart = thoracic duct (travels/crosses) 

    • Left side of heart = right lymphatic duct

Proximal branches of aorta

• Root of aorta 

• Right and left coronary arteries

• Ascending aorta

• Arch of aorta

  • Brachiocephalic artery* (ONLY ONE - no L vs R)

    • Right common carotid 

    • Right subclavian

      • Right internal thoracic

    • Left common carotid

    • Left subclavian 

      • Left internal thoracic 

Descending aorta branches

T4

• Bronchial arteries

• Posterior IC arteries

• Small branches to:

  • Mediastinium

  • Esophagus 

  • Pericardium 

Trajectory of aorta in thorax

• Begins at left ventricle of heart

  • Superior border of anterior surface of the heart

  • Between the SVC (right) and the pulmonary trunk (left) 

  • Anterior to the trachea and esophagus 

• Extends superiorly (ascending aorta) 

• 180 degree bend posteriorly and inferiorly (arch) 

  • Arches over pulmonary trunk, and left main bronchi, left recurrent laryngeal nerve

  • Slightly to the left 

Descending/thoracic aorta

• Continuation of thoracic aorta

• Begins at left side of T4 vertebrae 

• Descends in posterior mediastinum on left side of T5-T12

• Superior end is more lateral than inferior end 

• As it approaches the medial plane it displaces the esophagus to the right

• As the most inferior end it passes posterior to the esophagus as they both penetrate the diaphragm

  • T10 = esophagus 

  • T12 = aorta 

• Name change to abdominal aorta 

Main tributaries to SVC in thorax

• Right and left brachiocephalic veins

  • R and L internal thoracic veins

  • R and L vertebral veins

  • R and L external and internal jugular 

  • R and L subclavian vein 

• Azygos vein 

  • Right posterior IC vein

  • Accessory hemiazgos (left post IC 5-8) 

  • Hemiazygos (left post IC veins 9-12, lumbar v) 

Ligamentum arteriosum

Ligamentum arteriosum (connects aorta to pulmonary trunk via CT) 

• Remnant of fetal ductus arteriosus 

• Passes from root of left pulmonary artery to inferior surface of arch of aorta 

Mediastinum definition

• Medial cavity of thorax 

• Parts: superior and inferior (anterior, middle, and posterior) 

Borders of superior mediastinum

• Borders

  • Superior - superior thoracic outlet

  • Inferior - transverse thoracic plane (from sternal angle to T4/T5) 

  • Posterior - vertebrae T1-T4

  • Anterior - manubrium

  • Lateral - medial border of pleural sacs (left and right) 

• Conduit space to allow structures to pass between head/neck and thorax

• Large number of blood and lymph vessels 

contents of superior mediastinium

(organs, arteries, veins, lymphatics, nerves)

Contents of superior mediastinum 

• Organs

  • Thymus

  • Trachea

  • Esophagus

• Arteries

  • Arch of aorta

  • Brachiocephalic trunk

  • L common carotid artery

  • L subclavian artery

• Veins 

  • SVC 

  • R and L brachiocephalic veins

  • Arch of azygous vein

• Lymphatics 

  • Thoracic duct

• Nerves

  • L and R vagal

  • L recurrent laryngeal

  • L and R phrenic nerves 

Contents of superior mediastinum from anterior to posterior

• thymus/phrenics

• Great vessels 

  • Brachiocephalic veins

  • Brachiocephalic trunk/ left CC

  • Trachea / left subclavian 

  • Esopaghus 

  • Thoracic duct 

Left recurrent laryngeal nerve 

• Left vagal nerve descends inferiorly, passes anterior to the arch of the aorta

• Left recurrent laryngeal nerve is a branch that occurs just inferior to the arch of the aorta

• Passes inferior and posterior (under) the aorta then ascends superiorly

• Hooks under the aorta

• Immediately lateral and posterior to the ligamentum arteriosum

• Ascends to the larynx in the groove b/w the trachea and the esophagus

Right and left phrenic nerve

• Run just medial to the pleural cavities 

• Just lateral to the thymus 

• Anterior to the vagal nerves 

Anterior mediastinum borders

• Borders: 

  • Anterior- sternum (body and xiphoid), transversus thoracis muscles, and fifth, sixth, and seventh costal cartilages 

  • Posterior: pericardium 

  • Superior: transverse thoracic plane 

  • Inferior: diaphragm 

  • Lateral: medical border pleural sacs 

Contents of anterior mediastinum

• Organs: thymus (inferior portion)

• Arteries and veins: internal thoracic branches 

• Lymphatics: parasternal lymph nodes 

function of anterior mediastinum

• Smallest subdivision of the mediastinum 

• Protective function: filled with connective and fatty tissues that cushion and support the thymus and the vital structures posterior to it 

function of superior mediastinum

• Conduit space to allow structures to pass between head/neck and thorax

• Large number of blood and lymph vessels 

Border of middle mediastinum

• Boarders: (pericardium)

  • Anterior: anterior pericardial sac

  • Posterior: posterior pericardial sac

  • Superior: transverse thoracic plane

  • Inferior: diaphragm

  • Lateral: medial borders of pleural sacs

Function of middle mediastinum

• Houses heart and roots of great vessels

Contents of middle mediastinum

• Organs: 

  • Heart with its great vessel roots

  • Trachea with left and right main bronchi

• Arteries:

  • ascending aorta/ root 

  • pulmonary trunk

• Veins:

  • superior vena cava

  • pulmonary veins

• Nerves :

  • Right and left phrenic

Borders of posterior mediastinum

• Anterior: posterior pericardial sac

• Posterior: Bodies of T5-T12 vertebrae

• Superior: transverse thoracic plane

• Inferior: diaphragm

• Lateral: medial borders of pleural sacs

Contents of posterior mediastinum

• Organs:

  • Esophagus

• Arteries:

  • Thoracic aorta

• Veins:

  • Azygos/hemi-azygos veins

• Lymph:

  • Thoracic ducts/lymphatic trunks

• Nerves:

  • vagus, splanchnic, sympathetic chains

Function of inferior mediastinum

• Like the superior, it serves as a conduit , provides space for passage of structures between thorax and abdomen

Pericardium

• Located in the middle mediastinum

• It is a fibrous membrane covering the heart and the beginning of its great vessels

• Superior border is beyond the heart to the level of the sternal angle

• It is a closed sac composed of two layers:

  • Fibrous membrane (fibrous pericardium)

  • Serous membrane

    • parietal layer of serous pericardium

      • Pericardial cavity 

    • visceral layer of the serous pericardium

      • Epicardium 

Fibrous pericardium

(functions and borders)

• Tough external layer

• Protects the heart against sudden overfilling b/c it is so unyielding & closely related to the great vessels that pierce it superiorly

• Boarders

  • Continuous superiorly with the tunica adventitia of the great vessels entering and leaving the heart and with the pre-tracheal layer of deep cervical fascia

  • Attached anteriorly to the posterior surface of the sternum by the sternopericardial ligament

  • Bound posteriorly by loose connective tissue to structures in the posterior mediastinum

  • The inferior wall (floor) is firmly attached and confluent centrally with the central tendon of the diaphragm (pericardiacophrenic ligament)

Serous layer 

• Composed mainly of mesothelium, a single layer of flattened cells forming an

• Two layers, separated by small amount of fluid

• Parietal layer of serous pericardium

  • Lines the internal/deep surface of the fibrous pericardium

• Visceral layer of serous pericardium

  • Lines the myocardium of the heart, also called the epicardium

  • Forms the epicardium, the outermost of the three layers of the heart wall

  • Extends onto the beginning of the great vessels, blind pouch at the end, as it is continuous with the parietal layer

• Pericardial cavity formed between these 2 layers

  • contains pericardial fluid 

  • enables the heart to move and beat in a frictionless environment

Pericardial sinus

• Transverse pericardial sinus 

  • Can only be seen when pericardium is resected

  • Transverse passage within the pericardial cavity running between the arterial vessels (Aorta, pulm trunk) and the venous vessels (SVC, pulm veins)

• Oblique pericaridial sinus 

  • Wide pocket-like recess in the pericardial cavity posterior to the heart

  • Bounded laterally on one side with the IVC (inferior), and two right pulmonary veins

  • Bound laterally on other side by two left pulmonary veins

  • Bound anteriorly by the epicardium covering the left atrium

Vasculature of pericardium and nerve supply

Arterial supply

• Mainly Pericardiacophrenic arteries

  • Branch of the internal thoracic artery

  • Runs alongside phrenic nerve

Venous supply 

• Pericardiacophrenic veins 

  • Tributaries of the brachiocephalic veins 

• Variable tributaries of the azygos venous system 

Nerve supply 

• Fibrous and parietal pericardium 

  • Phrenic nerves (C3-C5)

  • Pain/ somatic sensation

  • Pain may refer to shoulder due to origin (dermatome is supraclavicular shoulder/lateral neck)

• Visceral pericardium

  • Minor contributions

  • Vagus nerves

  • Sympathetic trunks (T1-T4)

    • vasomotor

Heart shape and stats

• Heart about the size of person’s fist

• Weighs less than 1 pound

• Hollow, cone shaped

• Extends obliquely from 2nd rib to 5th intercostal space (12-14 cm)

• Center of heart lies to the left of the sternal edge

  • 2/3 of mass is to the left of MSL

External landmarks of heart

• The superior border 

  • Corresponds to a line connecting the inferior border of the 2nd left costal cartilage to the superior border of the 3rd right costal cartilage

• Inferior border 

  • Corresponds to a line drawn from medial aspect right 5th IC space to the left MCL

  • The 5th IC space on the left at the MCL is where the apex of the heart is anticipated to lie

Internal borders of heart

• Superior border:

  • Formed by the right / left atria and auricles

  • Ascending aorta and pulmonary trunk emerge from this border

  • SVC enters its right side

  • Level of 2nd costal cartilages

• Inferior border:

  • Formed mainly by the right ventricle

  • Slightly by the left ventricle

  • Left side ends as the apex

  • Level of 5th intercostal space

• Right border:

  • Slightly convex

  • Formed by the right atrium

  • Extending from the SVC to the IVC

• Left border:

  • Oblique, nearly vertical

  • Formed mainly by the left ventricle and slightly by the left auricle

Surfaces of heart

• Right pulmonary surface:

  • Formed mainly by the right atrium

• Left pulmonary surface:

  • Formed mainly by the left ventricle

  • It forms the cardiac impression in the left lung

• Anterior (sternocostal) surface:

  • Formed mainly by the right ventricle

• Diaphragmatic (inferior) surface:

  • Formed mainly by the left ventricle /partly by the right ventricle

  • It lies on the central tendon of the diaphragm

Base of heart

•  Is the posterior surface

• Broad flat base

• 9 cm wide, from T5-T8

• Directed toward the right shoulder

  • (base of the cone- opposite the apex)

• Formed mainly by the left atrium

(of note base sometimes is used to refer to the superior edge of this surface, at the 3rd intercostal space)

apex of heart

• Formed by the inferolateral part of the left ventricle

• Lies deep to the left 5th intercostal space in adults, at left MCL

  • average 9cm from the median plane

• Points inferior, points toward left hip

• Movement can be palpated from thoracic wall - apex beat

  • PMI- point of max impulse 

Layers of heart wall

• Epicardium 

  • A thin external layer (mesothelium) also known as visceral layer of serous pericardium 

• Myocardium 

  • Thickest layer of heart wall, especially thick in ventricles

  • Where contraction happens

  • Branching cardiac muscle cells, cardiomyocytes, run in in circular or helical shaped arrangements

  • Tethered together by crisscrossing connective tissue fibers arranged in spiral or circular bundles

  • Interlacing bundles link all parts of the heart together

  • Ventricles produce a wringing motion during contraction due to the double helical arrangement of the cardiac muscle fiber

• Endothelium

  • Lining membrane of the heart that also covers its valves 

  • A thin internal layer (endothelium wall and subendothelial connective tissue)

Chambers of heart

• 2 atria separated by interatrial septum 

• 2 ventricles separated by interventricular septum 

• 2 grooves on surfaces of heart that indicate boundaries of the chambers 

  • Coronary sulcus or atrioventricular groove 

    • above= atria, below= ventricles 

    • Holds right and left coronary arteries 

  • Interventricular sulcus 

    • Marks the IV septum anteriorly and posteriorly 

    • Anterior portion holds the anterior interventricular artery 

    • Posterior portion holds the posterior interventricular artery 

valves of heart

• Atrioventricular (AV) valves 

  • Tricuspid 

  • Bicuspid (mitral) 

• Semilunar (SL) valves 

  • Pulmonic 

  • Aortic 

Atria of heart (overview)

• The receiving chambers 

• Surface features: 

  • Each has an auricle which can increase atrial volume somewhat 

• Internal features: 

  • Posterior part

    • Smooth walled 

  • Anterior part

    • Pectinate muscle: bundles of muscles tissue form ridges (pectin= teeth of a comb)

Interatrial septum 

• Separates the atria

Fossa ovalis- shallow depression that was once the fetal opening- foramen ovale

External right atrium

• Forms the right border of the heart 

• Receives venous blood from the SVC and IVC

• Receives blood from coronary sinus 

  • Opening of coronary sinus, at the junction posterior IV sulcus, and the coronary sulcus 

• Anterior- right auricle 

• Posterior- sulcus terminalis 

  • Smooth and rough parts of the atrial wall are separated externally by a shallow vertical groove 

  • Runs vertically near vena cava 

Interior right atrium

• Posterior part is smooth (the sinus verarum) 

  • SVC/ IVC and coronary sinus open, bringing deoxygenated blood into the heart 

• Rough, muscular anterior wall composed of pectinate muscles 

• Fossa ovalis on posterior wall 

• Posterior and anterior walls  join together at a C shaped ridge- crista terminalis (terminal crest) 

  • Side of atria nearest vena cava 

• Right AV orifice to tricuspid valve 

• Discharges the deoxygenated blood into the right ventricle 

• Houses the sinoatrial (SA) node 

Left atrium

• For most of the base (posterior) of the heart 

• Receives oxygenated blood from the pulmonary veins 

• Four pulmonary veins (2 superior/ 2 inferior) entering posterior wall 

• External 

  • Left auricle 

• Inferior of the left atrium 

  • Mostly smooth walled 

  • Pectinate muscles going only in auricle 

  • Fossa ovalis on anterior wall

  • AV orifice to mitral valve 

• Discharges oxygenated blood into left ventricle 

Ventricles overview

• The discharging chambers 

  • Pumping 

  • Walls much thicker than atria 

• Make up most of the volume of the heart 

• Trabeculae carneae

  • Irregular ridges of muscle 

  • Mark internal wall of chambers 

• Papillary muscles 

  • Muscle bundles that play a role in valve function 

Interventricular septum 

•  Composed of muscle and membranous parts 

• Strong, obliquely placed 

• Right and left ventricles each forming part of the wall 

• Lies to right of midline due to higher pressures in LV 

• Muscular part of the interventricular septum forms the majority of the septum 

• Membranous part of the interventricular septum is smaller, thin membrane, part of the fibrous skeleton of heart found superiorly/ posteriorly 

Right ventricle

• Forms the largest part of the anterior surface of the heart, smallest part of the diaphragmatic surface 

• Receives deoxygenated blood from the right atrium 

• Right AV orifice

  • Surrounded by one of the fibrous rings of the fibrous skeleton of the heart 

  • Fibrous ring keeps the caliber of the orifice constant resisting the dilation 

Tricuspid valve (overview)

• Tricuspid valve → chordae tendineae → 3 papillary muscles 

• Has trabeculae carneae along the walls 

• Chordae tendineae (tendinous cords) 

  • Numerous tiny white collagen cords that originate from a papillary muscle and connect it to a flap of the valve 

• Papillary muscles- tricuspid 

  • Conical muscular projections with bases attached to the ventricular wall 

  • 3 papillary muscles corresponds to the cusps of tricuspid valves 

    • Anterior papillary muscle 

    • Posterior papillary muscle 

    • Septal papillary muscle 

Papillary muscle of tricuspid valve

Anterior papillary muscle 

• Largest and most prominent of the 3 

• Arises from the anterolateral wall of the right ventricle 

• It is tendinous cords attach to the anterior and inferior cusps of the tricuspid valve 

Inferior papillary muscle 

• Arises from the inferior wall of the right ventricle 

• Its tendinous cords attach to the inferior and septal cusps of the tricuspid valve 

septal/ medial papillary muscle 

• Arises from the interventricular septum 

• Its tendinous cords attach to the anterior and septal cusps of the tricuspid valve 

Septomarginal trebecula

• Connects IV septum to anterior wall of RV 

• Carries part of the right branch of the AV bundle (conducting system) 

Supraventricular crest-

•  thick muscular ridge superior aspect of chamber 

  • Separates the ridged muscular wall of the inflow part of the chamber from the smooth wall of the conus arteriosus, or outflow part 

Pulmonic orifice

• Surrounded by one of the fibrous rings of the fibrous skeleton of the heart 

• Fibrous rings keep the caliber of the orifice constant resisting the dilation 

• Pulmonary trunk begin at orifice 

• Discharges deoxygenated blood into pulmonary trunk → lungs 

Tricuspid valve

• Atroventricular (AV) valve 

• Has 3 flexible cusps or flaps 

  • Anterior (anterosuperior) 

  • Septal 

  • Inferior (mural) 

• Endocardium reinforced with connective tissue cores 

• Prevents backflow from the right ventricle to the right atria 

• Located in the 5th IC space 

  • Posterior to sternum, extends just past right border sternum 

Pulmonic valve

Pulmonic valve (semilunar (SL) valve)

• Prevents backflow from the pulmonary trunk to the right ventricle 

• 3 pocketlike cusps, each shaped like a crescent moon 

• Connective tissue with endocardial membrane 

• Open and close due to pressure changes 

• Occurs at the apex of the conus arteriosus 

• Posterior to sternum at the level of the left 3rd costal cartilage 

Left ventricle

• Forms the apex of the heart, nearly all its left (pulmonary) surface, and border and most of the diaphragmatic surface 

• Performs more work than the right ventricle, due to much higher pressure 

• 2-3x thicker walls 

• Cavity longer than the right ventricle 

• Receives oxygenated blood from left atrium 

Trabeculae carneae + Left AV orifice

• Left ventricle

• Trabeculae carneae 

  • Finer and more numerous than those of the right ventricle 

• Left AV orifice 

  • Surrounded by one of the fibrous rings of the fibrous skeleton of the heart 

  • Fibrous ring keeps the caliber of the orifice constant resisting the dilation  

Mitral/bicupsid valve papillary muscle

• Mitral valve → chordae tendineae → 2 papillary muscles 

• Papillary muscles are longer than those in the right ventricle 

  • Only 2 papillary muscles 

  • Anterior 

  • Posterior 

• Two cusps (anterior/ posterior)

• Prevents backflow from the left ventricle to the left atria 

• Each cusps receives tendinous cords from both papillary muscles

PM Chords Cusps 

A —----->             A

A —------>   P

P —------>   A

P —------->   P

• Endocardium reinforced with connective tissue cores 

• Located to the left of the sternum at 4th costal cartilage 

aortic vestibule + aortic orifice

• Aortic vestibule- smooth walled, superior outflow part, leading to the aortic orifice and aortic valve 

• Aortic orifice surrounded by a fibrous ring to which the right posterior and left cusps of the aortic valve are attached 

  • Fibrous ring keeps the caliber of the orifice constant resisting the dilation

  • Ascending aorta begins at the aortic orifice

• Discharges of oxygenated blood into aorta → body 

Aortic valve

Aortic Valve (semilunar valve)

• Prevents backflow from the aorta into the left ventricle

• 3 pocketlike cusps, each shaped like a crescent moon

• Connective tissue with endocardial membrane

• Open and close due to pressure changes

• Occurs at the apex of the aortic vestibule

• Located posterior to the left side of the sternum at the level of the 3rd intercostal space