Mediastinum: Heart, Superior & Inferior

what are the two primary regions of the mediastinum?

superior: trachea, esophagus, large blood vessels

inferior: anterior → thymus, middle → pericardium, heart, ascending aorta, pulmonary trunk, primary bronchi, posterior → esophagus, large blood & lymphatic vessels

where is the superior mediastinum located?

above the line from sternal angle to lower border of T4, below superior aperture

what are the contents of the superior mediastinum?

thymus, veins, arteries, airway, alimentary tract, lymphatic trunk

what are the characteristics of the thymus?

small, glandular organ (becomes fatty tissue by ~age 60)

consisting mainly of lymphatic tissue and serving as the site of T-cell differentiation

located in the inferior part of the neck & anterior part of the superior mediastinum

rich arterial supply

anterior mediastinal branches of the internal thoracic arteries

what are the veins of the superior mediastinum?

brachiocephalic veins

superior vena cava

what are the arteries of the superior mediastinum?

ascending arch of aorta

R brachiocephalic trunk → R subclavian, R common carotid

L common carotid

L subclavian

what are the airway and alimentary components of the superior mediastinum?

trachea → anterior

esophagus → posterior

what are the nerves of the superior mediastinum?

parasympathetic → vagus

right vagus → right recurrent laryngeal nerve

left vagus → left recurrent laryngeal nerve

sympathetic nerves from T1-T4

phrenic nerve

what are the divisions of the inferior mediastinum?

anterior, posterior, middle

what are the contents of the anterior-inferior mediastinum?

inferior thymus in young (remnants in old)

branches of the internal thoracic artery

lymph nodes and lymph fat

sternopericardial ligaments

what are the contents of the posterior-inferior mediastinum?

thoracic aorta

thoracic duct and lymphatic trunks

azygos and hemiazygos veins

posterior intercostals arteries

esophagus and its plexus

sympathetic chain

where does the thoracic aorta travel in the inferior mediastinum?

begins on the left side ~T4 vertebra and descends on the left sides of the T5-T12 vertebrae

what are the branches of the thoracic aorta in the inferior mediastinum?

paired posterior intercostal arteries (all but upper two intercostal spaces)

subcostal artery

paired phrenic artery (superior)

singular visceral branches (esophageal, bronchial)

what are the characteristics of the thoracic duct?

it is the largest lymphatic vessel in the body

it collects most of the lymph in the body and drains into the systemic (blood) circulation at the left subclavian vein

typically 38-45cm in length and an average diameter of about 5mm

where does the azygos vein travel?

right side and enters the thorax through the aortic hiatus in the diaphragm

where does the hemiazygos vein travel?

left inferior side enters the thorax through the left crus of the diaphragm

where does the accessory hemiazygos vein travel?

left superior side, ribs 4-8, 9

where does the left superior intercostal vein travel?

left superior ribs 1-4

what is the azygos vein system?

an H-shaped venous system that drains deoxygenated blood from the thorax and abdominal wall into the superior vena cava

what are the posterior intercostal arteries?

11 pairs of intercostal arteries

located in the intercostal spaces

where does the esophagus travel?

passes from the superior mediastinum posterior to and to the right of the arch of the aorta and posterior to the pericardium and left atrium and passes through the esophageal hiatus in the diaphragm at the level of T10

what is the esophageal plexus?

vagus nerve, sympathetic ganglia, greater splanchnic nerve, which is distal to the bifurcation of the trachea

what are the components of the pericardium?

layered closed sac surrounding and protecting the heart

fibrous pericardium

serious pericardium

pericardial cavity

what are the functions of the fibrous pericardium?

prevents overstretching of heart

protects and anchors heart in mediastinum

what are the characteristics of the serious pericardium?

double layer

parietal pericardium (superficial) connects to fibrous pericardium

visceral pericardium tightly adheres to heart muscle

where is the pericardial cavity located?

between the parietal & visceral pericardium & houses pericardial fluid

what are the characteristics of the heart?

central component of CV system

functions to propel blood through vessels

hollow, cone-shaped organ the size of a person’s fist

housed within the middle-inferior mediastinum

what are the characteristics and function of the right ventricle?

pumps blood to lungs (deoxygenated)

thinner, lower pressure

triangular shape

what are the characteristics and function of the left ventricle?

pumps blood to the body (oxygenated)

thicker/higher pressure

conical shape

what are the names of the AV valves?

right AV (tricuspid)

left AV (mitral or bicuspid)

what are the names of the semilunar valves?

pulmonary semilunar

aortic semilunar

what are the papillary muscles?

conical muscles attached to the wall of the ventricle that insert on the chordae tendinae

contract just prior to ventricular contraction

what are the chordae tendinae?

tendon-like chords connecting papillary muscles to ventricular surfaces of valves

cusps close due to pressure associated with ventricular contraction (prevent backflow into atrium)

CT prevents valve cusps from opening into atria during ventricular contraction

what are the branches of the right coronary artery?

supplies small branches to atrium and divides inferiorly

SA nodal branch

right marginal branch (in coronary sulcus, supplies only right ventricle)

posterior interventricular branch (PAD - posterior descending artery) → follows posterior interventricular sulcus, supplies walls of the two ventricles

what are the branches of the left coronary artery?

left anterior interventricular branch (LAD - left anterior descending) (both ventricles)

left diagonal branch (off LAD)

left marginal branch

circumflex branch (left atrium & left ventricle)

how do the functional syncytiums of the heart functions (atrial & ventricular)?

stimulation of one syncytium causes contraction of all cells in that network → atria contract as a unit, ventricles contract as a unit

atrial and ventricular syncytiums are exclusive except for a small area in floor of right atrium (where AV node is)

what are the characteristics of the SA node?

located near right atrial wall

“pacemaker” sets HR

initiates each heartbeat

stimulation from SA node spread over both atria and to AV node

electrical signal → atrial musculature within atrial syncytium, travels via gap junctions within intercalated discs

what are the characteristics of the AV node?

located in interatrial septum (floor of right atrium)

small area on right atria floor where atrial and ventricular syncytia are not exclusive

only normal electrical pathway between atrial and ventricular syncytia

delayed signal transmission, allows for complete atrial emptying and ventricular filling prior to contraction

what are the characteristics of the atrioventricular bundle?

aka AV bundle or Bundle of His

large group of fibers sitting in interventricular septum

receives electrical signal from AV node

spreads electrical signal over ventricles

does not cause ventricular contraction

AV bundle enters upper part of interventricular septum and divides into right and left bundle branches

what are the characteristics of the right and left bundle branches?

receives electrical signal from AV Bundle of His

sends electrical signal to purkinje fibers

what are the characteristics of the Purkinje fibers?

arise from bundle branches midway down septum

large sized fibers → allows for rapid signal transmission to distant regions of ventricular myocardium compared to cell-to-cell conduction, high speed of transmission allows for functional syncytium

initially, transmits signal to apex of heart

facilitates ventricular contraction → contraction begins at apex, thus pushes blood superiorly toward aortic and pulmonary semilunar valves

opposite would occur with cell-cell transmission

what is meant by the heart’s self-excitability?

specialized cardiac muscle fibers are auto-rhythmic

spontaneously generate action potentials (electrical signals)

what is the extrinsic conduction system of the heart?

autonomic nervous system (cardiac plexus)

increase or decrease HR, does not initiate contraction

sympathetic (T1-4 maybe T5) & parasympathetic divisions (CN X Vagus)

what components provide chemical regulation for the heart?

circulating hormones, especially from the adrenal medulla & ions