Middle Mediastinum

Question 1

Boundaries

Answer 1

Anterior – anterior pericardium
Posterior – posterior pericardium
Lateral – mediastinal pleura
Inferior - diaphragm

Question 2

Contents

Answer 2

Heart & pericardium
Ascending aorta
Pulmonary trunk & arteries 
Pulmonary veins
Nerves
Lymph nodes

Question 3

Heart development

Answer 3

Angiogenic clusters form in the mesoderm and these canalise to form early blood vessels.
Two large blood vessels form in the cardiogenic area known as the heart tubes.
The heart tubes grow and fuse and by day 23 the single heart tube begins to fold
Resulting in ventricles and outflow tracks positioned anteriorly and atria and veins posteriorly - the heart as we know it!
Septa then grow to divide the heart into 4 chambers.

Question 4

Heart Chambers

Answer 4

Right atrium receives poorly oxygenated blood from the SVC and IVC
Right ventricle discharges this to the lungs via the pulmonary trunk
Left atrium receives well oxygenated blood from the pulmonary veins
Left ventricle discharges this to the body initially through the aorta

Coordinated contractions of the Atria followed by the Ventricles - Cardiac cycle

Question 5

Fibrous skeleton

Answer 5

Framework of 4 fibrous rings of dense connective tissue
Provides attachment for AV and semi-lunar valves
Forms an electrical insulator between the atria and ventricles

Question 6

Pericardium

Answer 6

Layers: 
Fibrous pericardium (external)
Serous pericardium (internal)
- Parietal layer 
- Visceral layer (or Epicardium)

3 layered sac which heart is suspended in by the great vessels
Protects heart from overfilling
Influenced by movement of the great vessels, sternum & diaphragm

Attaches to first part of great vessels which creates the Transverse Sinus: space between outflow (AA + PT) and inflow vessels (SVC)

Question 7

Coronary circulation

Answer 7

Heart requires its own blood supply and this is provided by coronary arteries and veins

Located in the atrioventricular groove and interventricular grooves

RCA gives rise to branches: 
- Sinu-atrial nodal 
- Right marginal 
- Posterior interventricular
LCA gives rise to branches:
- Anterior interventricular
- Diagonal
- Circumflex
- Left marginal

Question 8

Dominance

Answer 8

Dominance defined by the coronary artery which gives rise to the Posterior IV artery.
RCA dominant - 67% of individuals.
15% individuals - LCA dominant where Posterior IV artery branches from the Circumflex artery.

Question 9

Variation

Answer 9

Variation in the LCA and RCA common.
4% individuals have an accessory CA.
40% population the SA nodal branch comes off the circumflex

Question 10

Venous drainage

Answer 10

The Coronary Sinus: largest coronary vein
Drains into right atrium via its own opening
Receives four major Cardiac veins:
Great
Middle
Small
Posterior

Question 11

Nerve impulse conduction

Answer 11

Impulse commences at Sinu-atrial (SA) node – aka The Pacemaker.

SA node: collection of specialised cells at junction of SVC and right atrium.

Excitation signals produced by the SA node cause atrial muscle to contract.

Impulses spread through atrial wall to atrio-ventricular (AV) node via internodal tracts.

AV node located in interatrial septum close to coronary sinus opening.

AV node continues directly into AV bundle (of His) penetrating the fibrous skeleton.

AV bundle divides in muscular inter-ventricular septum and continues into the walls of the ventricles to the Purkinje Fibers.

Question 12

Regulation of conducting system

Answer 12

Innervation of the myocardium is regulated by ANS

Nerves from the ANS destined for the heart enter the Cardiac Plexus
preganglionic parasympathetic fibres (Vagus nerve) enter and synapse in ganglia of cardiac plexus – postganglionic fibers exit plexus for the SA node
postganglionic sympathetic fibers (T1-5 paravertebral ganglia) enter and exit the cardiac plexus for the SA node

Question 13

Ascending Aorta

Answer 13

Oxygenated blood leaves left ventricle via the aorta

• Ascending (middle mediastinum)
• Arch (superior mediastinum)
• Descending thoracic (posterior mediastinum)
• Descending abdominal

Question 14

Pulmonary trunk

Answer 14

Deoxygenated blood leaves heart via pulmonary trunk
Branches into left and right pulmonary arteries
Enter hilum/root the lung

Question 15

Pulmonary veins

Answer 15

Oxygenated blood from lungs to left atrium

Two veins from each side Inferior and Superior

Question 16

Vena Cavae

Answer 16

Blood enters the heart via right atrium

Receives deoxygenated blood from Sup. and Inf. vena cava

Question 17

Angiogenic clusters

Answer 17

Angiogenic clusters form in the mesoderm and these canalise to form early blood vessels.

Question 18

Transverse Sinus

Answer 18

Pericardium attaches to first part of great vessels which creates the Transverse Sinus: space between outflow (AA + PT) and inflow vessels (SVC)