Middle Mediastinum

Question 1

Boundaries

Answer 1

Anterior: anterior pericardium 
Posterior: Posterior pericardium 
Lateral: Mediastinal pleura 
Inferior: diaphragm 
Superior: Transverse thoracic plane

Question 2

Contents

Answer 2

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

Question 3

Heart Development

Answer 3

Angiogenic clusters form in the mesoderm and these canalise to from early blood vessels
2 large blood vessels form in the cardiogenic area known as heart tubes
The heart tubes grow and fuse- by 23 the single heart tube begins to fold, which results in ventricles and outflow tracks positioned anteriorly and atria and veins posteriorly
Septa then grow and divide the heart into 4 chambers

Question 4

Heart Chambers

Answer 4

2 receiving atria, 2 discharging ventricles
Right atrium receives poorly oxygenated blood from SVC and IVS
Right ventricle discharges this into the lungs via pulmonary trunk
Left atrium receives well oxygenated blood from the pulmonary veins
Left ventricle discharges this to the body through aorta
Coordinated contractions of the atrial followed by the ventricles

Question 5

3 layers of chamber

Answer 5

Endocardium
Myocardium
Epicardium

Question 6

Fibrous skeleton

Answer 6

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 7

Pericardium

Answer 7

3 layered sac where heart is suspended in by the great vessels
Protects the heart from overfilling
Influenced by movement of the great vessels, sternum and diaphragm
Attaches to first part of great vessels which creates the transverse sinus

Question 8

Layers of pericardium

Answer 8

1. Fibrous pericardium (external)
2. Serous Pericardium (internal)
   a. parietal layer
   b. Visceral layer (or epicardium)

Question 9

Transverse Sinus

Answer 9

Space between outflow (AA and PT) and inflow vessels (SVC)

Question 10

Where is pericardial cavity located?

Answer 10

Between the parietal and visceral pericardium and contains small amounts of serous fluid

Question 11

Coronary circulation

Answer 11

Located in AV groove and interventricular groove

Right and left coronary arteries

Question 12

Right coronary artery branches

Answer 12

Sinu-atrial nodal
Right marginal
Posterior interventricular

Question 13

Left coronary artery branches

Answer 13

Anterior interventricular
Diagonal
Circumflex
Left marginal

Question 14

Anastomoses

Answer 14

These exist between small coronary branches of the RCA and LCA
E.g. End branches of anterior interventricular artery with posterior interventricular artery
There are abundant anastomoses but these don’t maintain substantial collateral circulation if major branch is occluded suddenly

Question 15

Coronary Artery dominance

Answer 15

Defined by the coronary artery which gives rise to posterior interventricular artery
RCA dominant - 67% of people
15% - LCS dominant where posterior IV artery from the circumflex artery Variation in the LCA and RCA common
4% individuals have an accessory CA
The SA nodal branch comes off the circumflex in 40% of people

Question 16

Venous drainage of the heart

Answer 16

Coronary Sinus: largest coronary vein 
Drains into right atrium via its own opening 
Receives 4 major cardiac veins:
1. Great 
2. Middle 
3. Small 
4. Posterior

Question 17

Conduction of nerve impulses

Answer 17

Impulse starts at SA node (pacemaker)
Causes the atrial muscle to contract
Impulses spread through atrial wall to atrio-ventricular node via internodal tracts
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 fibres

Question 18

SA node

Answer 18

collection of specialised cells at junction of SVC and right atrium

Question 19

AV node

Answer 19

located in interatrial septum close to coronary sinus opening

Question 20

Regulation of the conducting system

Answer 20

Regulated by ANS
Nerves from ANS destined for the heart enter the cardiac plexus
Preganglionic parasympathetic fibres enter and synapse in ganglia of cardiac plexus - postganglionic fibres exit plexus for the SA node
Postganglionic sympathetic fibres (T1-5 paravertebral ganglia) enter and exit the cardiac plexus for the SA node

Question 21

Ascending aorta

Answer 21

Oxygenated blood leaves left ventricle via the aorta

Question 22

Pulmonary trunk

Answer 22

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

Question 23

Pulmonary veins

Answer 23

Oxygenated blood from lungs to left atrium

2 veins from each side inferior and superior

Question 24

Vena cava

Answer 24

Blood enters heart via right atrium

Receives deoxygenated blood from SVC and IVC