The middle mediastinum, the heart and its embryology

Question 1

The layers of the heart chambers

Answer 1

Endocardium- lines the inside of the the heart chamber.

Myocardium- The cardiac muscle tissue. The middle layer.

Epicardium- the outer layer of the heart.

Question 2

Pericardium

Answer 2

The the membrane that encloses the heart, composed of there layers.

The external layer- Fibrous pericardium. Tough tissue that minimises overstretching with prevents the heart from overfilling.

The serous pericardium:

• Parietal layer: adheres to the fibrous pericardium
• Visceral layer: adheres to the epicardium, shiny external appearance.

The serous pericardial layers are separated by the pericardial cavity.

The pericardium is attached to the great vessels of the heart, the sternum and the diaphragm. Therefore movement in those structures, move the pericardium.

Question 3

Transverse sinus

Answer 3

Within the pericardial sac, it is the space between the aortic arch and pulmonary trunk.

This space is clinically significant in surgeries as it allows a clamp to be placed to stop outflow of blood while still allowing inflow of blood.

Question 4

The middle mediastinum boundaries.

Answer 4

Anterior- anterior mediastinum.

Posterior- posterior mediastinum.

Laterally- the mediastinal pleura

Inferiorly- Diaphragm

Superior- thoracic transverse place

Question 5

Contents of the middle mediastinum

Answer 5

The heart and the pericardium

Ascending aorta

Pulmonary trunk and arteries.

Pulmonary veins

Nerves

Lymph nodes

Question 6

Embryological origin of the heart

Answer 6

Developed from the mesoderm layer:
1. Angiogenic clusters from mesoderm layer grow and for heart tubes.

2. The heart tube grows and then fuses. The superior region, truncus arteriosus, is the primitive outflow tracks. The sinus venosus, inferiorly, is the primitive inflow tracks.
3. The primitive ventricle is below the truncus arteriosus, with the primitive atria below. The tube becomes too large for the pericardial space.
4. This causes the tube to fold with the atria and inflow tracks folding to the back. Therefore the atria and veins are folded posteriorly.

5, The ventricles and arteries are placed anteriorly.

6. The septa grows and divide to form the 4 heart chambers.

The heart is functioning at 23 days.

Question 7

Fibrous skeleton of the heart

Answer 7

Framework of insulating fibrous tissue in the walls of the heart- Contains four rings.
Location- forms boundaries between the atria and ventricles. Surrounds the start of the pulmonary trunk and aorta.

1. Provides attachment for valve in the heart (4 rings= 4 valves).
2. Separates atria from ventricles.
3. Provides electrical insulation between the atria and ventricles.
4. Provides a framework for the attachment of cardiac tissue.

Question 8

Coronary arteries

Answer 8

Coronary arteries aire located in the atrioventricular and interventricular groove.

The coronary arteries arise from the ascending aorta: right and left coronary arteries.

Question 9

Right coronary arteries

Answer 9

Branches from the aorta and supply the right side of the heart. Located on the atrioventricular groove/ coronary sulcus

Branches:
Sinoatrial nodal branch of the RCA- next to the SAN
Marginal branch of the RCA
Posterior, interventricular branch of the RCA

Question 10

Left coronary arteries

Answer 10

Branches from the aorta before bifurcating and forming many branches to supply the left wall of the heart.
Located on the interventricular groove.

Branches:
Circumflex branch of the LCA
- Left marginal branch of the circumflex branch

Anterior interventricular branch of the LCA
- Diagonal branch of the interventricular branch.

Question 11

Anastomoses of coronary arteries

Answer 11

Exist between the small branches of the RCA and LCA.

Example- connection between the end branches of the anterior interventricular artery connect with the posterior interventricular artery.

Question 12

Coronary artery dominance

Answer 12

This is the artery that gives rise to the posterior interventricular artery.

In 67% of people, this is the RCA, making them RCA dominant.
In 15% of people, this occurs from the circumflex branch of the LCA, making them LCA dominant.

Question 13

Variation in coronary arteries

Answer 13

RCA/ LCA dominance- the artery that gives rise to the posterior interventricular artery.

4% of individuals have accessory coronary arteries.

40% of individuals have the sinu-atrial nodal branch from the circumflex artery.

Question 14

Anastomoses of coronary arteries

Answer 14

Exist between the small branches of the RCA and LCA.

Example- connection between the end branches of the anterior interventricular artery connect with the posterior interventricular artery.

Question 15

Coronary artery dominance

Answer 15

This is the artery that gives rise to the posterior interventricular artery.

In 67% of people, this is the RCA, making them RCA dominant.
In 15% of people, this occurs from the circumflex branch of the LCA, making them LCA dominant.

Question 16

Variation in coronary arteries

Answer 16

RCA/ LCA dominance- the artery that gives rise to the posterior interventricular artery.

4% of individuals have accessory coronary arteries.

40% of individuals have the sinu-atrial nodal branch from the circumflex artery.

Question 17

Venous drainage of the heart

Answer 17

The coronary sinus is the largest vein in the heart, located posteriorly that enters the right atrium in its own opening.

Connected to 4 major cardiac veins:

• Great vein—> drains the left wall anteriorly.
• Middle vein—> drains the right/middle, posteriorly
• Posterior vein—> drains the left wall posteriorly
• Small veins—> drains the right wall, anteriorly.

Question 18

Condution of nerve impulses in the heart

Answer 18

1. Impulse is initiated at the SAN (the pacemaker). SAN is located where the superior vena cava meets the right atrium.
2. This impulse is spread across the atrial walls causing them to contract simulateously. The impulse is stopped from spreading to the ventricle via fibrous skeleton.
3. Impulse travels to the AVN in the interatrial septum, near the coronary sinus, via internodal tracts.
4. AVN divides in the interventricular septum and impulse transmitted down the bundle of His and branch out into Purkinje fibres in ventricular wall. Causing the ventricular walls to contract simultaneously.

Question 19

Regulation of the conducting system in the heart

Answer 19

The ANS regulates the speed at which SAN generates the impulse but does not initiate the impulse. Nerves enter the cardiac plexus before entering the SAN.

Parasympathetic control:
Vagus nerve

Sympathetic control:
T1-5 paravertebral nerves.

Question 20

Aorta

Answer 20

Exit vessel for oxygenated blood from the heart.

Ascending aorta- middle mediastinum

Arch of aorta- superior mediastinum

Descending abdominal aorta

Descending thoaracic aorta

Question 21

Pulmonary trunk

Answer 21

Main exit vessel of deoxygenated blood from the heart, from the right ventricle . Located in the middle mediastinum.

Branches into the left and right pulmonary arteries which enter the hilum of the lung.

Question 22

Formation of the foramen ovale

Answer 22

In the primitive heart of a foetus, the foramen ovale is the hole between the atria.

1. The septum primum forms between both atria.
2. A section of the septum primum disintegrates to form the ostium secundum.
3. Another wall forms parallel to the septum primum called the septum secundum.
4. A hole forms in the septum secundum- foramen ovale. More of the septum primum breaks down. This wall then acts as a valve for the flow of blood from the right to left atrium.

Question 23

Vena Cavae

Answer 23

Main entry of deoxygenated blood to the heart.

Inferior- drains from the lower part of the body. Mainly in lumabr region, not that long in the abdominal region.

Superior- drains from the upper region of the body receives blood from the left and right brachiocephalic veins

Question 24

Circulation prebirth

Answer 24

Four main difference in foetus compared to adult circulation:

1. The intestines are bypassed as the the foetus receives nutrients from the mother’s blood through the umbilical artery.
2. The liver is mainly bypassed as the foetus also receives filtered blood from the mother. Therefore the umbilical vein is connected to the ductus venosus which connects directly to the liver
3. The blood enters the right atrium, some of the blood crosses to the left atrium via the foramen ovale.
4. When blood is ejected from the right atrium, some of the blood cross across the ductus arteriosus which connects the pulmonary vein to the aortic arch.

Question 25

Fossa ovalis

Answer 25

A depression in the right atrium left after the closing of the foramen ovale from the foetal heart.

Foramen ovale once allowed blood to bypass the lungs to the left atrium.

Question 26

Circulation prebirth

Answer 26

Four main difference in foetus compared to adult circulation:

1. The intestines are bypassed as the fetus receives nutrients from its mother and the blood goes to the umbilical veins.
2. The some of the blood from the liver is shunted from the left portal vein to the inferior vena cava, via ductus venosus.
3. The foramen ovale between the atria shunts blood from right to left atrium, allowing most of it to bypass the lungs.
4. The ductus arteriosus shunts blood from the pulmonary arteries to the aorta, which also bypasses the non-functioning lung.