S3: development of the CVS

Question 1

Describe the formation and looping of the primitive heart tube

Answer 1

A pair of tubes (endocardial tubes) develops within the cardiogenic field in the 3rd week
The endocardial tubes are brought together during embryonic folding and fuse in the mid-line to create the primitive heart tube
The primitive heart tube is linear at first, receiving blood (inflow) at its caudal pole and pumping blood (outflow) from its cranial pole

Question 2

What are the four sections of the primitive heart tube?

Answer 2

Truncus arteriosus
Primitive ventricle
Primitive atrium
Sinus venosus

Question 3

Describe the formation of the inter-atrial septum

Answer 3

• septum primum grows downwards towards the endocardial cushions
• gap between septum primum and endocardial cushions: ostium primum
• septum primum continues to grow until is binds to the endocardial cushions -> undergoes apoptosis to produce ostium secondum
• ostium secondum = blood can pass bi-directionally but only want a one-way shunt
• this is down through the growth of the septum secondum (hole formed is now called the foramen ovale)

Question 4

Describe the formation of interventricular septum

Answer 4

Large muscular component grows up from the ventricular wall

Small membranous portion grows down from the endocardial cushions (if something goes wrong, it is usually this part)

Question 5

Describe the concept of right to left shunting in the foetus

Answer 5

In the foetus there are certain tissues that can be bypassed since the placenta performs their functions for them
Eg. lungs can be bypassed as foetus does not breathe air, liver

Question 6

Describe the fate of the three foetal shunts

Answer 6

When the foetus takes its first breaths, resistance in pulmonary circulation drops and the pressure within the left atrium increases -> oval foramen closed
Foramen ovale = fossa ovalis
Ductus arteriosus = ligamentum arteriosum
Ductus venosus = ligamentum venosum

Question 7

Describe in brief the development of the great vessels

Answer 7

Ridges of tissues form the spiral septum which grows in a helical way partitioning the blood (within the truncus arteriosus)
Separates the blood into the aorta on the left and the pulmonary arteries on the right

Question 8

Describe what a patent ductus arteriosus is

Answer 8

Ductus arteriosus fails to close in a child -> excessive mixing of blood between pulmonary and systemic circuits

Question 9

Describe the transposition of the great arteries

Answer 9

Origins of the aorta and pulmonary trunk and aorta are swapped (aorta comes from RV, PT comes from LV)
Spiral septum adds an extra 180 degrees to its twist (defects in neural crest cell development may contribute)
Need to use pharmacological agents to maintain the potency of the ductus arteriosus keeping it open and allowing mixing of blood

Question 10

Describe cardiac looping of the primitive heart tube

Answer 10

The process by which the adult disposition of the heart is achieved begins with the process of looping of the primitive heart tube
Looping places both the inflow and outflow cranially with the inflow dorsal to (behind) the outflow

Question 11

Describe which arteries are developed from which aortic arches

Answer 11

There 5 aortic arches in the embryo
Both pulmonary arteries are developed from the sixth aortic arch
The adult aortic arch is developed from the 4th aortic arches
Right subclavian artery is developed from the 4th aortic arch
Common carotid is derived from the 3rd arch and portions of the ventral aorta

Question 12

Describe where the laryngeal nerves are in relation to the aortic arches

Answer 12

Left laryngeal nerve finds itself trapped hooked around the ductus arteriosus, but on the right side this has disappeared
Therefore, right laryngeal nerve loops around the right subclavian (sits higher up) and left laryngeal nerve loops around the arch of the aorta

Question 13

Name the 3 important shunts in foetal circulation and their location

Answer 13

• ductus arteriosus: pulmonary trunk to aorta
• foramen ovale: right to left atrium
• ductus venosus: bypass liver (blood goes straight to the inferior vena cava)

Question 14

Describe the septation of the atrioventricular canal

Answer 14

Endocardial cushions help to separate the atria and ventricles & help support the valves stay in place (grow out from the dorsal and ventral aspects of the AV canal and separate into two halves)
This will need to fuse with the interventricular and interatrial septa = full septation of the heart

Question 15

Describe what a patent foramen ovale is

Answer 15

Septum primum and septum secondum have failed to fuse together after birth
Can be seen in adults -> can manifest as DVT + presents as a stroke

Question 16

Describe what a hypoplastic left heart is

Answer 16

Congenital heart defect
Inadequate blood flow from right to left (left side receiving nowhere near as much blood as it needs)
Consequence: right ventricle becomes the sole ventricle supplying both circuits (grossly enlarged pulmonary trunk)
If diagnosed pre-natally: need to prevent ducutus arteriosus from closing

Question 17

Describe the Tetralogy of Fallot

Answer 17

1) Overriding aorta
2) Pulmonary stenosis (PT is too small)
3) Right ventricular hypertrophy (gets bigger as its tries to push blood through narrow outflow)
4) Ventricular septal defect (spiral septum doesn’t line up with the endocardial cushions)