3. CVS Development

Question 1

Describe the formation of the primitive heart tube.

Answer 1

Cariogenic field is created during gastrulation, at the cranial end of the embryo before folding.
Endocardial tubes develop within cardiogenic field.
Endocardial tubes fuse together during embryonic folding, creating the primitive heart tube.

Question 2

What are the segments of the primitive heart tube?

Answer 2

truncus arteriosus
bulbus cordis 
ventricle
atrium
sinus venosus

Question 3

What is the importance of ‘looping’ and what triggers it?

Answer 3

As the tube elongation is confined to the pericardial space, it bends on itself. This places the atria above and behind the primitive ventricles.

Question 4

Endocardial tubes develop within the cardiogenic field in week ____.

Answer 4

3

Question 5

What does the RA develop from?

Answer 5

Most of primitive atrium

Part of sinus venosus

Question 6

What does the LA develop from?

Answer 6

Part of primitive atrium

Absorbs proximal parts of pulmonary veins

Question 7

Why is the RA rough lined yet the LA is smooth lined?

Answer 7

Primitive atrium has a rough lining, and makes up most of the RA. Whereas the pulmonary veins are smooth and make up the LA.

Question 8

What is the main difference between the fetal circulation and the mature circulation?

Answer 8

Fetal lungs do not work

Oxygenation and CO2 removal occurs at the placenta

Question 9

How is the circulatory system adapted to meet both metal and mature requirements?

Answer 9

Shunts in fetal circulation which are reversible at birth.

Question 10

3 fetal shunts are used to by-pass which areas?

Answer 10

1. By-pass liver after placenta
2. Shunt from RA to LA (by-pass RV and lungs)
3. Shunt from PT to aorta (by-pass lungs)

Question 11

Why is it important that a small amount of blood enters the RV in the fetus?

Answer 11

Ensure normal development of the ventricle.

Shunt from pulmonary trunk to aorta to avoid lungs.

Question 12

Name the 3 fetal shunts.

Answer 12

1. Ductus venosus (by-pass liver)
2. Foramen ovale (RA to LA)
3. Ductus arteriosus (PT to A)

Question 13

Describe how the shunts are shut at birth.

Answer 13

At first breath, LA pressure> RA pressure which pushes the septum premium against the septum secundum to close the foramen ovale.
Ductus artiosus contracts and closes
Placental removal causes ductus venosus to close.

Question 14

How is the early arterial system arranged?

Answer 14

Bilateral symmetrical system of arched vessels

Question 15

Which arch is remodelled to give the ‘pulmonary arch’?

Answer 15

6th arch gives R and L pulmonary artery

Question 16

Which arch is remodelled to give the arch of aorta and subclavian artery?

Answer 16

4th arch

Question 17

Which nerve becomes wrapped around the ductus arteriosus shunt during the caudal shift?

Answer 17

Left recurrent laryngeal nerve

Question 18

What is patent ductus arteriosus (PDA)?

Answer 18

Ductus arteriosus remains open due to failure of physiological closure.
Blood will shunt L to R from aorta into PT after birth.

Question 19

What forms an essential scaffold to both atrial and ventricle septum development?

Answer 19

Endocardial cushions - neural crest cells which migrate into the developing heart and allow chamber development.

Question 20

Describe the formation of the atrial septum and foramen ovale.

Answer 20

1. Septum primum extends down from primitive atrium.
   Ostium primum at its free edge fuses with the cushions
   Ostium secondum remains.
2. Septum second extends up from cushions. This has a hole which doesn’t line up with ostium secondum, creating a shunt called the foramen ovale.

Question 21

What direction does the foramen ovale shunt blood?

Answer 21

R to L in fetus

Question 22

Suggest mechanisms which could lead to atrial septal defect.

Answer 22

Septum primum reabsorbed or too short

Septum secondum too small

Question 23

What is hypoplastic left heart syndrome?

Answer 23

Underdeveloped left ventricle

Question 24

What could cause hypoplastic L heart syndrome?

Answer 24

• Defect in development of mitral and aortic valves, causing atresia and limiting flow to LV.
• ostium secondum too small, so R to L flow inadequate.

Question 25

What are the 2 components of the ventricular septum?

Answer 25

Muscular and Membranous

Question 26

Describe how the ventricular septum is formed.

Answer 26

Muscular portion grows upwards towards endocardial cushions, leaving a small gap - primary inter ventricular foramen.
Membranous portion grows down from cushions to close the foramen.

Question 27

Which component of the ventricular septum is usually the cause of a ventricular septal defect?

Answer 27

Membranous

Question 28

What would be the effect of a ventricular septal defect?

Answer 28

L to R shunt

Question 29

How is a spiral septum in the outflow tract developed?

Answer 29

Endocardial cushions in the truncus arteriosus grow towards each other and twist to form a spiral septum, separating the outflow tracts.