Development of heart and great vessels Flashcards
Where do the blood islands lie?
In mesoderm
on both sides
What are the blood islands surrounded by?
Cavity
What do the blood islands initially form?
Endocardial tubes
What do the blood islands eventually form?
Blood cells
Small vessels
Heart
What does the cavity form?
The pericardium
What happens to the endocardial tubes?
They meet each other in the midline
fuse together
to form the primitive heart tube
by lateral folding of the embryo
What are the zones of the primitive heart tube?
Sinus venosus Atrium Ventricle Bulbous cordis Truncus arteriosus Aortic roots
What is the direction of blood flow in the primitive heart tube?
From the sinus venosus to the aortic roots
What happens to the primitive heart tube after it has formed?
Undergoes cardiac looping
Why does cardiac looping occur?
Primitive heart tube is growing within pericardial sac
not enough space for it to keep growing vertically
folds up to maximise its use of space in the pericardial sac
How does cardiac looping occur?
Sinus venosus and atrium move superior and posteriorly
Ventricle, bulbous cordis, truncus arteriosus, aortic roots all move anteriorly and inferiorly
What has cardiac looping done to the position of the atrium, ventricle, inflow and outflow vessels?
Placed them at correct orientation to each other
What happens to the primitive heart tube after cardiac looping?
Needs to be divided into four chambers, two atria and two ventricles
Inflow and outflow tracts need to be remodelled
What does the right atrium develop from?
Most of the primitive atrium
Small part of sinus venosus
What does the left atrium develop from?
Small part of primitive atrium
Mostly from proximal part of primitive pulmonary veins
What is the texture of the right atrium? And the left atrium?
Rough and trabeculated
Left atrium is smooth
Why is the left atrium smooth compared to the right atrium?
Because the left atrium has developed mostly from blood vessels
which have smooth walls
What forms the endocardial cushions? Where do they come from?
Neural crest cells
Migrate into the heart from elsewhere
Where are the endocardial cushions located in the heart?
Base of atrium
in middle
What is the function of the endocardial cushions?
To act as scaffold
used to form the septum
How does the septum primum form?
Grows downwards from roof of atrium to endocardial cushion
What shape is the septum primum?
Crescent shaped - contains opening
What is the name of the opening in the septum primum?
The ostium primum
What happens to the ostium primum?
The septum primum continuing to grow downwards
closing up the ostium primum
What happens before the ostium primum is closed?
Upper septum primum
cells die by apoptosis
creating a second opening
What is the second opening in the septum primum called?
The ostium secundum
How does the septum secundum form?
Grows downwards from atrium to endocardial cushions
Where does the septum secundum lie relative to the septum primum?
Are adjacent to each other
What does the septum secundum contain? What is an important feature of this structure?
Contains an opening
Importantly, it does not line up with the ostium secundum
The opening in the septum secundum and the ostium secundum form what?
The foramen ovale
What keeps the foramen ovale open in the foetus?
Blood flowing through it
pushes the septum primum and septum secundum apart
What is the ventricular septum made up of? Which component forms most of the septum?
Muscle - forms most of the septum
Membrane
How does the muscular portion of the septum grow?
Muscular septum grows up from near apex of ventricle
to endocardial cushion
leaves small gap
What is the name of the small gap between the endocardial cushion and the muscular septum?
Primary interventricular foramen
What type of tissue is the membranous portion of the septum?
Connective tissue
How does the membranous portion of the septum grow?
Grows downwards from endocardial cushion
to muscular portion of septum
What happens to the primary interventricular foramen?
Closed by the membranous portion of the septum
How is the outflow tract separated into the aorta and pulmonary trunk?
Endocardial cushions in truncus arteriosus
grow towards each other
twist around each other
forming spiral septum
Why are the aorta and pulmonary trunk twisted around each other?
Because of the endocardial cushions twisting around each other
To form a spiral septum
Where does foetal blood become oxygenated?
Placenta
What carries oxygenated blood from the placenta to the foetus?
The umbilical vein
What are the three circulatory shunts in the foetus?
Foramen ovale
Ductus venosus
Ductus arteriosus
What is the ductus venosus?
Shunt that bypasses the liver
What circulatory shunt comes after the umbilican vein?
Ductus venosus
Why is the liver bypassed in foetal circulation?
Very metabolically active
would consume most of oxygen and nutrients, leaving little for rest of foetus
What carries the blood after the ductus venosus?
The IVC
What circulatory shunt comes after the IVC?
The foramen ovale
What direction does the blood flow through the foramen ovale?
RA to LA
What is the state of the lungs in the foetus?
Filled with amniotic fluid
What happens to the pulmonary vessels in the foetus? Why?
Lack of oxygen in the alveoli gives hypoxic pulmonary vasoconstriction
What is the relative pulmonary vascular resistance in the foetus? Why?
High
due to the pulmonary vessels being vasoconstricted, decreases their radius
Does some blood flow from the RA to the RV?
Yes
leaks into it from RA
Why does blood flow from the RA to the LA in the foetal circulation?
RA pressure is higher than LA pressure
because of high pulmonary vascular resistance, increasing pressure in the RV and RA
Why is it important that some blood leaks into the RV?
To give it blood to contract against
allowing for proper development of RV
What happens to the blood in the RV?
RV contracts
ejects blood into pulmonary trunk
blood passes from pulmonary trunk to aorta through ductus arteriosus
Why does blood move from the pulmonary trunk to the aorta?
High pulmonary vascular resistance
less resistance to blood flow from ductus arteriosus into aorta
Where does blood travel after it leaves the aorta?
To foetal body
to placenta
What type of blood is travelling to the placenta?
Deoxygenated blood
What happens to the placental circulation after birth?
Removed
What happens to systemic vascular resistance after birth? Why?
Increases
Because a major route of blood flow, the placental circulation, has been removed
What happens to the ductus venosus after birth?
Closes up
What happens to the pulmonary vessels after birth? Why?
Pulmonary vessels vasodilate
due to the presence of oxygen in the alveoli
What happens to the lungs after birth?
Inflate with air
fluid is pushed out
What happens to pulmonary vascular resistance after birth? Why?
Pulmonary vascular resistance decreases
because of vasodilation of pulmonary vessels
What happens to blood flow through the pulmonary circulation after birth? Why?
Blood flow in pulmonary circulation increases
due to decrease in pulmonary vascular resistance
What happens to pressure in the LA after birth? Why?
Increases
due to high systemic vascular resistance
and increased blood flow from pulmonary veins
What effect does increased LA pressure have on the foreman ovale? How?
Increased LA pressure pushes septum primum against septum secundum
fuse together
closing foramen ovale
Why was it important that the ostium secundum and the hole in the septum secundum didn’t align?
So when the septum primum is pushed against the septum secundum
Both openings are blocked
What happens to the ductus arteriosus?
Contracts and closes up
What is the remnant of the foramen ovale called?
Fossa ovalis
What is the remnant of the ductus arteriosus called?
The ligamentum arteriosum
What does the early arterial system look like?
Many arched vessels
Bilaterally symmetrical
What happens to these arched vessels?
Remodelled to form the major arteries that branch off the aorta
How many arches are there in the early arterial system?
6
What does the 4th arch on the right side form?
Proximal part of subclavian artery
What does the 4th arch on the left side form?
The aortic arch
What does the 6th arch on the right side form?
Right pulmonary artery
What does the 6th arch on the left side form?
Left pulmonary artery, ductus arteriosus
What is the nerve corresponding to the 6th arch?
The recurrent laryngeal nerve
What is unusual about the left recurrent laryngeal nerve?
It descends from the neck
Hooks around ductus arteriosus/ligamentum arteriosum
Then ascends back up to neck
What causes the left recurrent laryngeal nerve to hook around the ductus arteriosum?
Tangling of nerves as heart descends from embryonic neck to thorax during development
What is the crista dividens?
Border of septum secundum in the right atrium
What is the function of the crista dividens?
Create two routes of blood flow in the right atrium
What are the two routes of blood flow in the right atrium?
Majority of blood flow into left atrium
Small amount of blood flow into right ventricle
Blood from what flows from the right atrium into the right ventricle?
Blood from the superior vena cava
Where does the ductus arteriosus arise from the aorta?
After the three branches of the aortic arch
- brachiocephalic trunk
- left cartoid
- left subclavian
