Fetal Circulation

Question 1

Learning outcomes

Answer 1

After this lecture, time spent in the dissecting room, and further private study you should be able to:

1. Explain in detail how blood circulates through the foetal heart
2. Explain in general terms the circulation of blood in the foetus and placenta
3. Explain the functions of the foramen ovale, ductus arteriosus and ductus venosus
4. Explain the changes that occur in the circulation of blood in the foetus at the time of birth
5. Explain briefly the consequences of non-closure of foramen ovale and ductus arteriosus
6. Review and describe the development of trilaminar disc, the 3 germ layers (ecto, endo and mesoderm), and folding
7. Review and describe, briefly, the embryonic development of the pharynx and of its adult derivatives

Question 2

Give a broad overview of postnatal circulation

Answer 2

Involves separate pulmonary and systemic circulations
No mixing of oxygenated and deoxygenated blood
Oxygenation of the blood occurs in the lungs
There is extensive hepatic blood supply

Question 3

What are the special considerations to have when thinking about the circulation of the foetus?

Answer 3

A developing foetus has special needs
− Lungs are not functioning as in adults
− Digestive system is not functioning as in adults -> has a restricted hepatic circulation

The foetal circulation is modified by three shunts or by-passes to avoid the lungs and the liver
− By-pass of the hepatic circulation
− By-pass of the pulmonary circulation via the foramen ovale
− By-pass of the pulmonary circulation via the ductus arteriosus

Question 4

Describe the placenta in relation to foetal circulation

Answer 4

O2 and nutrients diffuse across the placental barrier from mother to foetus while CO2 and metabolic waste are removed via the placenta

Maternal and foetal blood never mix

Usually located at the fundus of the uterus

Question 5

Describe the umbilical cord in relation to foetal circulation

Answer 5

The umbilical cord is surrounded by the fetal membrane, amnion, and contains Wharton’s jelly.

Embedded in this jelly are two umbilical arteries and single umbilical vein

Question 6

What is the ductus venosus of the umbilical cord?

Answer 6

The umbilical vein

• carries oxygen- and nutrient-rich blood from the placenta

Divides into two branches:
− Smaller branch to the liver
− Larger branch, ductus venosus by-passes liver and drains into inferior vena cava

Oxygenated blood mixes with venous blood from the lower limbs.

Question 7

What is the purpose of the foramen ovale?

Answer 7

Most of the blood coming through inferior vena cava passes from right atrium into left atrium through for. ovale -> left ventricle -> aorta

• as lungs are useless at this point so pulmonary circulation is not needed

Question 8

What is the valve of the inferior vena cava?

Answer 8

The valve of the inferior vena cava (eustachian valve)

In prenatal development, the eustachian valve helps direct the flow of oxygen-rich blood through the right atrium into the left atrium and away from the right ventricle. Before birth, the fetal circulation directs oxygen-rich blood returning from the placenta to mix with blood from the hepatic veins in the inferior vena cava. Streaming this blood across the atrial septum via the foramen ovale increases the oxygen content of blood in the left atrium. This in turn increases the oxygen concentration of blood in the left ventricle, the aorta, the coronary circulation and the circulation of the developing brain

You still have an eustachian valve in adulthood but it serves no real purpose

Question 9

How does the foramen ovale close?

Answer 9

Post birth as blood flow to the lungs increases, the amount of blood flow entering the left atrium increases. When the pressure in the left atrium exceeds the pressure in the right atrium, the foramen ovale begins to close and limits the blood flow between the left and right atrium

Basically:
Once pulmonary circulation is established, blood from the pulmonary circulation (lungs) is returned to the LA
Pressure rises in the LA
Valve of the foramen ovale is pushed rightward and closes the foramen
It becomes structurally closed by 4 months -> fossa ovalis and falx septi

Question 10

What is the ductus arteriosus?

Answer 10

A blood vessel in the developing fetus connecting the trunk of the pulmonary artery to the proximal descending aorta - skips the non functional lungs

Question 11

Describe the path of deoxygenated blood from right ventricle to aorta in the foetus

Answer 11

Blood from SVC as blood from IVC comes from umbilical through hepatic system and is oxygenated and sent through foramen ovale:

• > RA
• > RV
• > pulmonary trunk
• > ductus arteriosus (90%)
• > aorta

Small amount

• > pulmonary trunk
• > pulmonary arteries (10%)
• > lungs
• > pulmonary veins
• > LA ->LV ->aorta

Question 12

Describe the umbilical arteries

Answer 12

One third of the blood in the descending aorta is distributed in the abdomen, pelvis and lower limbs

Two thirds of the blood goes to the placenta via internal iliac artery -> umbilical artery -> Placenta

Question 13

Describe foetal circulation

Answer 13

Oxygenated blood comes from the placenta through the umbilical vein, where it will either go to the liver or the ductus venosus which will join back at the inferior vena cava.
Blood will then travel to the right atrium and be directed into the foramen ovale by the eustachian valve, however some will pass into the right ventricle

The blood that passes into the foramen ovale will then pass through the left atrium and ventricle to the aorta

The blood that passes to the right ventricle will pass into the pulmonary trunk where the majority will join the aorta via the ductus arteriosus. Some will go through the pulmonary circulation and pass back into the left atrium then onto the aorta

From the aorta blood will go to the systemic circulation, then to the umbilical arteries and back out to the placenta to be oxygenated

Question 14

Describe the tying of the umbilical cord

Answer 14

At birth, the cord is tied off close to the umbilicus. About 2 in (5 cm) of cord is left between the umbilicus and the ligature, since a piece of intestine may be present as an umbilical hernia in the remains of the extraembryonic coelom. After application of the ligature, the umbilical vessels constrict and thrombose. Later, the stump of the cord is shed and the umbilical scar tissue becomes retracted and assumes the shape of the umbilicus, or navel

Question 15

Describe postnatal circulation

Answer 15

Shunts (by-passes) must be obliterated at birth when the pulmonary circulation comes on stream!!

The umbilical cord is tied and cut
Loss of the blood flow through the placenta
Increased systemic vascular resistance
Increased aortic -> left ventricular -> left atrial pressures

First breath
Expansion of the lungs (alveoli)
Decreased pulmonary vascular resistance
Reduced pulmonary arterial -> Reduced right ventricular -> Reduced right atrial pressures

Question 16

What is a patent foramen ovale?

Answer 16

Patent foramen ovale (PFO) is a consequence of non-closure
− Usually asymptomatic
− May cause paradoxical emboli

Question 17

Describe obliteration of the ductus arteriosus

Answer 17

Functional closure (first hour): Constriction of the smooth muscle on the wall of ductus arteriosus
Increased oxygen
Fall in Prostaglandins

Structural closure (between 1 and 4 months): Anatomical closure by thickening of the tunica intima

Question 18

What is patent ductus arteriosus?

Answer 18

Postnatally the aortic pressure is greater than in the pulmonary trunk
Blood will flow back into the pulmonary circulation
• Left to right shunt
• Increased pulmonary blood flow -> pulmonary hypertension
• Congestive cardiac failure

Question 19

Describe obliteration of ductus venosus

Answer 19

Blood flow through the umbilical vein ceases.
The muscle wall of the ductus venosus contracts
Portal venous blood flows through the hepatic sinuses
Functionally closes within 1 to 3 hours of birth
Obliterates and becomes Lig. venosum

Question 20

Describe obliteration of umbilical vessels

Answer 20

• Umbilical vein obliterates and forms the round ligament of the liver (lig. teres hepatis)
• Distal parts of the umbilical arteries atrophy around 2-3 months to become the medial umbilical ligaments (within medial umbilical folds)

Question 21

Describe the path from a zygote to a morula

Answer 21

2 cell stage is reached after ~30h
4 cell stage is reached after ~40h after fertilization
Morula stage is reached after ~72h after fertilization

During this time it is surrounded by zona pellucida. zona pellucida disappears after 4 days

Question 22

Describe the structure of a blastocyst

Answer 22

The centre is a large blastocyst cavity called the blastocele
The inner cell mass is also known as an embryo blast
The outer cell mass is a trophoblast

Question 23

Describe the transition from morula to blastocyst

Answer 23

The morula travels down the Fallopian tube, enters the uterine cavity and implants into the endometrium

At about the time of implantation, fluid begins to accumulate in the intracellular spaces of the morula
Gradually the intercellular spaces merge to form a single cavity.
(Have a peek at a diagram or something)

This cavity is called blastocele (blastocyst cavity)
We have an “inner cell mass” at one pole and these cells start to differentiate to form the primary germ layers (endoderm and ectoderm)
– This process is called gastrulation
At this stage the embryo is called a blastocyst

Question 24

Describe the blastocyst at day 8

Answer 24

On day 8, the inner cell mass (or embryoblast) differentiates into two layers
– Epiblast
– Hypoblast

These two layers form a flat disc (Bilaminar disc)

A small cavity appears within the epiblast
– This cavity will become the amniotic cavity
– Epiblast is adjacent to amniotic cavity
– Hypoblast is adjacent to blastocyst cavity (primitive yolk sac)

Question 25

What is a primitive streak?

Answer 25

Other than me when I see housing prices:

There is a shallow groove called primitive streak on the dorsal side of the bilaminar disc, at its caudal region

Question 26

Describe the formation of the trilaminar disk

Answer 26

When we take a cross section of the bilaminar disk, we can see the primitive node and streak superiorly, the epiblast layer and lateral amnioblasts, then the hypoblast and lateral yolk sac (endoderm?)

Cells of the epiblast migrate through the primitive streak and occupy and create a third layer between the epiblast and hypoblast
– Epiblast will eventually differentiate to ectoderm
– Hypoblast will eventually differentiate to endoderm
– And this middle layer will become to mesoderm

Question 27

Describe the structure of the trilaminar disc at 17 days.

Answer 27

Caudally there are three layers, with a central notochordal plate bisecting the mesoderm and attaching to the endoderm
Cranially the structure is the same, but the notochord is now separate

The embryo has prenotochordal cells and these cells migrate through the primitive streak to form the notochordal plate within the endoderm

Then the notochordal plate detachs from the endoderm and forms the notochord (derived from axial mesoderm)
– Notochord activates a signaling pathway involved in tissue differentiation and development

Because these events occur in a cranial-to-caudal sequence, portions of the notochord are established in the head region first

Superior to the ectoderm is the amniotic cavity, and inferior to the endoderm is the yolk sac cavity

Question 28

What are the three parts of the mesoderm in the trilaminar embryo?

Answer 28

Paraxial (proximal to notochord)
Intermediate mesoderm (between paraxial and lateral plate)
Lateral plate mesoderm (most lateral)

Question 29

Describe the mesoderm of the trilaminar embryo

Answer 29

The mesoderm will be subdivided into paraxial mesoderm, intermediate mesoderm and lateral plate mesoderm
– Paraxial mesoderm will differentiate into somites (sclerotome, myotome, dermatome) and head mesenchyme
– Intermediate mesoderm will develop into the urogenital system
– Lateral plate mesoderm will become the walls of the body cavities, serous membranes (parietal and visceral peritoneum) and circulatory system (with extraembryonic mesoderm)