Fetal Circulation

Question 1

Learning outcomes

Answer 1

• Explain in detail how blood circulates through the fetal heart
• Explain in general terms the circulation of blood in fetus and placenta
• Explain the functions of the foramen ovale, ductus arteriosus and ductus venosus
• Explain the changes that occur in the circulation of blood in the fetus at the time of birth
• Explain briefly the consequences of non-closure of foramen ovale, ductus arteriosus and ductus venosus

Question 2

Why does a developing foetus’ circulation have special needs?

Answer 2

• Foetus’ circulation has special needs because:
  1) Lungs are not functioning as in adults
  2) The digestive system is not functioning as in adults – leads to restricted hepatic circulation in foetus

Question 3

What are the 3 shunts/bypasses on foetal circulation?

Answer 3

• 3 shunts/bypasses on foetal circulation:
  1) Bypass of hepatic circulation (bypass liver) via ductus venosus
  2) Bypass of pulmonary circulation via foramen ovale (bypass lungs)
  3) Bypass of the pulmonary circulation via ductus arteriosus (connection between pulmonary artery and aorta

Question 4

Why are veins and arteries of foetal circulation strange?

Answer 4

• Foetal veins and arteries are strange as veins carry oxygenated blood and arteries carry deoxygenated blood

Question 5

What is the role of the placenta?

How is this done?

Answer 5

The role of the placenta is to deliver O2 and nutrients from the mother to the foetus, while removing CO2 and metabolic waste
* This is done via diffusion through the placental barrier

Question 6

Does foetal and maternal blood mix?

Where is the placenta found?

Answer 6

• Foetal and maternal blood never mix
• The placenta is usually in the posterior-fundal part of the uterus
• The fundal-posterior position is located on the top of the womb but extends to the back of the womb

Question 7

What does the umbilical cord connect?

What 2 things is the umbilical cord surrounded by?

What does the umbilical cord contain?

What is embedded within this?

What do these vessels extend between and then enter?

Answer 7

• The umbilical cord connects the placenta to the baby
• The umbilical cord is surrounded by the foetal membrane and amnion (outer membrane)
• The umbilical cord contains Wharton’s Jelly
• Embedded in this jelly are two umbilical arteries and a single umbilical vein
• These vessels extend between the placenta and umbilicus (naval/belly button), then enters the abdominal cavity

Question 8

Where is the umbilical vein found?

Answer 8

• The umbilical vein is found on the lower border of the falciform ligament, which is part of the embryological foregut, and forms a connection between the ventral abdominal wall and the liver

Question 9

What does the umbilical vein deliver?

Where does the umbilical vein enter?

What structure does it run towards?

When does it bifurcate?

Answer 9

• The umbilical vein carries oxygen and nutrient rich blood from the placenta
• The umbilical vein enters through the umbilicus, then moves into the abdominal cavity and starts to run towards the liver
• When at the liver, the umbilical vein bifurcates

Question 10

What 2 branches does the umbilical vein split into?

When does oxygenated and deoxygenated blood mix?

Answer 10

• Branches of the umbilical vein:
  1) Smaller branch to the liver – used to keep the liver alive
  2) Ductus venosus - Larger branch that bypasses the liver and drains into the inferior vena cava (IVC)
• Oxygenated blood from the ductus venosus and deoxygenated blood from the lower limbs in the IVC mix

Question 11

What Is the foramen ovale?

What is its purpose?

Where does most of the blood from the IVC go to when entering the heart?

What structure supports this?

What blood does not follow this route?

Answer 11

• The foramen ovale is an interatrial hole found in foetuses
• It allows blood to move from the right atrium to the left atrium to bypass the lungs
• Most of the blood from the IVC goes from the right atrium to the left atrium, left ventricle, then aorta when entering the heart
• This route is supported by the valve of inferior vena cava
• Anterior to the opening of the IVC, there is the crescent shaped valve of IVC, which directs blood flow towards the foramen ovale
• Some blood from the IVC and blood from the SVC go into the right ventricle

Question 12

Describe the 2 routes that deoxygenated blood from the superior vena cava can take when entering the right side of the heart?

Where does oxygenated and deoxygenated blood mix in these routes?

Answer 12

• Routes of blood from Super Vena Cava in foetal heart:
• Most common route:
  1) Right atrium
  2) Right ventricle
  3) Pulmonary trunk
  4) Ductus Arteriosus (90% of blood – connects pulmonary trunk to the aorta and acts as 2nd bypass of the lungs, with foramen ovale being the 1st )
  5) Aorta (deoxygenated blood mixes with oxygenated blood)
• Alternative route:
  1) Right atrium
  2) Right ventricle
  3) Pulmonary trunk
  4) Pulmonary arteries (10% of blood)
  5) Lungs
  6) Pulmonary veins
  7) Left atrium
  8) Left ventricle
  9) Aorta (deoxygenated blood mixes with oxygenated blood

Question 13

What is the role of ductus arteriosus?

Why is where it opens significant?

Why is the alternative route still important?

Answer 13

• The ductus arteriosus opens after the aortic arch, meaning deoxygenated blood is not being mixed with oxygenated blood and sent to the brain
• It is important that some blood still gets sent to lungs along the alternative route, as this prevents the lungs from totally collapsing

Question 14

Where does blood in the descending aorta end up?

Answer 14

• 1/3rd of the blood from the descending aorta is distributed to:
  1) Abdomen
  2) Pelvis
  3) Lower limbs
• 2/3rd of blood goes to the placenta via the route:
  1) Internal iliac artery
  2) Umbilical artery
  3) Placenta

Question 15

Foetal Circulation diagram

Answer 15

Question 16

Where is the umbilical cord tied and cut after birth?

Why is this?

Answer 16

• The umbilical cord is tied and cut 5cm away from the naval
• This is because a piece of intestine may be present as an umbilical hernia

Question 17

What 3 effects does the cutting and clamping of the remaining umbilical cord have on postnatal circulation?

Answer 17

• Effects the cutting and clamping of the remaining umbilical cord has on postnatal circulation:
  1) Loss of blood flow through the placenta
  2) Increased systemic resistance
  3) Increased aortic, left ventricular and left atrial pressures, because blood flow cant go back to the placenta

Question 18

What 3 effects does the first breath have on postnatal circulation?

What must happen after birth to bypasses?

Answer 18

• Effects the first breath has on postnatal circulation:
  1) Expansion of the lungs (alveoli)
  2) Decreased pulmonary resistance – in utero, the lungs are filled with fluid which compresses pulmonary vessels, leading to high pulmonary resistance
  3) Reduced pulmonary arterial, right ventricular, and right atrial pressures – when the baby takes their first breath, the lungs expand and the baby vomits the fluid that filled the lungs, leading to a decrease in pulmonary vessel circulation pressures
• After birth shunts/bypasses must be obliterated

Question 19

When does functional closure of the ductus arteriosus occur?

How does this happen?

Answer 19

• Functional closure of the ductus arteriosus occurs within the first hour after birth
• Closure of the ductus arteriosus happens through the constriction of the smooth muscle of the ductus arteriosus
• This happens because of
  1) Increased oxygen – oxygen acts as a vasoconstrictor
  2) Fall in prostaglandins – chemicals that act as vasodilators and keep the ductus arteriosus open in utero

Question 20

When does structural closure of the ductus arteriosus occur?

How does this happen?*

Answer 20

• Structural closure of the ductus arteriosus occurs between 1 and 4 months
• It is an anatomical closure though the thickening of the tunica intima
• The ductus arteriosus becomes the ligamentum arteriosum when closed

Question 21

What does the ductus arteriosum become when closed?

What can happen if the ductus arteriosus doesn’t close?

What can this lead to?

What can be used to treat this?

Answer 21

• If the ductus arteriosus doesn’t close, this will cause patent ductus arteriosus
• Postnatally, the aortic pressure is greater than in the pulmonary trunk
• This will lead to blood flowing from systemic circulation back into pulmonary circulation through the ductus arteriosus
• This will lead to increased pulmonary blood flow and pulmonary hypertension
• Too much blood will flow into the lungs, which will cause pulmonary oedema and possibly congestive heart failure, as the right ventricle needs to work harder
• NSAIDs such as ibuprofen can be used to treat patent ductus arteriosus, as they act as prostaglandin inhibitors

Question 22

Describe 4 the steps of the closure of the foramen ovale after birth.

Answer 22

• Describe 4 the steps of the closure of the foramen ovale after birth.
  1) Once pulmonary circulation is established, blood from the pulmonary circulation (lungs) is returned to the left atrium after the first breath
  2) This causes the pressure in the LA to rise and exceed the pressure in the right atrium
  3) This pushes the valve of the foramen ovale rightward, which closes the foramen
  4) It becomes structurally closed by 4 months, with the valve fusing with the IA septum, leaving an impression on the IA septum called the fossa ovalis. The margin of the fossa ovalis is called the falx septi

Question 23

What s a consequence of non-closure of the foramen ovale?

What are symptoms of this condition?

Answer 23

• A consequence of non-closure of the foramen ovale is patent foramen ovale (PFO)
• This condition is usually asymptomatic, but may cause paradoxical emboli (when a thrombus (clot) crosses intracardiac defect into systemic circulation)
• This condition will cause oxygenated blood to be mixed into deoxygenated blood

Question 24

Describe the 5 steps in the obliteration of the ductus venosus.

When does it functionally close?

When does it obliterate?

What does it become?

Answer 24

• Steps in the obliteration of the ductus venosus:
  1) Blood flow ceases through the umbilical vein
  2) The muscle wall of the ductus venosus constricts through increased oxygen and decrease prostaglandins
  3) Portal venous blood then flows through the hepatic sinuses
  4) Ductus venosus functionally closes within 1 to 3 hours of birth
  5) Obliterates in 2 weeks to 3 months and becomes the ligamentum venosum

Question 25

What does the umbilical vein obliterate and form?

When do distal parts of the umbilical arteries atrophy?

What do proximal parts of the umbilical arteries supply?

Answer 25

• The Umbilical vein obliterates and forms the round ligament of the liver (ligamentum teres hepatis)
• Distal parts of the umbilical arteries atrophy around 2-3 months
• They form the medial umbilical ligaments which, when covered in peritoneum, form medial umbilical folds
• We still have proximal parts of the umbilical arteries (superior vesicular arteries) that supply the superior surface of the bladder, with the rest closing after we get this supply.

Question 26

Fetal circulation diagram

Answer 26

Question 27

What occurs during gastrulation?

How is the 3rd layer formed?

What does each layer differentiate to?

Answer 27

• During gastrulation, the formation of the ectoderm, mesoderm, and endoderm take place to form the tr-laminar disk at 17 days
• Cells of the epiblast migrate through the primitive steak to form a third layer between the epiblast and hypoblast
• Epiblast will eventually differentiate to ectoderm
• Hypoblast will eventually differentiate to endoderm
• Middle layer will become the mesoderm

Question 28

How many cells are present after 30 hours from fertilisation?

How many after 40 hours?

When does the zygote become a morula?

What is the zona pellucida?

When does it disappear?

Answer 28

• After 30 hours from fertilisation, the zygote consists of 2 cells
• After 40 hours, the zygote consists of 4 cells
• After 72 hours, the zygote consists of 16 cells, and becomes known as a morula
• The zona pellucida extracellular matrix is present up to the 96-hour point before disappearing

Question 29

When does the morula/blastocyst implant into the uterine wall?

Answer 29

• After about 2-4 days, the morula/blastocyst will travel down the fallopian tube and enter the uterine cavity and implant into the endometrium of the uterine wall (fresh morula implants 8% of the time, Early blastocyst 25%, Advanced blastocyst 53%)

Question 30

Describe the 5 steps how the morula turns into a blastocyst

Answer 30

• Process of morula turning into blastocyst:
  1) Fluid begins to accumulate in the intercellular spaces of the morula
  2) Gradually, the intercellular spaces merge to form a single cavity (called the blastocele/blastocyst cavity)
  3) We have an inner cell mass at one pole which will differentiate later to form the primary germ layers (endoderm and ectoderm) in a process called gastrulation
  4) The embryo is called a blastocyst at this point
  5) The inner cell mass will develop into a baby

Question 31

What occurs 8 days after fertilization?

What do these 2 new layer form?

Answer 31

• After 8 days from fertilisation, two layers form in the embryo – epiblast (dorsal/posterior) and hypoblast (ventral/anterior)
• These 2 layers form a flat bilaminar disk, which has a primitive streak on the dorsal (posterior) side of the bilaminar disc at the caudal region that leads to the primitive node (primitive node is a pit)

Question 32

when does the amniotic cavity begin to form and where, and at this point how many cavities are there?

Answer 32

• The amniotic cavity begins to form as a space within the epiblast
• 2 cavities now present in the embryo – The amniotic cavity lined by epiblast and the exocoelomic cavity/primitive yolk sac lined by the hypoblast (after the blastocyst cavity in the blastocyst changes its name)

Question 33

Describe the 2 steps in notochord formation?

Answer 33

• Notochord formation:
  1) The notochord is formed by prenotochordal cells that migrate through the primitive streak to form the notochordal plate in the endoderm
  2) The notochordal plate then detaches from the endoderm to form the notochord, which is derived from paraxial mesoderm

Question 34

In what direction does the notochord develop?

What is the role of the notochord?

Answer 34

• The notochord develops in the cranial to caudal direction
• The role of the notochord is to activate a signalling pathway involved in tissue differentiation and development (notochord conducts development)

Question 35

What does the amniotic cavity do?

Answer 35

• The amniotic cavity will eventually surround the entire embryo
• The mesoderm layers:

Question 36

What are the 3 layers of the mesoderm?

What do these 3 layers become in the body?

Answer 36

1) Paraxial mesoderm
* Differentiates into somites (sclerotome, myotome, dermatome) and head mesenchyme

2) Intermediate mesoderm
* Develops into urogenital system

3) Lateral plate mesoderm (divides into 2 parts)
* Becomes the walls of cavities, serous membranes (parietal and visceral peritoneum) and vascular system