Fetal Circulation

Question 1

POSTNATAL circulation feutres

Answer 1

✓ 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 2

what are the special requirements of the foetus

Answer 2

• the lungs aren’t functioning as in adults

- the digestive system is not functioning as in adults and therefore has a restricted hepatic circulation

Question 3

The foetal circulation is modified by three shunts or by-passes to avoid the lungs and the liver

Answer 3

• by pass of the hectic circulation
• by pass of the pulmonary circulation via the foramen ovale
• by pass of the pulmonary circulation via the ductus arteriosus

Question 4

PLACENTA

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

UMBILICAL CORD

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

UMBILICAL VEIN

Answer 6

also known as ductus venosus
carries oxygen- and nutrient-rich blood from the placenta

divides into two
branches:
- smaller branch tot he liver
- larger branch, ductus venosus by passes the liver and drains into the inferior vena cava

✓ Arterial blood mixes with venous blood from the lower limbs.

Question 7

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

Question 8

DUCTUS ARTERIOSUS

Answer 8

✓ Blood carried by superior vena cava (de-oxygenated)
→RA
→RV
→pulmonary trunk →ductus arteriosus (90%) → aorta

✓ Small amount
→pulmonary trunk →pulmonary arteries (10%) → lungs
→pulmonary veins →LA→LV→aorta

Question 9

UMBILICAL ARTERIES

Answer 9

✓ 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 10

TYING THE CORD

Answer 10

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 11

POSTNATAL CIRCULATION

Answer 11

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

Question 12

The umbilical cord is tied and cut

Answer 12

✓ Loss of the blood flow through the placenta
✓ Increased systemic vascular resistance
✓ Increased aortic → left ventricular → left atrial pressures

Question 13

First breath

Answer 13

✓ Expansion of the lungs (alveoli)
✓ Decreased pulmonary vascular resistance
✓ Reduced pulmonary arterial → Reduced right ventricular → Reduced right atrial pressures

Question 14

CLOSURE of FORAMEN OVALE

Answer 14

✓ 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 15

PATENT FORAMEN OVALE

Answer 15

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

Question 16

OBLITERATION of the DUCTUS ARTERIOSUS - Functional closure

Answer 16

✓ 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 17

OBLITERATION of the DUCTUS ARTERIOSUS - Structural closure

Answer 17

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

Question 18

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

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

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

ZYGOTE → MORULA

Answer 21

✓ This figure shows the development of the zygote from the 2-
cell stage to morula stage
✓ 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

MORULA → BLASTOCYST

Answer 22

✓ 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.
✓ If we take a section from the blastocyst, that is how it looks (right
figure) 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 23

BLASTOCYST

Answer 23

✓ 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 24

BILAMINAR DISC

Answer 24

✓ Here on the left, we have the blastocyst. If we open the amniotic cavity, as shown by the dashed line, we would be able to see how the bilaminar disc looks like
✓ Note that there is a shallow groove called primitive streak on the dorsal side of the bilaminar disc, at its caudal region

Question 25

TRILAMINAR DISC

Answer 25

✓ If we get a cross section from this region (dashed line on the upper figure) of the bilaminar disc, below figure is what we get. We can see the primitive streak and the two layers
✓ However, sometime later, 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 26

TRILAMINAR DISC (17-day-old embryo)

Answer 26

✓ This is transverse section from a 17-day old embryo.
✓ 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

Question 27

TRILAMINAR EMBRYO

Answer 27

✓ There is our trilaminar disc or embryo. We have the endoderm, mesoderm and the ectoderm. These layers will develop into the entire body

Question 28

TRILAMINAR EMBRYO

Answer 28

✓ 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)