7.2 Foetal Physiology

Question 1

What acts as the organ of respiration for the developing foetus?

Answer 1

The placenta

Question 2

How does the placenta establish connection between mother and foetus?

Answer 2

It attaches to the uterine wall and establishes connection between mother and foetus via the umbilical cord

Question 3

What are the core functions of the placenta?

Answer 3

Provides nutrition, gas exchange, waste removal, a source of hematopoietic stem cells, endocrine and immune support

Question 4

What are the two different surfaces of the placenta?

Answer 4

1) the maternal surface ➞ facing the towards the outside

2) the foetal surface ➞ facing towards the inside (or the foetus)

Question 5

What is found on the foetal surface of the placenta?

Answer 5

1) the umbilical cord (link between the placenta and fetus) near the center of the organ. Contains the umbilical veins and arteries
2) amniotic membrane

Question 6

What gives the foetal side of the placenta the shiny appearance?

Answer 6

It is covered by the amniotic membrane

Question 7

What is the function of the amniotic membrane?

Answer 7

Secretes amniotic fluid which contains cells from foetus and amnion which high in protein ➞ this is swallowed by the foetus

It functions to:

• protect/cushion against the walls of the uterus
• maintain pressures and temperatures
• allows space for foetal growth
• protects against infection

Question 8

What is found underlying the amnion?

What is this + describe its stucture

Answer 8

The chorion ➞ a thicker membrane that is a structure of the placenta, continuous with the lining of the uterine wall.

Emerging from the chorion are the villi containing a system of foetal capillaries to allow maximum contact area with the maternal blood (also known as the intervillous space) for gas, nutrient and waste exchange.

Question 9

What is the intervillous space?

Answer 9

Villi emerging from the chorion (located below the amnion) that contain a system of foetal capillaries that allow maximum contact with maternal blood for gas, nutrient and waste exchange

Question 10

What comprises the maternal surface of the placenta during pregnancy?

Previous to pregnancy what is this structure more commonly known as?

What appearance does this give the placenta

Answer 10

It Is composed of the decidua (what is known as the uterine lining during pregnancy)

Previous to pregnancy the decidua is more commonly known as the endometrial lining of the uterus.

It is this portion that gives this structure a dark red, blood like appearance.

Question 11

What gives the maternal side of the placenta the dark red, blood like appearance?

Answer 11

The Decidua

Question 12

Describe the specific structure of the Decidua (include all 5 layers)

Answer 12

Different portions to the decidua have specific names according to their location and function:

• decidua capsularis
• decidua basalis
• decidua placentalis
• decidua vera
• decidua parietalis

Question 13

What other features are visible on the maternal side of the placenta?

Answer 13

15-20 lobules called cotyledons which are divided by deep channels known as sulci

Each individual lobule is divided into smaller sections containing one villi ➞ These villi are the same ones emerging from the chorion

These villi contain foetal capillaries, which bathe in the intervillous space

Question 14

Explain how foetal and maternal blood mix?

Answer 14

TRICK QUESTION!!! foetal and maternal blood NEVER mix

Question 15

How does O2 diffusion take place across the placenta from maternal blood?

Answer 15

The Barrier is small due to the foetal villi in contact with maternal blood

A gradient of partial pressures must be formed (Bohr effect):
• Maternal pO2 increases
• Umbilical venous pO2 is much lower

As the maternal blood has higher O2 and lower CO2 ➞ this creates pressure gradient which favours oxygen transfer to foetal circulation

Question 16

What determines the O2 transport RATE across the placenta?

Answer 16

determined by umbilical arterial pO2

Question 17

List 4 factors that can influence the maternal and foetal circulations

Answer 17

1) blood pressure
2) medication
3) uterine contractions
4) hormones

Question 18

1) Describe the levels of CO2 in the foetus and the mother
2) What acid/base changes are created through the diffusion of CO2 across placenta and how is this corrected
3) How do these changes facilitate uptake of O2 by foetus?

Answer 18

1) Foetus cannot tolerate higher pCO2 than mother. Hence, the mother has lower pCO2 which facilitates placental transfer of CO2 from foetus into maternal blood
2) The raised CO2 in the mother causes local acidosis, hence hyperventilation is stimulated by progesterone to maintain acid/base balance
3) The decreasing pH causes maternal Hb to have a lesser affinity for O2, enhancing placental transfer of maternal O2 offloading to foetus (increase in foetal pH due to removal of CO2 also facilitates uptake of O2)

Question 19

What is the structure of HbF + 3 comparisons of this to adult Hb

What is the overall effect of this?

Answer 19

HbF: 2α2γ

• low affinity for organic phosphates (2,3 DPG and ATP)
• higher affinity for O2 than adult Hb
• higher concentration

Favour rapid uptake of oxygen by foetus across the placenta at low PO2 levels

Question 20

What stimulates hyperventilation during pregnancy?

Answer 20

Progesterone

Question 21

ETA 1 recap

1) What is the Bohr effect?
2) describe the effect of increased temp, 2,3 BPG, pCO2 and H+ on the curve
3) What are the X and Y axis?

Answer 21

1) The extent to which haemoglobin can be saturated with oxygen and how it is affected by [H+], temperature and CO2
2) O2 dissociation curve is shifted to right by increased [H+], PCO2 and temperature
3) X axis = pO2, Y axis = %Hb saturation

Question 22

Explain the Bohr effect in terms of foetal and maternal O2 (include the shift)

Answer 22

Increased deoxygenated blood arriving in placenta (from foetus) favours release of maternal oxygen into fetoplacental bed because of higher concentration of HbF + other adaptations)

O2 dissociation curve of foetal RBC’s is shifted to the left compared to adult Hb

Question 23

Where does the umbilical cord emerge and end up?

What is contained within the cord?

Answer 23

Emerges from the foetal side of the placenta to the belly button region of the fetus (0 to 60cm length and 2 to 3cm width)

Contains 2 arteries and 1 vein that are in continuation with the foetal circulation

Question 24

Describe the pathway of BF through the adult heart (include arteries, veins and valves)

Answer 24

SVC ➞ RA ➞ tricuspid valve ➞ RV ➞pulmonary valve ➞ pulmonary artery ➞ lungs ➞ pulmonary vein ➞LA ➞ mitral valve ➞ LV ➞ aortic valve ➞ aorta ➞ rest of body

Question 25

Describe the pathway of BF through the foetal heart (include arteries, veins and valves)

Answer 25

1) oxygenated blood arrives in placenta and flows into umbilical vein
2) from left umbilical vein the ductus venosus shunts a portion of blood directly to IVC bypassing the liver
4) from IVC blood flows into RA
5) most shunts through the foramen ovale into LA ➞ LV ➞ aorta ➞ around the body
5) some continues to RV where it bypasses the pulmonary circulation via the ductus arterioles into aorta
7) from body deoxygenated blood enters umbilical arteries and travels back to the placenta

Question 26

What are the 5 vascular structures that differ between foetal and adult circulation?

Answer 26

1) placenta
2) umbilical vein/artery
3) ductus venosus
4) foramen ovale
5) ductus arteriosus

Question 27

What are the 3 ways (NOT structures) in which foetal circulation differs?

Answer 27

1) oxygenated blood enters the circulation from placental transfer
2) pulmonary blood flow accounts for less then 20% of total cardiac output due to the high pulmonary vasculature resistance
3) foetal vascular structures exist to direct blood flow

Question 28

Where do the following receive blood from and direct blood too?

• umbilical arteries and vein
• ductus venosus
• foramen ovale
• ductus arteriosus

Answer 28

Umbilical arteries and vein: carries deoxygenated blood to an oxygenated blood away from the placental respectively

Ductus venosus: receives oxygenated blood from the umbilical vein and directs it to the inferior vena cava and right atrium thereby bypassing the liver

Foramen ovale: a communication between the two atria that directs blood flow returning to the right atrium brought the septal wall into the left atrium bypassing the lungs

Ductus arteriosus: connects the pulmonary artery to the aorta and shunts blood right to left, thereby diverting flow away from the foetal lungs

Question 29

What may cause the appearance of cyanosis at birth?

Is this normal?

Answer 29

Neonatal Oxygen is LOW, blood arrives umbilical vein only 70% saturated

It shunts around liver and joins ascending flow in inferior vena cava

It then mixes with small amount of desaturated blood and saturation drops to 65%

This blood travels to aorta to carotids and reaches brain 60% saturated

This is normal for neonates as they dont NEED 100% saturation due to above shunts etc… BUT baby should pink up within 10 mins of birth when shunt begin closing and lung begin functioning

Question 30

Explain how the shunts are functionally closed in order

Answer 30

1) baby takes first breath causing a decrease in PVR (lungs inflate) ➞ greater pulmonary flow (increased pO2) along with the fall in prostaglandins cause the ductus arteriosus to close
2) the fall in PVR leads to an increase in LA pressure compared to RA and this closes foramen ovale
3) the cutting of the umbilical cord which detaches connection between placenta and foetus causes a lack of flow leading to closure of the ductus venosus

Question 31

What is responsible for keeping the ductus arteriosus open?

Answer 31

prostaglandins

Question 32

What are the adult remnants of the foetal structures?

Answer 32

Umbilical veins ➞ ligamentum teres
Umbilical arteries ➞ medial umbilical ligament
Ductus venosus ➞ ligamentum venosum
Foramen ovale ➞ fossa ovalis
Ductus arteriosus ➞ ligamentum arteriosus

Question 33

What are the 3 process that occur in order for the respiratory system to begin correctly functioning during/after birth

Answer 33

1) mechanical: when baby is passing through birth canal the lungs are compressed causing removal and fluid and establishment of lung volume
2) thermal: baby being wet and cold at birth stimulates first breath allowing air to replace lung fluid
3) chemical: cutting of the cord removes foetal O2 supply causing asphyxia to occur. The ↑C02 and ↓02 and ↓pH = ACIDOSIS. Acidosis stimulates the respiratory center in the medulla and chemoreceptors in carotid artery to initiate breathing

Question 34

How does crying at birth aid respiratory system function?

Answer 34

crying at birth increases the thoracic pressure, causing air to replace remaining fluid

Question 35

Give the two reasons why baby takes its first breath

Answer 35

Thermal and Chemical above

Question 36

What is the purpose of surfactant

Answer 36

lowers surface tension

Question 37

Describe the production of amniotic fluid in early vs late pregnancy

What can it be useful in?

Answer 37

Early in pregnancy ➞ formed from maternal and foetal ECF via diffusion across non-keratinised skin

Later in pregnancy ➞ formed from foetuses urine, it is re-swallowed which allows absorption of water and electrolytes

Diagnostically useful Amniocentesis

Question 38

What is Meconium?

Answer 38

Collection of secretions comprised of desquamated cells from the digestive tract and waste products from ingested amniotic fluid.

Appears ~beginning of T2 and accumulates in the colon until birth, usually passed in the first 24 h after birth