Where does materno-fetal exchange occur?
At the placenta
What does circulation around the fetal body culminate in?
Umbilical arteries and umbilical veins
How is oxygenated blood carried to the fetal body?
Via the umbilical vein
How is deoxygenated blood and products of fetal metabolism carried from the fetal body?
Where do the umbilical arteries and veins join with the fetal capillaries?
At the chorionic villi
Where does gas exchange between the mother and fetus occur?
At the tips of the chorionic villi
What is the function of the chorionic villi?
Increase the surface area
What happens to uterine arteries?
They branch into the spiral arteries, which spill maternal blood into blood lakes
What does the uterine vein do?
Drains maternal blood lakes into the intervillous spaces
What are the requirements for gas exchange at the placenta?
- Diffusion barrier must be small
- Gradient of partial pressures required
What happens to the size of the fetal barrier as pregnancy proceeds?
It decreases - the structure of the placenta adapts as the metabolic needs increase
How does the structure of the placenta adapt to optimise gas exchange?
Number of layers decreases to the minimum amount required to keep the circulations seperate
How much does maternal pO2 increase during pregnancy?
What is the result of the maternal pO2 increasing only marginally during pregnancy?
The fetal pO2 must be lower than the maternal pO2 to make the gradient work, and so is adapted to tolerate a much lower pO2
What is the pO2 of the fetus?
What factors increase fetal O2 content?
- Fetal haemoglobin variant
- Fetal haematocrit is increased over that of the adult
- Increased maternal production of 2,3 DPG
- Double Bohr effect
- Increased concentration gradient by physiological hyperventiation
- Double Haldane effect
- Increased maternal production of 2,3 DPG
What is the predominant form of haemoglobin in the fetus from 12 weeks - term?
What subunits are in HbG?
2 alpha and 2 gamma
Why does HbG have a greater affinity for oxygen than adult haemoglobin?
Because it doesn't bind 2,3-DPG as effectively as HbA
What is the increased maternal production of 2,3-DPG secondary to?
Respiratory alkalosis of pregnancy
What is the result of the double Bohr effect?
Speed sup process of oxygen transfer
How is the double Bohr effect produced in the fetus?
- On the maternal side, as CO2 passes into the intervillous blood, pH decreases, producing the Bohr effect, which decreases the affinity of Hb for O2, so more likely to give O2 up
- On the fetal side, CO2 is list, and pH rises. giving the Bohr effect and increasing addinity of Hb for O2
What drives hyperventilation in pregnancy?
What is the result of the physiological hyperventilation in pregnancy on gas exchange?
There is a lower pCO2 in maternal blood, as more CO2 has been blown off, and so there is an increased concentration gradient because the fetus is producing CO2, so relatively higher [pCO2] in fetal blood
What gives the double Haldane effect?
As Hb gives up O2, it can accept increasing amounts of CO2, and the fetus gives up CO2 as O2 is accepted
How does the fetal circulation receive oxygenated blood?
From the mother, via the placenta in the umbilical vein
Why does the fetal circulation not get its oxygen from the lungs?
Because the lungs are non-functional, and so they are bypassed
How does blood in the fetal circulation return to the placenta?
Via the umbilical arteries
What circulatory shunts does the fetus have?
- Ductus venosus
- Foramen ovale
- Ductus arteriosus
What does the ductus venosus bypass?
What path does blood take due to the ductus venosus?
The DV connects the umbilical vein carrying oxygenated blood to the IVC, then blood enters the right atrium
What is the purpose of shunting blood around the liver?
It ensures that saturation is mostly maintained, dropping from 70% to 65%
Why is shunting around the liver required to maintain oxygen saturation of the blood?
Becasue the liver is very metabolically active, so if the blood with the highest pO2 passed through it, would take most of the oxygen
What does the foramen ovale bypass?
The right ventricle and lungs
How does the foramen ovale function?
Right atrial pressure is greater than that in the left atrium, so forces the leaves of the FO apart, and blood flows into the LA
What forms the crista dividens?
The free border of the septum secundum
What is the crista dividens?
A small anatomical specialisation which optimises flow to the LA, by creating two streams of flow
Where does the majority of the blood go from the right atrium?
The left atrium
What is the saturation of the blood entering the LA?
What happens to the blood shunted to the left atrium?
It is pumped to the left ventricle, and then the aorta
What is the result of the shunting of blood into the left atrium?
The heart and brain get a large share of the oxygen
Other than from the RA, where else does the LA recieve blood from?
Recieves a small amount form the pulmonary venous return
Is the blood entering the left atrium from the pulmonary venous return oxygenated or deoxygenated?
Deoxygenated, it is the blood from the metabolic output of the developing lungs
What happens to the minor portion of blood from the RA that flows to the RV?
It mixes with deoxygenated blood from the SVC
Why must some blood flow into the RV?
To allow development, as the muscle needs something to pump against
What does the ductus arteriosus do?
Bypasses the lungs from the right ventricle
Where does the ductus arteriosus shunt blood from?
The RV and PT, to the aorta
Where does the blood shunted by the ductus arteriosus enter the aorta?
Distal to the supply to the head
Why is it important that the blood shunted by the ductus arteriosus enters the aorta distal to the supply to the head?
It minimises drop in O2 saturation, and therefore minimises dilution by deoxygenated blood
What adaptations does the fetus have to manage transient decreases in oxygenation?
- HbF and transient decreases in oxygenation
- Redistribution of flow to protect supply to the heart and brain
- Fetal heart rate slows in response to hypoxia, to reduce O2 demand
What is the trade-off with redistribution of flow to protect the supply to the heart and brain?
Reduces supply to GIT, kidneys, and limbs
What does vagal stimulation lead to in the fetus?
What is the clinical importance of vagal stimulation leading to bradyvardia in the fetus?
During delivery, contractions of the vagina constricts the spiral arteries, and thus constricting the blood lakes, reducing flow to the fetus, so must check fetal HR during delivery
What can chronic hypoxia of the fetus be caused by?
Anything impairing normal blood supply to the placenta
Give an example of where chronic hypoxia of the fetus may occur?
In women who smoke
What does chronic hypoxia of the fetus lead to?
- Growth restriction
- Behavioural changes
What behavioural changes are seen in chronic hypoxia of the fetus?
- Less movement
- Less evidence of REM sleep state
What hormones are required for fetal growth?
- IGF I and IGF II
is IGF I nutrient dependant or independant?
When does IGF I dominate?
In T2 and T3
Is IGF II nutrient dependant or independant?
When does IGF II dominate?
What is the function of leptin in the fetus?
Promotes utilisation of nutrients
What produces leptin in the fetus?
How does fetal dependancy on growth hormone differ from that of the newborn?
Fetus less dependant
What can have an effect on fetal growth during pregnancy?
What can malnutrition cause in pregnancy?
Symmetrical or asymmetrical growth restriction
What is Barkers hypothesis, or the 'developmental origins of health and disease' hypothesis?
Maternal nutrition can cause symmetrical or asymmetrical growth restriction
What mechanisms underlie Barkers hypothesis?
Mechanisms not well understood, but could be placental adapative responses to alterations in hormonal and/or nutritional status - changes in fetal physiological to match nutrient availability
What is the dominant cellular growth mechanism from 0-20 weeks?
What is the dominant cellular growth mechanism from 20-28 weeks?
Hyperplasia and hypertrophy
What is the dominant cellular growth mechanism from 28 weeks - term?
What does the amniotic sac do?
Encloses the embryo/fetus in amniotic fluid
What is the role of amniotic fluid?
- Contributes to the development of the lungs
How does amniotic fluid contribute to the development of the lungs?
Practice breathing movements takes fluid into the lungs
What is the volume of the amniotic fluid at 9 weeks?
What is the volume of amniotic fluid at 38 weeks?
What happens to the volume of amniotic fluid post-EDD?
Why does the volume of amniotic fluid fall post-EDD?
Because the placenta is designed to last the gestational period, and after that, the function declines
When does urine production start in the fetus?
How much urine is produced per day in T3?
Up to 800ml
Draw a diagram illustrating amniotic fluid production and recycling
What is the result of the amniotic fluid being inhaled into the fetal lungs?
The lungs bathe in the fluid
What happens in the intramembranous pathway?
The placeta and fetal mmebranes take amniotic fluid back into the fetus
What happens when amniotic fluid is swallowed?
- Absorbs water and electrolytes
- Debris accumulates in the gut as meconium
What does meconium contain?
Debris from the AF, plus intestinal secretions including bile
When should meconium be passed?
What is meconium stained amniotic fluid a sign of?
What can meconium in the amniotic fluid cause?
What % of amniotic fluid is water?
Other than water, what is in amniotic fluid?
- Bile pigments
- Fetal cells
- Vernix caseosa
What is the purpose of the vernix caseosa?
Waterproofs the fetus, ensuring that fetal skin is not damaged by exposure to fluid during gestation
What is amniocentesis?
Sampling of amniotic fluid to allow for collection of fetal cells
What is amniocentesis uesd for?
Diagnostic testing, particularly fetal karyotyping
What is the problem with amniocentesis?
How does the risk of amniocentesis compare with chorionic villi sampling?
Less invasive, so lower risk of miscarriage with amniocentesis
How is clearance of bilirubin conducted in the fetus?
Handled efficiently by the placenta
Why does fetal bilirubin need to be handled by the placenta?
Because the fetus cannot conjugate bilirubin due to immaturity of liver and intestinal processes for metabolism, conjugation, and excretion
What is the result of a baby being unable to conjugate bilirubin?
Physiological jaundice is common