Where does materno-foetal exchange occur?
Materno-fetal exchange happens at the placenta
Identify the vascular structures involved in materno-foetal exchange
- Umbilical arteries
- Umbilical vein
- Fetal capillaries within chorionic villi
- Uterine arteries
- Uterine veins
Describe the foetal blood supply and drainage
- Oxygenated blood carried via the umbilical vein
- Deoxygenated blood carried via the umbilical arteries
Identify the factors increasing foetal pO2 content
- Fetal haemoglobin variant
- Fetal haematocrit is increased over that in the adult
What is necessary for gas exchange to occur at the placenta?
- Gradient of partial pressures required
- Maternal pO2 increases only marginally, so fetal pO2 must be lower than maternal pO2
What is the value of foetal pO2 compared to maternal pO2?
- pO2 approx. 4kPa compared to normal adult pO2 of 11 – 13kPa
Identify 3 additional factors promoting oxygen exchange to the foetus at the placenta
- Increased maternal production of 2,3 BPG
- Fetal haemoglobin
- Double Bohr effect
Why is there an increased maternal production of 2,3 BPG?
Secondary to physiological respiratory alkalosis of pregnancy
The predominant form of haemoglobin from weeks 12 is HbF.
Describe how its structure is adapted to its function
- HbF consists of 2 alpha subunits & 2 gamma subunits
- HbF has a greater affinity for oxygen as it doesn’t bind 2,3-BPG as effectively as HbA
The double Bohr effect speeds up the process of O2 transfer.
Describe how this process works
- As CO2 passes into intervillous blood, pH decreases (Bohr effect)
⇒ Decreasing affinity of Hb for O2
- At the same time, as CO2 is lost, pH rises (Bohr effect)
⇒ Increasing affinity of Hb for O2
Explain how maternal physiological adaptations to pregnancy promotes CO2 transfer from the foetus
- Progesterone-driven hyperventilation lowers pCO2 in maternal blood
- Concentration gradient created
Describe how CO2 is transferred to the maternal blood from the foetus through the Double Haldane effect
- As maternal Hb gives up O2, it can accept increasing amounts of CO2
- Foetus gives up CO2 as O2 is accepted
- No alterations in local pCO2 occur
Briefly, outline foetal circulation
- Receives oxygenated blood from mother via placenta in umbilical vein
- Lungs are non-functional so blood by-passes the lungs
- Returns blood to the placenta via umbilical arteries
In terms of foetal shunts and vasculature, describe the anatomical course of foetal circulation in detail
In terms of foetal shunts and vasculature, label the key structures of foetal circulation in the diagram below
Why is the ductus venosus needed?
- DV connects umbilical vein carrying oxygenated blood to the IVC so blood enters right atrium
- By ensuring shunting of blood around the liver, saturation is mostly maintained
Why is the foramen ovale needed?
- Right atrial pressure > left atrium
- Forces leaves of FO apart and blood flows into left atrium
Why is the ductus arteriosus needed?
- DA joins aorta distal to the supply to the head (and heart)
- DA shunts blood from right ventricle and pulmonary trunk to aorta, minimising drop in O2 saturation
Describe the foetal response to hypoxia
- Foetal chemoreceptors detect decreased pO2 or increased pCO2
- Redistribution of flow (reduced to limbs & intestines) to protect supply to heart and brain
- Vagal stimulation leads to bradycardia to reduce O2 demand
What are the effects of chronic hypoxaemia on the foetus?
- Growth restriction
- Behavioural changes
- Impact on development
Identify some hormones necessary for foetal growth
- Leptin (placental production)
Distinguish between IGF I and II
- IGF II nutrient independent, dominant in first trimester
- IGF I nutrient dependent, dominates in second and third trimester
Nutritional and hormonal status during foetal life can influence health in later life.
What is the effect of malnutrition on foetal growth?
Malnutrition can cause symmetrical or asymmetrical growth restriction
Identify and describe the dominant cellular growth mechanisms at different stages of pregnancy
What is the structure and function of the amniotic sac?
- Structure: encloses embryo / fetus in amniotic fluid
- Function: protection and also contributes to development of lungs
What is the volume of aminiotic fluid?
- 10 ml at 8 weeks
- 1 litre at 38 weeks
Most of the amniotic fluid is swallowed.
Identify the structures involved in the production and recycling of amniotic fluid
- Foetal urinary tract (urine production by 9 weeks)
- Foetal lungs
- Foetal GI tract
- Placenta and foetal membranes
Describe the composition of amniotic fluid
- 98% water
- 2% electrolytes, creatinine, urea, bile pigments, renin, glucose, hormones and fetal cells, lanugo and vernix caseosa
What is amniocentesis and what does it do?
Amniocentesis is a useful diagnostic test involving the sampling of amniotic fluid and allows for collection of fetal cells
Debris accumulates in the foetal gut and the presence of meconium in amniotic fluid during delivery is a sign of foetal stress and asphyxia.
What is meconium?
Meconium is debris from the amniotic fluid and intestinal secretions including bile
In terms of bilirubin metabolism during pregnancy and after birth, explain why physiological jaundice is common in infants
- During gestation: clearance of foetal bilirubin is handled efficiently by the placenta as foetus cannot conjugate bilirubin
- At birth: the liver, intestinal processes for metabolism, conjugation and excretion are immature