Foetal physiology Flashcards
What vessels make up the materno-foetal circulation?
- Foetal circulation
- Umbilical arteries
- Umbilical vein
- Foetal capillaries within chorionic villi increase SA
- Uterine arteries
- Uterine veins
- Maternal blood lakes in intervillous spaces
Outline foetal circulation
- Oxygenated blood carried via umbilical vein
- Deoxygenated blood carried to placenta via umbilical arteries
- Lungs are non-functional
- Blood by-passes lungs and liver
How is the movement of oxygen from maternal to foetal red cells is facilitated by 3 factors?
- Relative concentration difference between foetal and maternal blood
- A higher affinity of foetal haemoglobin for oxygen than adult haemoglobin
- Double Bohr affect
Outline foetal PO2 concentration
- Foetal blood has low PO2
- 4kPa compared to adult pO2 of 11-13kPa
Compare foetal haemoglobin to adult haemoglobin
- Predominant form from weeks 12-term is HbF
- 2 alpha subunits + 2 gamma subunits (instead of beta)
Why does foetal haemoglobin have a greater affinity for oxygen?
- Doesn’t bind 2,3-DPG as effectively as HbA
What is meant by a ‘double Bohr effect’?
- As CO2 passes into intervillous blood, pH decreases
- This decreases affinity of maternal Hb for O2
- Relative decrease in CO2 on foetal side causes foetal blood to become slightly more alkaline
- Increasing affinity of foetal Hb for O2
What is the double Haldane effect?
- As maternal Hb gives up O2, it can accept increasing amounts of CO2
- Deoxygenated blood has higher affinity for CO2
- Foetus gives up CO2 as O2 is accepted
What are the foetal circulatory shunts?
- Foramen ovale
- Ductus venosus
- Ductus arteriorsus
Why is ductus venosus needed?
- Ductus venosus connects umbilical vein carrying oxygenated blood to IVC of foetus
- Blood enters right atrium
- By ensuring shunting of blood around liver, saturation is mostly maintained
Why is foramen ovale needed?
- Creates a shunt between right atrium and left atrium
- Allows oxygenated blood to enter left ventricle and then aorta to brain
- Small amount enters right ventricle to the pulmonary trunk
- Blood bypasses lungs
What allows the heart and brain to get the lion’s share of oxygen?
- Blood is pumped to aorta from left atrium
- Blood reaching left atrium has saturation of approx. 60%
Why is ductus arteriosus needed?
- Shunts blood from pulmonary trunk to aorta
- Joins aorta distal to supply to head and heart
- Minimises drop in O2 saturation
- Bypasses inactive lungs
How are foetuses adapted to overcome hypoxia?
- HbF
- Increased [Hb]
- Flow can redistribute to protect supply to heart and brain
- Foetal heart rate slows in response to hypoxia to reduce O2 demand
How do foetuses detect changes in CO2 or O2?
- Chemoreceptors detect changes in PO2 or PCO2
- Vagal stimulation leads to bradycardia
What happens to a foetus in response to chronic hypoxaemia?
- Growth restriction
- Behavioural changes
- Impact on development
Outline the properties of the amniotic fluid
- Amniotic sac encloses embryo/foetus in amniotic fluid
- Protection
- Contributes to development of lungs
- Volume is 10 ml at 8 weeks
- Volume is approx 1L at 38 weeks
How is amniotic fluid produced?
- Initially transudation
- Then from foetal urinary tract from week 9 onwards
- Foetal lungs
How is amniotic fluid reabsorbed?
- Foetal GI tract
- Placenta and foetal membranes
What is oligohydramnios?
- Too little amniotic fluid
- Due to placental insufficiency (pre-eclampsia)
- Foetal renal impairment
- Premature rupture of membranes
What is polyhydramnios?
- Foetal abnormality
- Oesophageal atresia, duodenal atresia, anencephaly
- Gestational diabetes
- Foetal anaemia
- Multiple pregnancy
What is the composition of amniotic fluid?
- 98% water
- Electrolytes
- Creatinine
- Urea
- Bile pigments
- Renin
- Glucose
- Hormones and foetal cells
- Lanugo and vernix caseosa
What happens when a foetus swallows amniotic fluid?
- Absorbs water and electrolytes
- Debris accumulates in gut
What is meconium?
- Debris from amniotic fluid plus intestinal secretions including bile
What is amniocentesis?
- Sampling of amniotic fluid
- Allows for collection of foetal cells
- Useful diagnostic test for foetal karyotyping
Outline bilirubin metabolism
- During gestation clearance of foetal bilirubin is handled efficiently by placenta
- Foetus cannot conjugate bilirubin
- Immaturity of liver and intestinal processes for metabolism, conjugation and excretion
- Physiological jaundice common
What is measured in a cardiotocograph?
- DR: define risk
- C: contractions
- BRa: baseline rate
- V: variability
- A: accelerations
- D: decelerations
- O: overall impression
