Fetal Physiology Flashcards
Describe how materno-fetal exchange occurs
in placenta
Maternal circulation (endometrial arteries and veins) join with cytotrophoblastic shells -> blood lakes in the intervillous spaces -> diffuses into fetal capillaries within chorionic villi -> umbilical vein carries O2 blood to fetal cirulstion and umbilical arteries carry DO blood back to maternal circulation
In order for gas exchange to occur at placenta gradient of partial pressures is required, how does this occur?
PO2 foetus 4KPa (adult 11-13) but:
Fetal Hb variant - predominant form from week 12- term HbF (2alpha, 2 gamma) greater affinity for O2 bc doesn’t bind to 2,3- DPG as effectively as HbA (maternal production 2,3DPG increased secondary resp alkalosis pregnancy)
Double Bohr effect - as CO2 passed into intervillous blood - (PH decreases so maternal blood gives up O2) (PH increases as foetus blood loses CO2)
HbF ODC shifts left, HbA shifts right
-> double haldane effect as HbA gives up O2 can accept more CO2 and foetus more O2, no alterations in local PCO2
Fetal haematocrit is increased (increased RBC: blood)
Progesterone drives maternal hyperventilation so lowers PCO2 in maternal blood
What shunts does oxygenated blood have to pass through from endometrial arteries to umbilical vein? State the saturation of blood as it changes.
70% Placenta ->
- Ductus Venosus - by-pass liver 65%
IVC -> RA
-foramen ovale - by-pass RV & lungs
LA (minor proportion to RV*) 60% -> LV -> aorta -> 🧠 & ❤️ ->
rest body -> deoxygenated
Placenta
- foramen ovale -> RV -> PT
- ductus arteriosus - by-pass lungs -> joins aorta distal to 🧠❤️ supply to minimise drop in O2 saturation
What is the crista dividens and what’s its function?
Free border of septum secundum forms a crest = crista dividens
Crete’s two streams of blood flow from foramen ovale
- majority to LA
- minor proportion flows to RV (joins with small volume deoxygenated blood from SVC) keeps pressure in pulmonary circulation and prevent atrophy of muscles in RV
Shunts function
maintain blood saturation by avoiding unnecessary structures
E.g. ductus venosus by-Pass liver, foramen ovale and ductus arteriosus by-pass lungs
Ductus arteriosus also shunts blood RV and PT to aorta so helps minimise drop in O2 saturation after 🧠❤️
What adaptations does the foetus have to manage hypoxia? What if these don’t work?
- HbF and increased [Hb]
- redistribution of flow to pretext supply to heart and brain (reduction to GIT, kidneys, limbs)
- fetal HR slows to reduce O2 demand.
fetal chemorecptors detect decrease pO2/ increased pCO2 -> vagal stimulation -> bradycardia (opp adults)
Chronic hypoxaemia:
- growth restriction
- behavioural changes (impact on development)
What hormones are necessary for fetal growth, where are they produced?
Fetal liver:
IGFI nutrient dependent (dominates T2 & T3)
IGF2 nutrient independent (dominates 1st trimester)
IGF = Growth hormone
Placenta:
Leptin
EGF
TGF
Effects of nutrients on foetus
Malnutrition -> symmetrical (entire fetus) or asymmetrical growth (sacrifice parts e.g. brain tends preserves but trunk/ limbs reduced)
Nutritional and hormonal status during fetal life can influence health in later life ‘developmental origins of health and disease hypothesis’
May be placental adaptive response to alterations?
What’s the dominant cellular growth mechanism in each trimester?
1st weeks 0-20 hyperplasia
2nd 20-28 hyperplasia + hypertrophy
3rd 28-term hypertrophy
How does amniotic fluid get produced and recycled?
Placenta and fetal membranes production dominates early pregnancy
Fetal urinary tract production dominate slater pregnancy (urine production by 9 weeks, 800ml/ day in T3)
Recycled through: fetal lungs/ GI tract
Swallowed - absorbs H20 and electrolytes, debris accumulates in gut
and inhaled
Composition of amniotic fluid
98% water
+ electrolytes, creatinine, urea, bile pigments, renin, glucose, hormones, fetal cells,
lanugo (fine hair protects),
vernix caseosa (white waxy substance protects)
What is a meconium?
First bowel move to - should be after birth (if green secreted into amniotic fluid early - cause problems)
What is amniocentesis and why is it done? What’s the alternative?
Sampling of amniotic fluid
low risk
Allows for collection of fetal cells
Useful diagnostic testing e.g. fetal karyotyping
Can do chorionic vili sampling from placenta in earlier pregnancy but higher risk
How is fetal bilirubin metabolised?
During gestation clearance of fetal bilirubin handled by placenta - can’t conjugate bilirubin due to immaturity of liver and cant excite due to immaturity of intestinal processes for metabolism
- physiological jaundice at birth therefore common - if premature greater risk
