Download Maternal Physiology

Fetal Physiology
Dr Hettiarachchi
Anaesthesia & SICU
SBSCH- Peradeniya
Is uterine blood flow
auto-regulated ?
Highly
muscula
r
spiral
arteries
Increasing from
~50 mL/min at 10 weeks’ gestation
to
>500 mL/min at term
Simple diffusion, driven by an oxygen pressure
gradient from the mother’s blood to the fetus’s blood
Po2 gradient for diffusion of oxygen through
the placental membrane is about 20 mm Hg
Fetal blood leaving the placenta
has a Po2 of only 30 mm Hg
Fetal hemoglobin (HbF) can carry 20 to 50
more oxygen than maternal hemoglobin can
%
Adult haemoglobin
Hemoglobin concentration
of fetal blood is about 50 %
greater than that of the mother
Bohr effect
The decrease in O2 affinity of hemoglobin when the pH of blood falls
Pco2 rises --> P50 rises
UNsaturation of hemoglobin
Secondary to
the decline in the Po2
1–2% unsaturation
is due to the rise in Pco2
What is the ‘double Bohr effect’ ?
Bohr effect is operative in both the maternal and foetal
circulations
The double Bohr effect means that the oxygen
dissociation curves for maternal HbA and foetal
HbF move in opposite directions
The foetus requires an increasing
oxygen supply as it grows
How are these increased oxygen
needs met?
Increased maternal blood supply to the
placenta (Uterine flow increases 20
fold during pregnancy)
Increased foetal blood supply to the
placenta
Presence of HbF which has a higher oxygen affinity
than maternal HbA
Higher [Hb] concentration in the foetus (40%
higher than in adult)
Haldane effect
Increased capacity of deoxygenated hemoglobin to
bind and carry CO2
venous blood carries more CO2 than arterial
blood
CO2 uptake is facilitated in the tissues, and CO2
release is facilitated in the lungs
Fetal cardiovascular adaptations include:
1. Placenta : site of O2 and CO2 exchange
2. Ductus venosus : shunts blood around the liver
3. Foramen ovale : allows O2 -rich blood to bypass the
pulmonary circulation
4. Ductus arteriosus (DA): allows right ventricular output
to bypass the pulmonary Circulation
Neonatal
circulation
1. Blood in which of the following vessels
normally has the lowest PO2 ?
A. Maternal artery
B. Maternal uterine vein
C. Maternal femoral vein
D. Umbilical artery
E. Umbilical vein
CHANGES IN FETAL CIRCULATION
& RESPIRATION AT BIRTH
At birth, the placental circulation is cut off and
the peripheral resistance suddenly rises
Infant becomes increasingly asphyxial and
gasps several times
Then lungs expand
The markedly negative intrapleural pressure
(–30 to –50 mm Hg) is created
Once the lungs are expanded, the pulmonary
vascular resistance falls to less than 20% of the
value in utero, and pulmonary blood flow
increases markedly.
Placental transfusion
Sucking action of the first breath plus
constriction of the umbilical veins squeezes
as much as 100 mL of blood from the placenta
Closure of foramen ovale
Blood returning from the lungs
raises the pressure in the left atrium,
Closing the foramen ovale by
pushing the valve
that guards it against the interatrial septum
Ductus arteriosus
Constricts within a few hours after birth,
producing functional closure
Then permanent anatomic closure follows
in the next 24–48 h due to extensive
intimal Thickening
Mechanism
Increase in arterial O2 tension
Bradykinin, which is released from the lungs during
their initial inflation
Relatively high concentrations of vasodilators are
present in the ductus in utero—especially
prostaglandin F2a
Prostaglandin synthesis is blocked by inhibition of
cyclooxygenase at birth
Premature infants
- the ductus fails to close
spontaneously
Closure can be produced by infusion of
drugs (e.g., ibuprofen) that inhibit cyclooxygenase
Nitric oxide may also be involved in maintaining
ductal patency in this setting
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23. Features of Mendelson’s syndrome include:
a) urticarial rash
b) bronchospasm
c) hypoxia
d) hypotension
e) aspiration of at least 100 ml of gastric contents
23. FTTFF
• 38. The placenta:
•
• a) transports glucose from maternal to fetal
blood by facilitated diffusion
• b) can synthesise glycogen
• c) actively transports oxygen from maternal to
fetal blood
• d) allows protein molecules to pass from
maternal to fetal blood by pinocytosis
• e) secretes oestradiol
38. TFFTT