9.2 Flashcards
Describe the fetal circulation before birth.
Oxygenated blood enters fetus via umbilical vein from placenta.
Bypasses liver by ductus venous
IVC (mixed with desaturated)
From RA -> LA by foramen ovale (some to RV via crusta dividens)
LA–> LV
RV–>From pulmonary artery –> aorta via ductus arteriosus
Aorta –> Deoxygenated blood leaves via 2 umbilical arteries.
Why does blood not enter the lungs?
Resistance is very high due to hypoxic pulmonary vasoconstriction
What happens when an infant takes its first breath?
Hypoxic pulmonary vasoconstriction removed.
Resistance of lungs reduced.
Greater venous return to LA
Pressure in LA > RA
Closure of foramen ovale
Increased oxygen saturation of blood and removed prostaglandins (placenta removed) results in:
Constriction of ductus arterioles
Contraction of umbilical artery
Stasis of blood in umbilical vein and ductus venous leads to clotting of blood and closure due to fibrosis.
What is the function of amniotic fluid?
Surrounds fetus providing mechanical protection - shock absorption, and a moist environment so the fetus does not dehydrate.
How much amniotic fluid is there at 8 weeks, 38 weeks and 42 weeks
10 ml at 8 weeks
1 L at 38 weeks
Falls to 300 ml at 42 weeks
How much urine is produced at 25 weeks and at term? How much do adults?
25 weeks - 100 ml per day
Term 500 ml per day
Adults 1L per day
What does amniotic fluid contain?
98% water Electrolytes Creatinine Cells from fetus/amnion --> amniocentesis and karyotyping Urea Bile pigments Renin Glucose Hormones
What happens to swallowed amniotic fluid?
Fetus absorbs water and electrolytes.
Debris accumulates in fetal gut
Fluid washes this away forming meconium together.
This is debris from amniotic fluid plus intestinal secretions including bile
What happens to fetal bilirubin?
Fetus cannot conjugate bilirubin due to immaturity of the liver and intestinal processes for metabolism, conjugation and excretion
Bilirubin crosses the placenta after accumulating
Excreted by the mother.
After birth, what stimulates neonate liver to begin conjugation?
What if this does not establish quickly?
Light exposure (Phototherapy) Jaundice
How long are fetal oxygen stores
Very low - 2 mins
What is the haemomonochorial
Thin barrier that oxygen diffuses across in the placenta from the maternal blood.
What is the driving factor for this diffusion
Gradient of partial pressure between maternal and umbilical blood.
Why is the placenta well adapted for exchange?
Lass SA and low resistance to diffusion
What is used to indicate fetal oxygen saturation
Fetal heart rate
What is fetal pO2 and normal adult pO2 in arteries
Fetal 4kPa
Adult arterial 13.3kPa
How is fetal Hb adapted to work better at low pO2
Different Hb without beta chains and with gamma chains.
Doesn’t readily bind 2,3-BPG
Higher affinity for O2
70% saturated at 4kPa
Adult is 45% saturated at 4kPa
Means that fetal blood contains 7.5mM/L O2, simlar to adult blood.
More Hb (higher haematocrit)
What is the double Bohr effect
Increase in pCO2 or [H+] results in Hb losing affinity and releasing more O2. Happens in both mother and fetus
How does the mother blow off extra CO2 from the baby?
Hyperventilation stimulated by progesterone
Why ultrasound scan at 20 weeks?
Organs big enough to see but early enough to abort
Why should pregnant women take folic acid
Important for neural tube development
Fetal heart rate at term
140-160 bpm
What is fetal abdominal circumference assessing?
Growth of liver and amount of subcutaneous fat
What does meconium in the amniotic fluid indicate
Fetal distress
