Flashcards in Repro 9.1 Fetal Physiology Deck (46):
Where does blood enter and leave the foetus?
Through the liver in the umbilical artery and vein
Where does the foetal blood reconnect with the maternal circulation?
Umbilical vein returns from the placenta to combine with the venous drainage of the gut
Why does the foetal circulation need to be modified?
The lungs aren't functional so the foetus is dependent on the mother for oxygen. In the absence of modification the oxygenated blood passing to the foetus would pass through the liver and the lungs and mix with venous blood from the body and brain before it reaches the systemic arteries; thus losing most of its oxygen
How is the liver avoided when blood enters the foetus?
Shunt through the ductus venosus
What causes the decrease in oxygen saturation between entry to the foetus and arrival at the right atrium?
Mixing with venous blood from the lower body - this is ok because the lower body is relatively small and not that active metabolically (70% -> 65%)
How is oxygenated foetal blood prevented from mixing with the venous blood from the brian?
Crista dividens directs oxygenated blood towards the foramen ovale which shunts blood from the right atrium to the left atrium, bypassing the right ventricle
What happens to blood flowing from the foetal brain to the right heart?
Passes through the right heart towards the lungs
Why is there high resistance to blood flow in the lungs?
They are not yet active - collapsed
What happens to blood flowing from the right heart to the lungs as a result of this high resistance?
Shunts from the pulmonary artery to the aorta through the ductus arteriosus, joining the aorta distal to the arterial outflow of the brain.
Is the foetal left or right atrium at higher pressure?
Right - blood shunts right to left
What causes the pressure in the right atrium and pulmonary artery to be higher than that in the left and aorta?
How does the oxygen in the foetus differ to that in the adult?
PO2 is lower in foetus (4kPa compared to 13.3kPa in adults)
Foetus is adapted to a degree of hypoxia that would be fatal in a normal adult
How does foetal haemoglobin compare to that in adults?
Different haemoglobin with much higher affinity for O2 so will carry more at lower partial pressures (70% sat at 4kPa, only 45% in adults)
Higher levels of haemoglobin - 18g/dl
Has no beta chains so does not readily bind 2,3 DPG
What enables transfer of O2 to the foetus?
Low diffusion resistance - barrier small as vili are in contact with maternal blood
Partial pressure gradient of 9kPa
Higher affinity of foetal Hb
Double bohr effect in maternal and foetal circulations
What determines the transport rate of O2 to the foetus?
Umbilical arterial pO2 so it gets what it needs as foetal stores only last approx 2 mins
How do maternal CO2 levels change during pregnancy?
Lowered by hyperventilation stimulated by progesterone which enables the foetus to have relatively normal pCO2
(the foetus cannot tolerate any higher CO2 or acid-base problems arise)
how regularly does the foetus make breathing movements?
Every 1-4hrs each day
Why does the foetus make breathing movements?
'Practice'/build up muscles for life after birth
Draws amniotic fluid into and out of the lungs to 'flush' them
When do the lungs start secreting surfactant?
week 20 but production significantly increased after week 30 when the alveoli open in significant number and the surface area dramatically increases
What does surfactant do?
Lowers alveolar surface tension such that inspiration is made with less effort post-natally
What is respiratory distress of the new born?
Premature babies tend to have a deficiency of surfactant so breathing is much more difficult
How is the foetus provided with glucose?
Relatively high levels in mother - diffuses across placenta
When does the foetus begin to secrete insulin?
How does the foetus remove bilirubin?
Cannot be excreted by the foetal gut therefore it is not conjugated and so passes to the maternal circulation
Why is neonate jaundice fairly common?
Bilirubin is not conjugated by the foetus and it may not be able to conjugate it immediately after birth so jaundice results
How is the majority of the amniotic fluid produced in late pregnancy?
Foetal kidneys produce urine, particularly late in gestation
How much urine does the foetal kidneys produce?
@ wk 25, 100ml/day hypotonic urine
@ term, 500ml/day
What happens to the urine produced by the foetal kidneys?
Forms the amniotic fluid which is constantly swallowed so the gut absorbs water and electrolytes, leaving debris to accumulate and together with the debris formed from the developing gut accumulates in the foetal large bowel as meconium
When is menconium excreted?
Usually only when the foetus is in distress (e.g. if hypoxic)
What are the components of the amniotic fluid?
Cells from the foetus and amnion and a variety of proteins
What is the maximum volume of amniotic fluid and when is this reached?
1L at 38 weeks (may fall as labour nears)
What is an amniocentesis?
Removal of some of the amniotic fluid to perform biochemical and cytological studies on it to assess the presence of neural tube defects and chromosomal abnormalities
What is the amniotic fluid derived from in early pregnancy?
By dialysis of foetal and maternal extracellular components with some exchange occurring across the foetal skin
What is different about foetal skin?
It is non-keratinised
How is the amniotic fluid volume assessed?
What is polyhydraminos and what might cause it?
Excessive amniotic fluid
Oeophageal or duodenal atresia or CNS abnormalities
What is a low volume of amniotic fluid called and what might cause it?
Suggestive of poor or absent renal function or reduced placental function such as in pre-eclampsia
At what stage in development is withdrawal from pain recognised?
When do thalamo-coritical projections reach maturity?
When is myelination of corticospinal tracts complete?
Post-natally but musculoskeletal movements are essential for foetal growth
What promotes foetal corticosteroid production and why is this important?
The steoid is vital for foetal physiology esp CVS function
How and when are nervous system development, bone and hair growth stimulated?
Mediated via thyroid hormones active from week 12
What does the foetal liver do?
Stores large amounts of glycogen which is reflected in changes in foetal circumference
What induces the neonate to take its first breath?
Combination of physical trauma and cold temperature
What happens as a result of the neonate taking its first breath?
Dramatic reduction in pulmonary vascular resistance and a dramatic rise in arterial pO2.
The fall in resistance causes the left atrial pressure to rise in respect to right so closing the foramen oval
Smooth muscle sensitive to pO2 in the wall of the ductus arteriosus contracts to close the ductus
Both shunts close off completely within a few weeks