Foetal Adaptations to Birth Flashcards
Intrauterine environment - how is it different from outside world?
What is the basic direction of gas exchange across the placenta.
O2 + nutrients = from maternal blood –> into intervillous spaces –> into chorionic villi –> foetal blood
CO2 = from foetal blood –> into chorionic villi –> into intervillous space –> into maternal blood
Describe how O2 & nutrients are exchanged occurs across the placenta.
O2 & nutrients diffuse from spiral arteries (maternal blood) -> across trophoblast memb -> into branches of umbilical vein/chorionic vein (within placental villi) -> then flows into umbilical vein -> then through umbilical cord to foetal circulation O2 & nutrients distributed to heart, brain & all foetal organs & tissues
Describe how CO2 is exchanged occurs across the placenta.
CO2 & other waste products from foetus body -> umbilical arteries (x2) -> branches of umbilical artery/chorionic artery (within placental villi) -> across trophoblast memb -> through uterine veins (maternal blood)
How does O2 transport occur - what facilitates it?
-Placenta has low resistance
-Maternal blood PO2 is higher than foetal -> so O2 readily diffuses across to foetus = pressure gradient set up
How does CO2 transport occur - what facilitates it?
-Placenta has low resistance
-Foetal blood PCO2 is higher than maternal -> so CO2 readily diffuses across to mother = pressure gradient set up
What else assists in the movement of CO2 across placenta?
Haldane effect = the increased capacity of deoxy blood to carry CO2 compared to oxygenated blood
-As foetal blood absorbs O2 to form oxyhaemoglobin – it has reduced affinity for CO2 – so releases this CO2 to mother
-Whereas waste products e.g., urea, uric acid, creatinine – diffuse passively due to conc grads – from foetal to maternal blood
How is foetal haemoglobin different from adult haemoglobin -> describe process involved?
-Foetal = x2 alpha + x2 gamma chains
-Adult = x2 alpha + x2 beta chains
-Foetal Hb = has higher affinity for O2 (so is shifted to left on oxyhaemoglobin dissociation curve) - more readily binds/associated O2 - higher O2 carrying capacity
-Adult Hb = has lower affinity for O2 (so curve is further to right) - more readily dissociates O2 - lower O2 carrying capacity
–> more 2,3-DPG at placenta - beta chains (in HbA) bind to 2,3-DPG more readily than alpha (= stabilises deoxygenated Hb by allosteric binding & facilitates O2 release at tissue)
–> so 2,3-DPG enables more O2 release from HbA at placenta - to HbF - so HbF can then deliver O2 to foetal tissues
(as HbF is unaffected by 2,3-DPG)
Why is foetal haemoglobin (HbF) different from adult haemoglobin (HbA)?
Foetal arterial oxygen pressures are lower than adult arterial oxygen pressures
–> so HbF requires the ability to associate with more O2 (as there is less of it!)
Describe foetal circulation (& shunts involved)?
- Blood from placenta –> flows to umbilical vein (oxygenated blood)
- Ductus venosus shunts blood from umbilical vein to inferior vena cava
- Inferior vena cava flows to right side of heart – RA
- Blood will bypass lungs (as lungs are filled with fluid not air – so pressure is too high to get blood to flow to them – need pressure gradient) by:
- Foramen ovale –> shunts blood from RA to LA (as right side of heart has higher pressure than left)***
- Flows down to LV
- Up aorta
- Down descending aorta
- Joins up with umbilical artery (deoxygenated blood)
- –> to placenta
*** Another shunt – called ductus arteriosus:
-As some blood flows from RA -> RV
–> so shunt moves blood up pulmonary artery & to the aorta (this bit is the shunt) –> to aorta –> down descending aorta –> joins up with umbilical artery –> to placenta
Name the 3 shunts in foetal circulation & where they shunt blood from & to.
-Foramen ovale - shunts blood from RA to LA
-Ductus arteriosus - shunts blood up pulmonary artery & to aorta
-Ductus venosus - shunts blood from umbilical vein to inferior vena cava
What facilitates the movement of blood from right to left side of heart?
*Higher pressure on right side compared to left
-High pulmonary vascular resistance
-Hypoxaemia – pulmonary vasoconstriction
Why are the lungs bypassed in foetal circulation?
As lungs are filled with fluid not air – so pressure is too high to get blood to flow to them – need pressure gradient
What occurs at birth to baby’s circulation - simple description?
-Change in circulation occurs in first few breaths as infants makes a transition to extrauterine life
–> this occurs as umbilical cord is cut
-Pink lips & tongue, rest cyanosed (blue) as lower oxygen saturation in utero – takes several hours for peripheral circulation to make full transition
Give the process of the circulatory changes at birth.
-Circulatory changes – once umbilical cord is cut –> then gas exchange is no longer from placenta
-First breaths transforms circulation in infant
Inflation of lungs reduces pulmonary vascular pressure
-Gas exchange at lungs = rise in PO2
–> causes vasodilatation of pulmonary vasculature (factor causing higher pressure in right side than left – enabling blood shunts)
–> significantly reduces pulmonary vascular resistance (factor causing higher pressure in right side than left – enabling blood shunts)
-Reversal in interatrial pressure gradient causing a valve over foramen ovale to close = functional closure – leaves fossa ovalis (small depression)
-Reversal in pressure gradient between pulmonary artery & aorta causes flow to reverse through ductus arteriosus
-Loss of placental PGE2 & the increased aortic PO2 triggered by respiration also triggers contraction of ductus arteriosus – causing a functional closure of ductus arteriosus = forms ligamentum arteriosum
-Ductus venosus closes = forms ligamentum venosum