Lecture 12: fetal circulation and the challenges of oxygen delivery to the fetus Flashcards
1
Q
What is the function of the placenta
A
- Function: supplies all gas and nutrient exchange to fetus
2
Q
How does exchange take place at the placenta
A
- Exchange happens in medium of intervillous space
- Between: Placenta+ foetal capillary
- Barriers for exchange= chorion, endothelial cells of villi
- Chorion contains synctiotrophoblasts
3
Q
Is the blood of the intervillous space mixed?
A
- Blood of intervillous space= venous+ arteriole
4
Q
Explain the reason for fHb being different to aHb
A
- If foetal Hb were the same as adult Hb + equal number of RBCs in blood flow in both
- > oxygen would reach a balance between mum’s and baby’s bloodstream of 50% O2 saturation
- Foetus needs more + continuous uptake of O2=> foetal hb has higher affinity to O2 compared to adult hb
5
Q
Explain the strucuture of Hb
A
- 4 globins (2α and 2β)
- 1 haem for each globin
- O2 binds to haems
- Found inside red blood cells (in vertebrates)
6
Q
How can Hb affinity to be oxygen be reduced?
A
- Phosphate in β- globin has (-) charge
-> attracted to (+) charge of histidine in protein chain - 143 a.a in β- globin= histidine-> has (+) charge
- When 2,3-BPG present-> tugs on His= bends β- globin
= reduced affinity to O2
7
Q
Explain the difference in fHb structure
A
- Foetal Hb has different globins:
γ as opposed to β ( 2α+ 1β + 1γ)
Histidine is replaced with serine (neutral) as aa #143
= gamma globin does not bind well to 2,3-BPG
= foetal Hb has a higher affinity for O2
8
Q
Explain the dissociation curve of fHb
A
- Graph shows that at same PO2 (environment)-> O2 saturation of foetus is higher
- O2 diffused from adult hb to foetal hb
->because foetal hb has greater affinity to O2 - E.g- at PO2 of 26.8= 50% for mum + 75% for foetus
= foetal hb has higher affinity to O2 at any given PO2 - Foetal Hb has a left-shifted oxyHb dissociation curve-> final balance of O2 is not 50/50
9
Q
Name and explain the other feature of feotus that allows increased oxygen uptake
A
- Foetus has a high haematocrit (ratio of the volume of red blood cells to the total volume of blood)
-> 14-18 g/dL vs. 11.5-15.5 g/dL in adults
= lots of Hb molecules to bind oxygen - Mum just has to keep her RBCs moving through the intervillous space at a fast-enough rate to keep filling baby’s Hb with oxygen
->to maintain conc. gradient with replenishing constantly with O2 rich blood
10
Q
What are shunts the solution to
A
- Some organs not used-> mother already uses her organ to complete process
->e.g lungs- respiration, liver-filtration of blood + more
= Don’t want to send lots of blood to organs that don’t need much
= solution- SHUNTS
11
Q
Name and explain the function of the shunts in feotal circulation
A
- Ductus venosus (DV)
- Allows a larger portion of the blood to bypass the foetal liver
- Small portion of fresh blood from placenta supplied to liver for growth
- >mum’s liver is processing much of the blood on behalf of the baby
- slowly closes up in the weeks following birth - Foramen ovale (FO)
- Allows a larger portion of the blood to bypass the pulmonary circuit ->baby isn’t using its lung to breathe
- DV running next to + separately the inferior vena cava
- O2 rich blood coming from umbilical vein
- Umbilical vein-> DV-> FO->LA->LV->Aorta
- Skips going to right side of heart + pulmonary circuit
- As a foetus->lungs are collapsed
-> arterioles that supply them with blood are constricted
= creates great resistance to flow - To overcome this -> need a lot of force from the right ventricle
- Doesn’t benefit-> because lungs don’t need much blood at this
- Solution= FO
- Ductus arteriosus
- Allows a portion of the blood to bypass the lungs
- Connection between the pulmonary artery and the aorta
- Joins the aorta downstream of the branches leading to the carotid arteries-> blood from umbilical vein goes to brain via upstream
= Brain gets O2 rich blood
- Purpose of shun system = to ensure that the freshest blood gets to baby’s brain
12
Q
Explain the factors that contribute to baby’s first birth
A
- Drop in ambient temperature post delivery
- Hypercapnia (↑ CO2) during delivery
- Especially if labour difficult/prolonged
- PCO2 ↑ during & just after delivery - ↑ sensory input
- Tactile, auditory, visual may stimulate breathing
- Pulmonary Surfactant is vital (opposes surface tension)
Phospholipoprotein complex
↓ surface tension of water molecules lining alveoli & ↑ lung compliance
(accommodates stretch)
13
Q
Explain the changes to fetal circulation that happen at birth
A
- foetal circulation must adapt quickly after first breath
- IMPORTANTLY -blood no longer bypasses lungs & foetal shunts must close
1. When baby breathes -> pulmonary blood flow ↑↑↑ - Alveoli inflate as air moves into lungs
- ↑↑ PO2 of blood perfusing lungs
- vasodilation of pulmonary vessels (opposite response to ↑ O2 in systemic vessels)
- ↓ resistance
- When baby separated from placental blood supply- umbilical vessels shut down
- Spontaneous or clamping by obstetrician
- This changes blood flow pattern & contributes to closure of shunts
14
Q
Explain how the shunts close
A
Foramen Ovale
- during foetal life right atrial P similar to left atrial P
- At birth: pulmonary resistance ↓ pulmonary perfusion ↑
- Blood flow ↑ to left atrium
- END EFFECT = LEFT ATRIAL P EXCEEDS RIGHT ATRIAL P
- Foramen ovale = 2 unfused septa
- If right atrial P> left atrial P -> septa remain apart = shunt is OPEN
- If left atrial P > right atrial P -> septa are forced together = shunt is CLOSED
- Should close completely within few days of birth
Ductus Arteriosus and Ductus Venosus - Mechanisms not fully understood
- Current ‘winning’ hypothesis
- smooth muscle of DA constricts in response to ↑↑ PO2 with 1st few breaths of life
- DV constricts in similar fashion (but, due to what?)
- Permanent closure occurs at ~ 10 days
15
Q
Describe the clinical defects that can occur due to improper closing of shunts
A
- Forman ovale may remain patent (stays open after birth)
- 15-20% of congenital heart defects
= leads to Right-left shunting in atria - > Deoxygenated blood bypasses the lungs
- > Enters systemic circulation
- > Produces cyanosis = blue-purple discolouration of skin & mucous membranes
- Atrial septum does not develop properly
= leads to Left-right shunting in atria
->Extra blood may cause volume overload for right side of heart
->Right side hypertrophy
= Heart failure
