15. Fetal growth and development Flashcards
In which umbilical vessel is oxygenated blood carried?
- Umbilical arteries bring unoxygenated blood from the fetus to the placenta
- umbilical vein takes oxygenated blood from the placenta back to the fetus
what is needed to maintain the concentration gradient of oxygen at the placenta?
- Maternal pO2 increases only marginally
* Therefore to make the gradient work, fetal pO2 must be lower than maternal pO2
Describe the features of uteroplacental circulation
placenta as the main exchange unit between the
maternal and fetal circulations.
The chorionic villi allow for greater surface area for exchange, and are bathed in maternal blood from the
uterine arteries.
The uterine arteries develop to allow a high flow, low resistance circulation in the utero-placental unit.
what is the pO2 of fetal blood?
- Fetal blood has low pO2
* pO2 approx. 4kPa compared to normal adult pO2 of 11 – 13kPa
What adaptations does the fetus have to overcome the low pO2?
- Fetal haemoglobin variant - greater affinity for oxygen transfer
- Fetal haematocrit is increased over that in the adult
- Increased maternal production of 2,3 DPG
- Double Bohr effect
What ABG state is the mother in during pregnancy and how does this affect the Hb saturation curve?
- Progesterone causes a physiological hyperventilation, resulting –> physiological respiratory alkalosis.
- Lower CO2 levels to maintain conc gradient at placenta
- Increased pH would ordinarily increase maternal affinity for oxygen, shifting the oxygen dissociation curve to the left.
Why isn’t gas exchange compromised despite rise in mothers blood pH?
increased maternal 2,3-BPG is produced, which shifts the curve back to the right to reduce the Hb affinity for oxygen. This will ultimately promote release of oxygen to the fetus
What is the predominant form of Hb present in the fetus?
HbF (2 alpha and 2 gamma subunits), from week 12 to term
What produces Hb in the fetus?
Yolk sac -> Liver -> Bone
What is affinity of HbF for oxygen compared to HbA and why?
Greater affinity for oxygen because it doesn’t bind 2,3-BPG as effectively as HbA
What is the double bohr effect and why is it important?
Bohr effect on both the maternal and fetal circulations. This will speed up oxygen transfer
Describe how the double bohr effect works.
The mother:
As CO2 passes into the intervillous blood, the pH will decrease, causing a decreased affinity for oxygen for the mother so O2 more easily released for transfer.
The fetus:
The fetus will be giving up CO2 due to the gradient of transfer, causing increased pH, which results in increased affinity of Hb for oxygen.
What is the Haldane effect?
Describes the ability of hemoglobin to carry increased amounts of carbon dioxide (CO2) in the deoxygenated state
Describe the double haldane effect?
The mother:
As maternal Hb gives up oxygen, it can accept increasing amounts of CO2.
The fetus:
The fetal Hb gives up more CO2 as oxygen is accepted.
What is the function of the umbilical arteries and veins in the fetal circulation?
- Receives oxygenated blood from mother via placenta in umbilical vein
- Returns to the placenta via umbilical arteries
What is the function of the lungs in fetus circulation?
• Lungs are non-functional
What shunts are present in the fetal circulation?
Ductus venosus: bypasses the liver
Foramen ovale: bypasses the right ventricle & lungs
Ductus arteriosus: bypasses the lungs
What is the saturation of HbF in the umbilical vein?
70%
Why is the ductus venosus needed, what is oxygen saturation after this?
- DV connects umbilical vein carrying oxygenated blood to the IVC
- Blood enters right atrium
- By ensuring shunting of blood around the liver, saturation is mostly maintained
- Drops from 70% to 65
Why does the fetal circulation need to bypass the liver?
Very large and highly metabolic so could potentially engulf the whole fetal circulation.
Shunting the blood away from the liver maintains a high level of oxygen in the circulation that will be arriving to the brain and rest of the fetal body
Why is the foramen ovale needed, what is oxygen saturation after this?
Bypass the right ventricle and lungs
- drops from 65 to 60%
- Right atrial pressure is greater than that in the left atrium
- Forces leaves of FO apart and blood flows into LA
Why does some blood from the right atrium go into the right ventricle?
- Free border of septum secundum forms a “crest” - crista dividens
- Creates two streams of blood flow
- Majority flows to LA
- Minor proportion flows into RV, mixing with blood from SVC
What is the saturation of blood in LA and why is this important?
• only Small amount of pulmonary venous return
- deoxygenated
• Blood reaching left atrium has saturation approx. 60%
• Pumped by LV to aorta
• Heart and brain get lion’s share of oxygen
Why is the ductus arteriosus necessary, where does it join the aorta?
- Shunts blood from RV and PT to aorta
- joins aorta distal to the supply to the head (and heart)
- Minimising drop in O2 saturation
