Fetal Physiology Flashcards
How does gas exchange at the placenta work -
• Diffusion barrier
– Small, and decreases as pregnancy
proceeds
• Gradient of partial pressures required
• Maternal pO2 increases on marginally
• Therefore to make the gradient work, fetal pO2 must be lower than maternal pO2
Compare fetal to adult pO2
Fetal blood = low pO2 ~4kPa compared to normal adult 11-13 kPa
But factors increasing fetal o2 content
– Fetal haemoglobin variant
– Fetal haematocrit is increased over that in the adult
Ss
What are factors promoting O2 exchange at placenta
• Increased maternal production of 2,3 DPG
– Secondary to physiological respiratory alkalosis of pregnancy
• Fetal haemoglobin
• Double Bohr effect
– Fetal haemoglobin variant
– Fetal haematocrit is increased over that in the adult
– 0.513 – 0.56 l/l cf 0.4 – 0.54 l/l in adult males
– Fetal Hb = 166 – 175g/L cf , 95 – 140 g/L at 2 years and 130 – 180 g/L in adult males
Describe fetal Hb
- Predominant form from weeks 12 – term is HbF
- 2 alpha subunits plus 2 gamma subunits
- Greater affinity for oxygen because it doesn’t bind 2,3-DPG as effectively as HbA
Describe teh double Bohr effect
- Speeds up the process of O2 transfer
- As CO2 passes into intervillois blood, pH decreases
- Bohr effect
- Decreasing affinity of Hb for O2
- At the same time, as CO2 is lost from fetal circulation, pH rises
- Bohr effect
- Increasing affinity of Hb for O2
Describe CO2 transfer
- Maternal physiological adaptation to pregnancy
- Progesterone-driven hyperventilation
- Hence lower pCO2 in maternal blood - blows of CO2
- Concentration gradient
Descrbe ethe double haldane effect
- Double Haldane effect
- As Hb gives up O2, it can accept increasing amount of CO2
- Fetus gives up CO2 as O@ is accepted
- No alterations in local pCO2
Describ efetal circulation
• Receives oxygenated blood from mother via placenta in umbilical vein
• Lungs are non-functional
• By-passes the lungs
• Returns to the placenta via umbilical arteries
Shuts around liver - wrong side of heart 0 shunted to left side - round body
What are teh fetal circulation shunts
Ss
Why is the DV needed
• 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 is the FO needed
• Right atrial pressure is greater than that in
the left atrium
• Forces leaves of FO apart and blood flows into LA
• Free border of septum secundum forms a “crest” – crista dividens
• Creates to streams of blood flow
• Majority flows to LA
• Minor proportion flows to RV, mixing with blood from SVC (deoxygenated)
What happens in the left atrium
• Small amount of pulmonary venous return
– doxygenated
• 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 DA needed
- Shunts blood from RV and PT to aorta
- NB: joins aorta distal to the supply to the head (and heart)
- Minimising drop in O2 saturation
Describe the fetal response to hypoxia
• Adaptations to manage transient decreases in oxygenation
• HbF and increased [Hb]
• Redistribution of flow to protect supply to heart and brain (reducing supply to GIT, kidneys,
limbs)
• Fetal heart rate SLOWS in response to hypoxia to reduce O2 demand
• Fetal chemoreceptors detecting decreased pO2 or increased pCO2
– Vagal stimulation leading to bradycardia
– cf adult where vagal inhibition leads to tachycardia
• Chronic hypoxaemia
– Growth restriction
– Behavioural changes
• Impact on development
What are teh hormones necessary for fetal growth
– Insulin
– IGFI and IGFII
– IGF II nutrient independent, dominant in first trimester
– IGF1 nutrient dependent, dominates in T2 and T3
– Leptin
• Placental production
– Plus EGF, TGFa
