Fetal Physiology Flashcards
Describe the blood vessels present at placenta exchange
- Fetal capillaries within chorionic villi to increase surface area for exchange
- Uterine arteries and uterine veins on maternal side
- Maternal blood lakes in the intervillous spaces
- Umbilical arteries and umbilical vein on fetal side
Explain if oxygenated or deoxygenated blood carried in umbilical artery/vein
- Oxygenated blood carried via the umbilical vein
- Deoxygenated blood carried via the umbilical arteries
Explain the principle of ductus venosus
- DV connects umbilical vein carrying oxygenated blood to the IVC
- Blood enters right atrium
- Shunting around the liver maintains saturation of oxygen and nutrients in blood
Explain the principle of foramen ovale and crista dividens
- Right atrial pressure is greater than that in the left atrium
- Forces flaps of foramen ovale apart and blood flows into left atrium
- Free border of septum secundum forms a ‘crest’ - crista dividens
- Creates two streams of blood flow
- Majority flows to left atrium
- Minor proportion flows to right ventricle, mixing with blood from SVC (deoxygenated)
Explain the principle of ductus arteriosus
- Shunts blood from right ventricle and pulmonary trunk to aorta
- Joins aorta distal to the supply to the head and heart
- Minimizes drop in oxygen saturation
List factors promoting oxygen exchange to the fetus in the placenta
- Increased maternal production of 2,3 DPG
- Decrease affinity of Hb for oxygen, therefore release more oxygen at placenta
- Fetal haemoglobin
- HbF forms from week 12 (previously embryonic haemoglobin)
- 2 alpha subunits + 2 gamma subunits
- Greater affinity for oxygen because it doesn’t bind 2,3 DPG as effectively as HbA
- Double Bohr effect
Explain the double Bohr effect
- Speeds up the process of oxygen transfer
- As CO2 passes into intervillous blood, pH decreases
- Decreases affinity of Hb for oxygen causing more oxygen to be released
- At the same time, as CO2 is lost in the fetal circulation, pH rises
- Increases affinity for Hb for oxygen in the fetus
Explain CO2 transfer in placenta, including double Haldane effect
- Maternal physiological adaptation to pregnancy
- Progesterone driven hyperventilation
- Lowers pCO2 in maternal blood, creating a concentration gradient
- Double Haldane effect
- As Hb gives up oxygen, it can accept increasing amounts of CO2
- Fetus gives up CO2 as oxygen is accepted
- No alterations in local pCO2
Describe the fetal response to hypoxia
- HbF and increased [Hb]
- Redistribution of flow to protect supply to heart and brain (reduces flow to GI, kidneys and limbs)
- Fetal heart rate slows in response to hypoxia to reduce oxygen demand
- Fetal chemoreceptors detecting decreases pO2 or increased pCO2
- Vagal stimulation leading to bradycardia
- (in adults, vagal inhibition occurs leading to tachycardia)
What is normal heart rate in fetus
110-160 bpm
Explain the change in volume of amniotic fluid
- Amniotic fluid volume increases mainly through urine secretion from kidney
- Volume decreases through swallowing and absorption
- Some fluid reabsorbed into lungs to aid development
Describe the composition of amniotic fluid
- 98% water
- Electrolytes, creatinine, urea, bile pigments, renin, glucose, hormones, fetal cells, lanugo (soft hair covering fetus), vernix caseosa (wave covering fetal skin)
- Absorbs water and electrolytes
Explain meconium
- Accumulation of amniotic fluid substances such as bile
- Can be released in fetal distress and inhaled back into lungs
- Causes meconium aspiration syndrome
Distinguish between symmetrical and asymmetrical growth restriction
- Symmetrical - decrease in growth of all parts of the body
- Asymmetrical - head normal size but decreased abdominal growth
