Lecture 15 - Fetal Physiology Flashcards
Adaptation of low foetal pO2
HbF
Foetal haematocrit is higher
More Hb
More bloody supply to brain and heart
Foetal haemoglobin
Week 12 to term
2 alpha subunits and 2 gamma sub units
Greater affinity for oxygen because it doesn’t bind to 2,3 - DPG as effectively has HbA
Factors promoting oxygen exchange
Increased maternal 2,3 diphosphoglycerate - secondary to respiratory alkalosis
At higher partial pressures it unloads more
Double Bohr effect
- As CO2 passes into the intervillous blood pH decreases
- Affinity of oxygen decreases in maternal Hb (shift right)
- CO2 lost in foetus therefore pH rises
- Increases affinity for oxygen in Hb of foetus (shift left)
Why is pCO2 lower in maternal blood
Progesterone driven hyperventilation
Maintains concentration gradient so foetal CO2 is transferred to mother
Double Haldane effect
- Maternal Hb gives up oxygen and accepts more CO2
2. Total blood loads more oxygen and gives up more CO2
Foetal circulation
- Receives oxygenated blood from the umbilical vein.
- Bypasses the liver by the ductus venosus to the right atrium via the inferior vena cava
- Right atrium to the left atrium via the foramen ovale as bypasses the lungs
- From the aorta to the body to the umbilical artery to the maternal circulation
- Some blood enters the right ventricle and pulmonary trunk which enters the aorta via the ductus arteriosus
Ductus venosus
Umbilical vein to IVC
Bypasses liver as has a higher metabolic demand
Therefore maintains oxygen saturation
Foramen ovale
Right atrial pressure is greater than the left atrial pressure in the foetus
For aces foramen ovale leaves apart and blood flows into the left atrium
Crista dividens
Free border of septum secundum forms a crest
Minor proportion of blood flows into the right ventricle mixed with deoxygenated blood from the SVC
From the right ventricle it goes to the lungs to prevent right ventricular atrophy and supports lung development
Left atrium
Small amount of pulmonary venous return (deoxygenated)
60% saturation as mixed with right atrial oxygenated blood
Pumped to LV and aorta
Ductus arteriosus
Shunts blood from the right ventricle and pulmonary trunk to the aorta
Joins the aorta distal to the supply to the brain and heart to minimise drop in O2
Foetal response to hypoxia
Foetal heart rate decreases - reduce oxygen demand
Increased haemoglobin
Redistribution of flow to protect heart and brain
Foetal bradycardia
- Chemoreceptors detect hypoxia
2. Vagal stimulation leading to bradycardia
chronic hypoxia
Due to smoking
Effects:
Growth restriction
Behavioural change
IGF I
Insulin like growth factor nutrient dependent
Dominant in 2nd and 3rd trimester
IGF II
Nutrient independent
Dominant in 1st trimester
Cellular growth mechanisms in each trimester
Trimester 1 - hyperplasia
Trimester 2 (20 -28 weeks) - hyperplasia and hypertrophy
Trimester 3 - hypertrophy
Amniotic fluid
Amniotic sac encloses embryo and is filled with amniotic fluid
Provides mechanical protection and contains substances critical for lung development and surfactant production
Amniotic fluid is inhaled in the foetus to the foetal lungs
Swallowed - GI absorbs water and electrolytes and debris accumulates in the gut to form meconium
Production and recycling of amniotic fluid
In early pregnancy amniotic fluid is produced by the placental and foetal membranes - intermembranous pathway
Foetal urine production - 9 weeks - amniotic fluid
Also produced by the lungs and GI tract a little
Composition of amniotic fluid
98% water
Contains electrolytes + lanugo (fine hair) and vernix caseosa (white waxy substance that protects the skin)
Meconium
Debris from foetus and instestinal secretions including bile
Passed after delivery of baby - green due to bile
Amniocentesis
Sampling of amniotic fluid - collection of foetal cells
Used in foetal karyotyping and diagnostic test
Risk of miscarriage
Bilirubin metabolism
Placenta clears foetal bilirubin during gestation
Foetus cannot conjugate bilirubin due to immature liver
Physiological jaundice - delay in new borns ability to conjugate and excrete bilirubin
