7.2: Fetal Physiology Flashcards
What do the physiological functions of the baby depend on?
Maternal systems
Describe how oxygen reaches the fetus from maternal arteries and is circulated in the fetus!
Diffuses across placenta -> umbilical vein -> bypasses liver via ductus venosus -> IVC -> R atria-> Some to R ventricle and pulmonary trunk, SMALL amount into lungs due to high vascular pressure -> Most -> ductus arteriosus -> aorta (lower vascular resistance) OR R->L atria via foramen ovale -> L ventricle -> aorta (meets with blood entering from ductus arteriosus)Aorta-> systemic distribution->umbilical artery -> placenta
Name 4 factors that contribute to increasing oxygen saturation in fetal blood.
- Fetal hb has higher O2 affinity (no beta chains (only 2 alpha and 2 gamma)) 2. More hb 3. Low diffusion resistance4. Double Bohr effect (in maternal and fetal blood)
Why is even a short interruption to bloodflow possibly detrimental to the fetus and when can this be a problem in labour?
Fetus only stores O2 for ~2min. Frequent contractions in labor may constrict available bloodflow without giving much recovery time between
What should you do if you notice fetal deceleration?
Take blood sample from fetus head to check O2 *fetal deceleration is a decrease in fetal HR below the fetal baseline HR, measured with CTG.
How might you determine whether the fetus has acidosis?
Take lactate level
What enables the fetus to have a relatively normal pCO2?
Maternal CO2 levels are low due to physiological hyperventilation (progesterone)
What happens to O2 saturation as it travels from the umbilical vein to and throughout the fetus?
What is the O2 saturation when blood reaches the carotids?
Saturation drops slightly when blood mixes with deoxygenated blood (therefore neonatal O2 is low)
Drops when it mixes with
- 70-65% IVC
- to 60% pulmonary venous flow
- > reaches carotids 60% saturated
What is the role of the crista dividens?
Division in R atria that can direct bloodflow into foramen ovale
From the aorta blood is sent to the fetal brain, arms and heart muscle. Where does it go after?
Down SVC back to R ventricle -> pulmonary artery -> some to lungs but most to descending aorta via ductus arteriosus
What produces meconium and what is its role?
What happens if there are excessive amounts and why might this happen?
Fetal intestines, initially helps pass ‘earliest stool’
Excessive amounts released with stress (i.e hypoxia), can lead to
- > increased chances of bacterial infection (even though its sterile)
- > aspirated into lungs -> hinder O2 transfer
What is amniotic fluid composed of? WHen does it reach its max volume and how can its volume be assessed?
Maternal fluids and fetal extracellular fluid (that diffused across fetus’ non-keratinized skin).
Reaches max volume close to term, can be assessed with USS
How would you diagnose obstetric cholestasis and how might this affect the baby?
If maternal bile acids are high even though all infection screens and LFT come back negative.
Baby relies on mother’s liver to remove
bile acids from blood, if it doesn’t
-> crosses placenta -> stresses fetal liver
-> cause peristalsis -> excessive meconium passes -> aspiration -> stillbirth
Why might amniocentesis be performed?
Check for chromosomal abnormalities and neural tube defects
What are the risks of performing amniocentesis early and later in the pregnancy?
Early: miscarriage or vascular damage (if needle penetrates umbilical vessels)
Late: infection
How does the mother’s inability to conjugate bilirubin affect the fetus?
Jaundice (Fetus cannot conjugate bilirubin)
The placenta replaces the function of which fetal organs?
Fetal gut, lungs and kidneys
When does fetal insulin secretion begin?
Week 10
How does the fetal gut absorb water and electrolytes?
Constantly swallowing amniotic fluid
Describe when and how the fetal endocrine system plays a part in fetal development
- Placental progesterone -> promotes fetal corticosteroid production -> vital for CVS function
- Thyroid hormones -> development of nervous system, bone & hair growth
Describe how fetal circulation adapts once the baby is been born
*include the disappearance of fetal structures
- Trauma and cold temperatures -> first breath -> Lungs inflate -> fall in pulmonary vascular resistance -> increased bloodflow to pulmonary vessels -> L atrial pressure>R atria -> foramen ovale closes
Smooth muscle in ductus arteriosus is sensitive to high pO2 and prostaglandins -> contracts
Cord clamping -> lack of blood flow -> constriction in ductus venosus sphincter -> blood goes to liver sinusoids.
How is rhesus disease managed and when is this strategy INeffective?
Anti-D IgG injections remove RhD fetal blood cells before they cause sensitization.
Won’t work if she’s made anti-D IgG in previous pregnancy -> close monitoring of pregnancy and baby
Name three treatment methods for a baby born with rhesus disease
- Phototherapy
- Blood transfusions
- IV immunoglobulins (antibodies) to prevent RBCs from being destroyed
Define cholestasis, what causes it? (2)
Decrease in bile flow
- Impaired secretion by hepatocytes
- Obstruction
Which direction does the fetal O2 dissociation curve shift and why?
Left:
1. Bohr effect: CO2 crossing the placenta from fetus-mother creates an area of local acidity -> lowers maternal Hb’s O2 affinity -> increases diffusion across placenta
- Higher fetal Hb level and O2 affinity facilitates uptake from placenta-fetus.
Name three major ways in which fetal circulation differs from normal circulation
- Oxygenated blood enters circulation via placental transfer
- Pulmonary blood flow accounts for <20% of total CO
- Fetal vascular structures exist to direct blood flow
Name the adult structures that arise from the following embryological ones:
a) foramen ovaleb) ductus arteriosusc) ductus venosusd) umbilical veine) umbilical arteries
a) foramen ovalisb) ligamentum arteriosumc) ligamentum venosumd) ligamentum terese) medial umbilical ligaments (and superior vesical arteries to urinary bladder)
How much time passes after birth for the following shunts to undergo functional and anatomical closure? a) ductus arteriosusb) foramen ovalec) ductus venosus
a) ductus arteriosusFunctional: 10-96 hrs Anatomical: 2-3 weeks b) foramen ovaleFunctional: several mins Anatomical: 1 year c) ductus venosusFunctional: several mins Anatomical: 3-7 days
Describe the ‘trauma’ process that initiates breathing in a newborn
Cord is cut -> O2 drop -> asphyxia -> acidotic state stimulates respiratory centre in medulla and chemoreceptors in the carotid to initiate breathing
