Fetal Physiology Flashcards
What problems does a fetus face in regards to oxygen supply?
Oxygen diffuses across placenta from maternal blood
- Barrier small - fetal villi contact with maternal blood
Need a gradient of partial pressures
Maternal pO2 increased
But umbilical venous pO2 must be much lower
&
Umbilical venous blood is delivered to venous side of fetal circulation
So fetus must cope with low pO2 blood arriving in the wrong place
How is a fetus equipped to increase the oxygen content of it’s blood?
At best fetal pO2 is 4kPa (pO2 of adults = 13.3kPa) Fetus has different haemoglobin - 70% saturated at 4kPa - (Adult Hb 45% saturated) Fetus has more haemoglobin - 18-20 g.dl/\-1 (Fetal Hb has no beta chains - doesn't readily bind 2,3DPG)
What is the oxygen content of a fetus?
At 4kPa, fetal blood contains c7.5mmol.l/-1 oxygen
- Similar to adult blood
Describe the oxygen transfer across the placenta.
Low diffusion resistance
Partial pressure gradient c9kPa
Higher affinity of fetal Hb
Double Bohr effect, in both maternal & fetal blood
Oxygen transport rate determined by umbilical arterial pO2
Fetus gets oxygen it needs
What are the capacity of fetal oxygen stores like?
About 2 minutes worth
Can be a problem in labour
What is the significance of carbon dioxide in the fetus?
Fetus cannot tolerate higher pCO2 than mother
- Acid-base problems
Placental transfer of CO2 facilitated by lower maternal pCO2
Hyperventilation stimulated by progesterone
What is the problem with the delivery of umbilical venous blood to the fetus?
And why would this normally be a problem?
Umbilical venous blood effectively delivered to hepatic portal vein
If adult form of circulation would:
- Lose oxygen in liver
- Mix with deoxygenated venous blood from body in IVC
- Mix with deoxygenated blood from head in RA
- Lose oxygen in lungs
Which would leave desaturated blood for fetal brain
What are the different shunts a fetus has?
Ductus venosus - Around liver Foramen ovale - Flow from inferior vena cava directed selectively to left atrium Ductus arteriosus - Pulmonary artery to aorta - Distal to branch to head
What is the flow of oxygenated blood around the fetal body?
- Arrives umbilical vein 70% saturated
- Shunts around liver
- Joins ascending flow in inferior vena cava
- Mix with small amount od desaturated blood
- Saturation drops to 65%
- Ascending flow directed by crista dividens to foramen
ovale- Right atrial pressure >left
- Joins small pulmonary venous flow (deoxygenated)
- Small because most pulmonary flow diverted
- Saturation 60%
- Left atrium to left ventricle
- Aorta to carotids
- Reaches brain c60% saturated
- Oxygen content c7mmol.l/-1
- Supports brain growth and development
- Returns to superior vena cava
- Superior venous flow separated from inferior
- Superior to right ventricle
- Right ventricle to pulmonary artery
- Ductus arteriosus
- To aorta
- Distal to supply to head
- Aorta to umbilical artery
- Picks up oxygen
- And start again
In summary, flow goes left heart - right heart rather than vice versa
Tell me a little about the fetal lungs.
Once they are developed enough - about 20 weeks
The fetus makes breathing movements
- 1-4 hr each day
- Flush lungs with amniotic fluid
What happens once the fetus is born in relation to it’s circulation?
The circulation is the wrong way round for after birth
The now baby takes a breath and this moves the flap of the foramen ovale back and blocks off that shunt making the circulation go the right way round.
What are some of the functions of amniotic fluid?
Surrounds fetus:
- Mechanical protection
- Moist environment (skin is not keratinised early on)
- Other functions
How much amniotic fluid is there throughout gestation?
About 10ml at 8 weeks
Rising to 1L at 38 weeks
Then falls to 300ml at 42 weeks
How is amniotic fluid formed?
Early in pregnancy - Formed from maternal fluids - And from fetal extracellular fluid by diffusion across non- keratinised skin Later in pregnancy - Turnover via fetus
Tell me about fetal urine.
At 25 weeks fetus prduces about 100ml hypotonic urine per day
Rising to about 500ml at term
Fetus swallows amniotic fluid constantly (same rate as excretion)
Absorbs water and electrolytes
Debris accumulates in fetal gut
- Together with gut debris form meconium
