Oxygen delivery to the foetus
Simple diffusion across the placenta
Diffusion barrier is < 5 microns capillary endothelium + trophoblast
pO2 of maternal blood is 50mm Hg
pO2 of fetus is 30 mm Hg
Diffusion gradient = 20 mmHg
However foetal oxygen reserve is very low, can be a problem in labour.
Carbon dioxide transport from the foetus
Maternal hyperventilation stimulated by progesterone reduces maternal pCO2
Establishes a concentration gradient between maternal and foetal blood.
Facilitates placental transfer of CO2 by simple diffusion
Fetus cannot tolerate higher pCO2 than mother. Causes acid-base problems
Describe flow of blood in the foetal circulation
Oxygenated blood from the placenta travels to the foetus cia the umbilical vein (70% saturated)
Blood is shunted around the liver via the ductus venosus to the IVC where it is mixed with desaturated blood from the body (65%)
Blood passes into the right atrium and is shunted away from the RV to the foramen ovale by crista dividens. Right pressure > left pressure, which aids flow. Blood joins pulmonary venous flow (saturation (60%)
From the left atria blood enters the left ventricle and is pumped out of the aorta to the carotids to supply the brain.
Blood drains back into the right atria via the SVC, and from the RV into the pulmonary artery. The ductus arteriosus shunts blood from the pulmonary artery to the aorta which transports blood to the rest of the body and back to the placenta.
State 3 differences between adult and foetal circulation
Ventricles act in parallel instead instead of in series
Preferential flow of blood to the brain
Shunts act to direct blood flow away from certain structures - ductus venosus, ductus arteriosus, foramen ovale
How is the foetal cardiovascular system monitored?
Imaging: size, position, anatomy
What are teh circulatory adaptations and occur after birth
After the first breath, the pulmonary vascular resistance decreases causing left atrial pressure to rise above right atrial pressure. This closes the foramen ovale
The ductus arteriosus contracts due to the high pO2 sensitivity of the smooth muscle
Both shunts close within minutes after birth. Complete closure is within weeks.
Ductus venosus remains partially open but closes 2-3 months after birth.
Briefly describe the development of the lungs
Lungs development begins at 4 weeks as a respiratory diverticulum of the foregut which eventually divides into two bronchi. The main bronchi divide into segments which form the lobes of the lung, then continously divide.
The lung develops and matures in 4 phases -
Psudoglandular (6-16weeks): lung consists of terminal bronchi surrounded by connective tissue which contain capillaries.
Canalicular phase (16-24 weeks): terminal bronchioles divide to form alveolar ducts and capilaries. Primordial alveoli (terminal saccules) begin to develop
Saccular phase (24 weeks - birth): number of terminal saccules increaes and epithelium differentiates into type 1 and type 2 pneumocytes.
Alveolar phase (32 weeks - 8years): walls of the alveoli thin and capillaries bulge in. Alveoli mature and new alveoli are formed.
Function of foetal lungs
The foetus makes breathing movements irrigating lungs with amniotic fluid.
Movements stimulate lung development and help condition respiratory muscles.
Role of surfactant in lung development
Surfactant lowers the alveolar surface tension, increasing lung complance so the alveoli inflate easily during inspiration.
Produced by type II pneumocytes from week 20 which significantly increases after week 30 when the alveoli begin to open and surface area incerases.
Deficiency in surfactant leads to respiratory distress because the lungs are stiff and difficult to inflate.
How are foetal lungs monitored
Breathing movements (seen from week 12-14)
Name 4 foetal behaviours which can be used to monitor development of the nervous system
10 weeks: Local stimuli evoke response
12-14 weeks: Breathing Movements
18-20 weeks: Movements
> 24 weeks : Ability to suck
Briefly describe the development of the urinary system
Foetal kidney develops from the mesonephros.
Between weeks 9-12 uretic buds and nephrogenig tissue interact to produce the metanephros which is the embryonic kidney
Some function begins at week 10 and urine production begins at 12 weeks. Empties every 60mins into the amniotic fluid.
Kidneys have a lobulated form until 4-5 years of age.
Describe the role of the gut and kidneys in regulating amniotic fluid
Early in pregnancy fluid is formed by ultrafiltration of maternal plasma.
The foetal kidneys produce urine which forms a major part of the amniotic fluid.
This fluid is contantly swallowed. The gut absobs water and electrolytes while the debris accumulates in the foetal large bowel to form meconium.
Meconium is usually only excreted if the foetus is in distress. e.g. hypoxia
How is feotal urinary function monitored?
Bladder function on ultrasound scans
Why is amniotic fluid an important marker of foetal development
Amniotic fluid turns over constantly. Water replaced every 3hrs, electrolytes every 15hrs.
Early pregnancy: ultrafiltrate of maternal plasma and foetal extracellular fluid. Later in pregnancy turnover controlled by foetus, mostly foetal urine.
Polyhydramnios is associated with oesophageal/duodenal atresia and CNS abnormalities. Could also be a sign of GDM
Oliogohydramnios indicates poor or absent renal function or reduced placental function e.g. pre-eclampsia
Fluid also contatins cells from foetus, amnion and proteins. Can be used diagnostically.