9. The foetal circulation (workbook)

Question 1

which organs does the placenta replace the function of?

Answer 1

1. foetal gut
2. lungs
3. kidneys

Question 2

in the foetus, blood from the umbilical vein returns from the placenta to combine with what?

Answer 2

the venous drainage of the gut

Question 3

in the absence of modification to the circulation, what would happen to the oxygenated blood of the umbilical veins?

Answer 3

the oxygenated blood would pass through the liver and the lungs and mix with venous blood from the body and brain before it reaches the systematic arteries - thus loosing most of its oxygen

Question 4

how is umbilical venous blood shunted around the liver?

Answer 4

by ductus venosus

Question 5

what does umbilical venous blood mixes with?

Answer 5

venous blood from the lower body in the ascending vena cava

Question 6

why is umbilical venous blood mixing with venous blood from the lower body in the ascending vena cava not a major problem?

Answer 6

the lower body of a foetus is relatively small and not that metabolically active

Question 7

where does venous blood from the brain travel to in a foetus?

Answer 7

right atrium via the descending vena cava

Question 8

what prevents mixing of venous blood from umbilical vein and brain?

Answer 8

crista dividens

Question 9

what does crista dividens do?

Answer 9

directs the oxygenated blood towards the foramen ovale

Question 10

what is the foramen ovale?

Answer 10

a hole between the right and left atrium which allows blood to by-pass the right ventricle and the lungs and enter the left heart directly

Question 11

where does deoxygenated blood from the brain pass to?

Answer 11

right atrium, RV then lungs

Question 12

what do the collapsed lungs of a foetus offer?

Answer 12

a high resistance to blood flow, so most of the output of the right heart flows through the ductus arteriosus instead

Question 13

what is the ductus arteriosus?

Answer 13

links the pulmonary artery with the aorta

Question 14

where does the ductus arteriosus join the aorta?

Answer 14

at level below the arterial outflow to the brain

Question 15

where is the oxygenated blood in a foetus directed to?

Answer 15

left heart via the foramen ovale (L-R atrium)

therefore mixed with very little blood from the lungs in the left atrium and remains well oxygenated

Question 16

what happens to the oxygenated blood in the left ventricle?

Answer 16

pumped out to the aorta and up to the brain

Question 17

what does the remaining oxygenated arterial flow join?

Answer 17

the flow from the ductus arteriosus (PA - aorta) to supply the rest of the body (needs are less critical than heart and brain)

Question 18

where does deoxygenated blood return?

Answer 18

returns to the placenta in the umbilical artery for reoxygenation

Question 19

what does the pattern of foetal circulation require?

Answer 19

the pressure in the left atrium to be LOWER than the right (right higher)

Question 20

why does the the pressure in the left atrium need to be LOWER than the right?

Answer 20

1. so blood flows the right way through foramen ovale (R to L)
2. the pressure in the pulmonary artery is higher than aorta
3. flow in ductus arteriosus will be the right way (PA to aorta)

Question 21

with changes in the foetal circulation, what is the partial pressure of oxygen in foetal blood like?

Answer 21

the ppO2 of foetal blood is v LOW compared to adult

Question 22

how is the low ppO2 of foetal blood achieved?

Answer 22

1. a different haemoglobin

2. higher haemoglobin levels (within RBC)

Question 23

how is foetal haemoglobin different from adults?

Answer 23

2 gamma and 2 alpha instead of 2 alpha and 2xß chains

foetal haemoglobin has a much HIGHER affinity for oxygen - carry more O2 at lower pO2

Question 24

how does the foetus have a higher haemoglobin level?

Answer 24

the neonate has a haemoglobin level of at least 18g.dl-1

foetus has more haemoglobin compared to adults

Question 25

what is the foetal pCO2 like? why?

Answer 25

foetus has relatively normal pCO2
the maternal CO2 levels are LOWERED by hyPERventilation stimulated by progesterone
(hyperventilation of mother allows for normal pCO2 within foetus)

Question 26

does the foetus make breathing movements? why?

Answer 26

YES

to draw amniotic fluid in and out of lungs

Question 27

what is produced in foetal lungs from around week 20?

Answer 27

surfactant produced by type II pneumocytes (lung epithelium)

Question 28

when is production of surfactant in foetus significantly increased?

Answer 28

after week 30 when the alveoli open in significant number and the surface area dramatically increases

Question 29

what is the function surfactant?

Answer 29

lowers alveolar surface tension so that inspiration is made with LESS effort post-natally

Question 30

what does a deficiency in surfactant result in?

Answer 30

pre-term infants (pre-mature)

can lead to respiratory distress of the newborn