9. foetal physiology (lecture)

Question 1

where does materno-foetal exchange occur?

Answer 1

at the placenta

Question 2

what are the structures involved in materno-foetal exchange?

Answer 2

umbilical arteries
umbilical veins
foetal capillaries with chorionic villi
uterine arteries
uterine veins

Question 3

function of the foetal capillaries with chorionic villi?

Answer 3

increase surface area

Question 4

function of uterine veins?

Answer 4

maternal blood flows in the intervillous spaces

Question 5

what is carried via the umbilical vein and arteries?

Answer 5

oxygenated blood in u. vien

deoyxgenated blood in u. artery

Question 6

how does the umbilical circulation connect within the foetal circulation?

Answer 6

across membrane of chorionic villi

the 2 circulations do not mix

Question 7

describe the placenta as a diffusion barrier for gas exchange

Answer 7

the diffusion barrier of the placenta is small and decreases as pregnancy proceeds

Question 8

what is required for gas exchange at the placenta to occur?

Answer 8

gradient of partial pressures

Question 9

what happens to maternal pO2 within pregnancy?

Answer 9

maternal pO2 increases only marginally

Question 10

if maternal pO2 only increases marginally in pregnancy and there needs to be a gradient of partial pressure for gas exchange to occur, then what has to happen?

Answer 10

foetal pO2 must be lower than maternal pO2

Question 11

if foetal blood has a low pO2, then what are the factors increasing foetal O2 content?

Answer 11

foetal haemoglobin variant (gamma instead of ß)

foetal haematocrit is increased to greater than adult

Question 12

what is haematocrit?

Answer 12

ratio of RBC to blood volume

Question 13

what are additional factors promoting O2 exchange to the foetus at the placenta?

Answer 13

increased maternal production of 2,3 DPG (secondary to physiological respiratory alkalosis of pregnancy) - double bohr effect
foetal haemoglobin (gamma)

Question 14

function of 2,3-DPG?

Answer 14

O2 liberated from Hb to tissues easier

more difficult to pick up O2, but released a lot easier

Question 15

what is the Hb that foetus’ have?

Answer 15

HbF (2 alpha, 2 gamma)
greater affinity for oxygen, doesn’t bind to 2,3-DPG as effectively as HbA
(picks up O2 easier, but doesn’t release as easily - opposite to adults)

Question 16

purpose of double bohr effect?

Answer 16

speeds up the process of O2 transfer

Question 17

what is the double bohr effect?

Answer 17

as CO2 passes into intervillous blood, pH decreases (more acidic - Bohr effect maternal)
decreasing affinity of Hb for O2 (shifts curve to left)
at the same time, pH increases as CO2 is lost (from foetus - Bohr effect foetus)
increasing affinity of Hb for O2 (for foetus)

Question 18

what is the intervillous space?

Answer 18

the “space between the villi containing the vessels” of the mother and the embryo

Question 19

how does CO2 transfer occur?

Answer 19

via concentration gradient:
maternal physiological adaptation to pregnancy
progesterone-driven hyperventilation
lowering pCO2 in maternal blood (high conc in foetus - therefore diffuse across to maternal blood)

Question 20

how does the CO2 transfer help ensure foetus gets enough O2?

Answer 20

Double Haldane effect:
as Hb gives up O2, it can accept more CO2 (can only bind to one)
foetus gives up CO2 to maternal, as O2 is accepted
no alterations in local pCO2

Question 21

give a brief overview of foetal circulation

Answer 21

receives oxygenated blood from mother via placenta (umbilical vein)
lungs are non-functional, so has to bypass
returns to placenta via umbilical arteries

Question 22

what are the foetal circulatory shunts?

Answer 22

by-pass the liver: ductus venosus
by-pass lungs: ductus arteriosus
bypass right ventricle and lungs: fossa ovalis

Question 23

what does ductus venosus join?

Answer 23

placenta to IVC

Question 24

what does ductus arteriosus join?

Answer 24

pulmonary trunk to aorta

Question 25

what does fossa ovalis (foramen ovale) join?

Answer 25

RA to LA

RA higher pressure before birth

Question 26

why is ductus venosus needed?

Answer 26

DV connects umbilical vein carrying oxygenated blood to the IVC
blood enters RA
ensures saturation is mostly maintained (70% drop to 65%)

Question 27

why is foramen ovale needed?

Answer 27

RA pressure greater than in LA

forces leaves of FO apart and blood flows into LA

Question 28

what does the free border of septum secundum form?

Answer 28

a ‘crest’ - crista dividens

Question 29

function of crista dividens?

Answer 29

created to stream blood flow
majority flows to LA (and brain)
minor proportion flows to RV (for it to develop), mixing with blood from SVC (deoxygenated)

Question 30

what is the saturation of blood reaching LV?

Answer 30

around 60%

Question 31

why is the ductus arteriosus needed?

Answer 31

shunts blood from RV and PT to aorta

Question 32

where does the ductus arteriosus join the aorta?

Answer 32

distal to supply to the head (and heart) - doesn’t affected blood supply to brain
(minimises drop in O2 saturation)

Question 33

why is there a foetal response to hypoxia?

Answer 33

adaptations to manage transient decreases in oxygenation

Question 34

what is the foetal response to hypoxia? (circulation)

Answer 34

HbF and increased Hb conc.

redustribution of flow to protect supply to heart and brain (reducing supply to GIT, kidneys, limbs)

Question 35

what is the foetal response to hypoxia? (HR)

Answer 35

foetal HR slows in response to hypoxia

reduces O2 demand

Question 36

what is the foetal response to hypoxia? (chemoreceptors)

Answer 36

foetal chemoreceptors detecting decreased pO2 / increased pCO2
vagal stimulation leading to BRADYcardia (decreased use of limited supply)
COMPARED to adult where vagal inhibition leads to tachycardia (increased resp. to increased O2, breath off CO2)

Question 37

what are the impacts of chronic hpoxaemia on the foetus?

Answer 37

growth restriction

behavioural changes - impact on development (e.g. less movement)