9. foetal physiology (lecture) Flashcards
where does materno-foetal exchange occur?
at the placenta
what are the structures involved in materno-foetal exchange?
umbilical arteries umbilical veins foetal capillaries with chorionic villi uterine arteries uterine veins
function of the foetal capillaries with chorionic villi?
increase surface area
function of uterine veins?
maternal blood flows in the intervillous spaces
what is carried via the umbilical vein and arteries?
oxygenated blood in u. vien
deoyxgenated blood in u. artery
how does the umbilical circulation connect within the foetal circulation?
across membrane of chorionic villi
the 2 circulations do not mix
describe the placenta as a diffusion barrier for gas exchange
the diffusion barrier of the placenta is small and decreases as pregnancy proceeds
what is required for gas exchange at the placenta to occur?
gradient of partial pressures
what happens to maternal pO2 within pregnancy?
maternal pO2 increases only marginally
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?
foetal pO2 must be lower than maternal pO2
if foetal blood has a low pO2, then what are the factors increasing foetal O2 content?
foetal haemoglobin variant (gamma instead of ß)
foetal haematocrit is increased to greater than adult
what is haematocrit?
ratio of RBC to blood volume
what are additional factors promoting O2 exchange to the foetus at the placenta?
increased maternal production of 2,3 DPG (secondary to physiological respiratory alkalosis of pregnancy) - double bohr effect foetal haemoglobin (gamma)
function of 2,3-DPG?
O2 liberated from Hb to tissues easier
more difficult to pick up O2, but released a lot easier
what is the Hb that foetus’ have?
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)
purpose of double bohr effect?
speeds up the process of O2 transfer
what is the double bohr effect?
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)
what is the intervillous space?
the “space between the villi containing the vessels” of the mother and the embryo
how does CO2 transfer occur?
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)
how does the CO2 transfer help ensure foetus gets enough O2?
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
give a brief overview of foetal circulation
receives oxygenated blood from mother via placenta (umbilical vein)
lungs are non-functional, so has to bypass
returns to placenta via umbilical arteries
what are the foetal circulatory shunts?
by-pass the liver: ductus venosus
by-pass lungs: ductus arteriosus
bypass right ventricle and lungs: fossa ovalis
what does ductus venosus join?
placenta to IVC
what does ductus arteriosus join?
pulmonary trunk to aorta
what does fossa ovalis (foramen ovale) join?
RA to LA
RA higher pressure before birth
why is ductus venosus needed?
DV connects umbilical vein carrying oxygenated blood to the IVC
blood enters RA
ensures saturation is mostly maintained (70% drop to 65%)
why is foramen ovale needed?
RA pressure greater than in LA
forces leaves of FO apart and blood flows into LA
what does the free border of septum secundum form?
a ‘crest’ - crista dividens
function of crista dividens?
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)
what is the saturation of blood reaching LV?
around 60%
why is the ductus arteriosus needed?
shunts blood from RV and PT to aorta
where does the ductus arteriosus join the aorta?
distal to supply to the head (and heart) - doesn’t affected blood supply to brain
(minimises drop in O2 saturation)
why is there a foetal response to hypoxia?
adaptations to manage transient decreases in oxygenation
what is the foetal response to hypoxia? (circulation)
HbF and increased Hb conc.
redustribution of flow to protect supply to heart and brain (reducing supply to GIT, kidneys, limbs)
what is the foetal response to hypoxia? (HR)
foetal HR slows in response to hypoxia
reduces O2 demand
what is the foetal response to hypoxia? (chemoreceptors)
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)
what are the impacts of chronic hpoxaemia on the foetus?
growth restriction
behavioural changes - impact on development (e.g. less movement)
