Cardiovascular Physiology - Foetal Circulation Flashcards
Describe the Foetal circulation
A parallel cirucit circulatory system that allows the foetus to receive oxygen from the mother rather than its own lungs, which are collapsed and submerged in amniotic fluid.
Foetal blood is oxygenated to 90% in the placenta (as a result of the higher HbF affinity for O2)
It returns to the foetus via the umbilical vein, with 60% bypassing the liver through the ductus venosus after the left hepatic portal vein, and the remaining 40% oxygenating the liver.
Now at 65% saturation, the blood enters the RA from the IVC, and crosses the foramen ovale into the LA.
At 60% saturation, it passes through the LV, aorta and carotids, with some diversion ot the coronary arteries. The afterload to the LV is provided by the resistance of the upper body and cerebral circulations.
It returns to the RA via the SVC with 30% saturation, and down into the RV after mixing with blood from the IVC.
It is pumped into the pulmonary artery, but given the high resistance of the collapsed pulmonary circulation, almost 90% crosses the ductus arteriosus to the descending aorta to supply the rest of the body.
The umbilical arteries arise from the internal iliacs, returning deoxygenated blood to the placenta.
Streaming is the process by which the RA can manage two separate streams of blood.
What is streaming?
In context of foetal circulation
It is the process that allows oxygenated and deoxygenated blood to reach different destinations in the foetal RA.
Oxygenated blood from the placenta is guided by the Eustachian valve along the dorsal RA towards the foramen ovale.
Deoxygenated blood returning from the brain via the SVC is streamed anteriorly across the tricuspid into the RV.
The Eustachian valve is a flap of tissue between the IVC and the right atrium.
What is the foramen ovale?
The foramen ovale is a flap-valve in the inter-atrial septum that facilitates right-to-left flow of oxygenated blood from the placenta in-utero.
It closes functionally at birth, and completely by 6 weeks.
Up to 25% of adults have a patent foramen ovale (PFO), which can allow emboli from the right side of circulation (venous) to cause cerebro-vascular events.
Functional closure of the foramen ovale reflects the drop in right heart pressure caused by perfusion of the lung bed and reduced IVC inflow after clamping of the umbilical vein, resulting in left atrial pressure (which has increased with the blood entering from the pulmonary vein), exceeding that of the RA.
What is the ductus arteriosus?
A wide, muscular conduit allowing blood to flow between the pulmonary artery and aorta within foetal circulation.
It is a right-to-left shunt that allows blood to bypass the collapsed lungs (and therefore high pulmonary vascular resistance), prioritising bloodflow to the lower half of the body.
Its presence in-utero is maintained by low oxygen content, a high pulmonary:aortic pressure gradient, and vasodilatory Prostaglandin (PGE2).
It closes functionally after 3 days, and permanently as a result of thrombosis and fibrosis at 21 days.
What changes occur immediately after birth?
With the first breath, lung volume increases, PVR drops significantly, (further decreased by a reduction in hypoxic pulmonary vasoconstriction) - RV pressures drop.
RA pressure drops because of reduced RV afterload and reduced IVC inflow (from clamping of umbilical vein).
LA pressure increases with increased pulmonary vein inflow.
LA > RA pressure for the first time, and the foramen ovale closes functionally
This can be reversed if the pressure balance changes, and only closes irreversibly at 6 weeks.
The ductus venosus closes via unclear mechanisms after a few hours, and completely by 10 days.
Clamped umbilical arteries raise SVR and aortic pressures.
Aortic > Pulmonary artery pressure for the first time, causing flow reversal in the ductus arteriosus, which closes functionally after 3 days, and anatomically after 2 weeks.
This is the final step in the process, and marks the conversion of a parallel system to one where the ventricles are in series with each other.
Pulmonary artery pressures, blood flow, and vascular resistance reach adult values at 4-6 weeks.
