Foetal circulation + changes at birth Flashcards
What drives oxygen transfer from maternal to foetal blood?
partial pressure differences
maternal uterine arterial blood pO2 = 80-100mmHg
foetal umbilical arterial blood pO2 = 20mmHg
What week of gestation does the foetal heart begin to beat?
4 weeks
Name the 3 shunts present in a foetus
ductus venosus
ductus arteriosus
foramen ovale
[holistic aim to bypass liver and lungs and supply oxygenated blood to body]
Describe the role of the ductus venosus
serves to bypass around the liver
connects umbilical vein and inferior vena cava
oxygenated blood from placenta flows:
- umbilical vein
- ductus venosus
- inferior vena cava
- right atrium
Describe the role of the foramen ovale
shunts right atrium directly to left atrium
bypass around the pulmonary circuit
high vascular resistance in lungs mediates use of this lower pressure shunt pathway
Describe role of ductus arteriosus
secondary bypass around lungs
connects pulmonary artery and aorta
vasoactive - held in state of vasodilation and mediated by prostaglandins
What stimuli prompt a newborn’s first breath?
reduction in skin temperature
hypoxia
hypercapnia
tactile stimulation
What vascular resistance changes occur at birth?
removal of placenta increases systemic vascular resistance
lung expansion decreases pulmonary vascular resistance
What causes closure of foramen ovale at birth?
rise in left atrial pressure (due to increased pulmonary blood flow, closing of ductus arteriosus + increased systemic vascular resistance due to placenta removal) and fall in right atrial pressure (due to reduced resistance in pulmonary circuit)
reversal of pressure gradient across atrial septum
functionally closes foramen ovale
What causes closure of ductus arteriosus?
altered balance in vascular resistance reduces blood flow through ductus arteriosus
decreased circulating prostaglandins (removed placenta + increased pulmonary uptake) reduces vasodilation in ductus arteriosus
What causes closure of ductus venosus?
in days following birth, progressive vasoconstriction of ductus venosus and blood is diverted through liver
mechanisms currently unknown
Which anatomic landmark in the foetus has the highest pO2?
ductus venosus
returning blood from the placenta through the umbilical vein passes through the ductus venosus - blood coming from placenta has highest concentration of oxygen found in foetus
In the circulatory system of the foetus, what is greater before birth than after?
right atrial pressure
right atrial pressure falls after onset of breathing because of a reduction in pulmonary vascular resistance, pulmonary arterial pressure and right ventricular pressure
What is markedly higher in the neonate than in a foetus?
aortic pressure
loss of blood flow through the placenta causes systemic vascular resistance to double at birth which increases aortic pressure as well as pressure in left ventricle and left atrium
