What happens to the lungs in the foetus?
They are non functional until after birth
What is the result of foetal lungs being non-functional until after birth?
The foetus must receive oxygenated blood from the mother via the placenta
What happens at the placenta?
How is blood passed from placenta to body?
By the umbilical vein
How does the circulation bypass the lungs and liver?
By action of shunts
Why must the umbilical vein bypass the liver?
Because the first thing the oxygenated blood from the umbilical vein encounters, but the foetal liver is very metabolically active, and so would use up all the oxygen and there would be none left for the rest of the body
What do we need to ensure happens of the shunts?
They close immediately after birth
What route does oxygenated blood from the mother take?
It enters the placenta, bypasses the liver, and enters the right atrium through the inferior vena cava
Does oxygenated blood enter the right ventricle in the foetal circulation?
A small amount does
Why does a small amount of blood enter the right ventricle in the foetal circulation?
They pass to the pulmonary trunk to give the lungs the nutrition they need to develop
How does the blood thats gone to the pulmonary trunk bypass the lungs?
By the ductus arteriosus
What does the ductus arteriosus connect?
The pulmonary trunk to the aorta
What happens to the majority of blood in the pulmonary circulation?
It bypasses the right ventricle and lungs, and goes directly to the left atrium, by action of the foramen ovale shunt, and then to the left atrium, to the aorta, which passes oxygenated blood to the body
What happens to deoxygenated blood?
It returns to the placenta and goes back to the mother
Why does blood in the foetal circulation bypass the lungs?
Partially because there is no point, as the blood is already oxygenated, and partially because we need to protect the developing lungs, as they are behind in their developmental programme, and so would not be able to cope if we were to send all of the cardiac output to the lungs
What does the outflow of the primitive heart tube feed into?
A structure known as the aortic sac
What does the aortic sac feed into?
An early arterial system
What does the early arterial system begin as?
A bilaterally symmetrical system of arched vessels
What happens to the early arterial system?
It undergoes extensive remodelling to create the major arteries of the heart
What does the early arterial system consist of?
5 arches, number 1-4, and 6
What happens during remodelling of the early arterial system?
There is a loss of all or parts of some of the arches
What happens to the 4th arch of the early arterial system?
It is remodelled so that the right part becomes the subclavian artery, and the left becomes the arch of the aorta
What is the 6th arch of the early arterial system known as?
The pulmonary arch
What happens to the pulmonary arch?
The right part becomes the right pulmonary artery, and the left part becomes the left pulmonary artery and ductus arteriosus
What does each aortic arch have?
A corresponding nerve
What is the nerve corresponding to the 6th aortic arch?
The recurrent laryngeal nerve (br vagus, CN X)
Where does the right branch of the recurrent laryngeal nerve descend?
Where does the left branch of the recurrent laryngeal nerve descend?
What do the branches of the recurrent laryngeal nerve innervate?
Why are the branches of the recurrent laryngeal nerve anatomically unusual?
They descend down the thorax, then come back on themselves to innervate the muscles