Vasculature of the neonate and changes at parturition Flashcards
Explain the key concepts of fetal circulation
Fetus ‘breathes’ amniotic fluid (not air, as lungs deflated)
-Therefore pulmonary artery doesn’t carry oxygen
Fetal blood is oxygen-poor (hypoxaemic)
-This is normal, but haemoglobin is ‘different’
Fetus swims in amniotic fluid
-Stable temperature
-Water helps protect from bumps
Fetus is fed parenterally (umbilical cord), not orally
-Therefore hepatic portal vein is not needed
Explain the role of the placenta
Interface for exchange of gases, food and waste removal
- repeated production of fuel substrates and hormones
- filtration of potentially toxic substances
Fetal lungs and livers:
-Net “receivers” of oxygen and nutrients during fetal life, to help growth.
-In adults, lungs liver are net “providers” of oxygen and nutrients
Fetal lungs
-Collapsed as they are unnecessary, and have high resistance
Fetal liver
Not required to excrete waste, but becomes important over late gestation (storing glycogen for use post-birth)
Developing organs, tissues are fragile therefore most blood kept away and pressure low (40-50 mmHg MAP vs. 90-100 mmHg) to prevent tissue damage
Identify the major vessels of the fetal circulation
From placenta via Umbilical vein to liver via…
Ductus venosus to
IVC to right atrium in fetal heart via
Foramen Ovale to left atrium to aortic arch, carotid, SVC back to RA
…back through Ductus Arteriosus to descending aorta
Umbilical arteries back to placenta
List the three shunts present in a fetus that are not found within adults
1.Ductus venous
2.Foramen ovale
3.Ductus arteriosus
Outline the importance of fetal shunts
During physiologic ‘stress’ (hypoxemia, low blood flow, etc such as during birth) then remaining blood volume is redistributed to organs essential for immediate survival
The 3 shunts help make this happen
Identify and describe the foramen ovale explain its function
Foramen Ovale to left atrium. Aortic arch, carotid, SVC to RA. Shunts oxygenated blood straight through RA to LA. Pressure difference across FO keeps it open in fetus. Patency maintained by high blood flow
Back through Ductus Arteriosus to descending aorta.
Identify and describe the ductus arteriosus and explain its function
Shunts blood in pulmonary artery straight to descending aorta
Pressure differential between lungs and lower body streams flow back to placenta
Allows equivalent ventricular function in fetus – helps develops ventricular musculature and vasculature
Trace the path of the ductus venosus and explain the function of this structure
a bypass of hepatic circulation (not used a lot in fetus as no oral intake of food, hepatic portal vein). Get nutrient/oxygen rich blood up to heart.
Umbilical vein transports blood full of oxygen and nutrients, it joins onto the caudal vena cava though the shun ductus venous, providing a shortcut so that the blood is able t reach the heart and be circulated around the body much quicker
Identify structures contained within the umbilicus and explain their function
Umbilical vein delivers nutrients TO FETUS…
Umbilical arteries delivers blood back TO PLACENTA
The urachus- fibrous remnant of allantois. Was a canal that drains the urinary bladder of the fetus
Explain the consequences of parturition upon the circulatory system and list all the major circulatory changes that occur
all shunts close. Placenta is separated.
Outline the changes that happen at birth in relation to placenta function
Fetal lungs are collapsed so that respiration can begin
Fetal circulation is now a closed system Fetus now has no oral intake of nutrients and so digestion begins Thermoregulation is introduced as fetus had no requirement Kidneys are able to excrete waste which fetal kidneys have no requirement for These features must be functional at birth but will take time to mature
Describe what happens to the ductus venous at birth
Umbilical cord is ligated as placenta detaches meaning there is no blood in umbilical vein
- Therefore ductus venous sphincter constricts and blood is diverted through liver (hepatic portal vein)
Remnant of DV called ligamentum venosum.
Explain how the ductus venous is shut
The fetal circulation has very high prostaglandin E2 (PGE2) levels as there is no ventilation, so no clearance has occurred.
Rapidly disappears at birth, as blood oxygenated Neonatal ventilation means that there is pulmonary clearance of PGE2.
Its removal means shunts constrict naturally as no stimulation of vasodilation occurs. Additional during birth, noradrenaline levels are high which causes vasoconstriction
Describe what happens to the foramen ovale at birth
Lungs open creating low pulmonary resistance.
Placenta gone which leads to high systemic pressure
This pressure differential causes septa to close together and fuse
Blood is then diverted to lungs through right ventricle
remnant is called the
FOSSA OVALIS
Explain how the foramen ovale is shut
As the lungs begin to ventilate resistance within the pulmonary vasculature drops and blood (via line of least resistance) goes to lungs via pulmonary artery.
More blood into lungs means that more blood returns into left atrium. Blood pressure therefore increase in left atrium.
Flow from left to right atrium prevented by flap like valve but prolonged contact between valve of the Foramen Ovale and Septum Secundum results in fibrous connections being made between the two and permanent closure.
Describe what happens to the ductus arteriosus at birth
Big muscular conduit
Patency maintained by:
Low pO2
High PGE2 (and PGI2)
PGE2 3-5 fold higher in fetus vs neonate
During the first breath the level of pO2 increase, leads to the reduction of PGE2
These lead to the constriction of the ductus arteriosus
Explain how the ductus arteriosus is shut
Closure occurs as result of two coordinated process:
Towards term the sensitivity the DA to the constrictive effects of O2 increases. So during the first few hours of breathing the higher levels of pO2 depolarise smooth muscle of DA, leading to its constriction
Concentrations of PGE2 rapidly decrease after birth due to the increased clearance of the circulatory system.
Blood flow in opened lungs,
- pressure lower in lung
- pressure higher in body
- also helps to close the DA
- but can take a while for full closure
Describe the mechanisms that regulate changes in circulation at birth
pressure changes secondary to lung inflation (PaO2 increases)
Changes in hormones (cortisol, catecholamines, prostaglandins)
Vascular resistance changes
Tissue layers pushed together fuse and fibrose, become ligaments
Appreciate the time span by which the adult circulation has developed in relation to ductus venous
Functional closure in minutes due to
Improved pulmonary clearance
No umbilical blood supply
Anatomic closure within days.
Appreciate the time span by which the adult circulation has developed in relation to foramen ovale
Functional closure is relatively quick i.e. hours
Anatomic closure is relatively slow (weeks-years) with a small opening persisting in about 25% mammals.
Failure to close results in atrial-septal defect
‘Hole in the heart’
Appreciate the time span by which the adult circulation has developed in relation to ductus arteriosus
Functional closure in mins to hours.
Anatomic closure may take 2-7 days
If closure does not occur, ventricular septal defect = patent ductus arteriosus.
-Relatively common condition
-Non-life threatening
-Reduces efficiency of heart
-Exertional incompetence
List the anatomic remnants of the fetus in the adult
Internal umbilical arteries atrophy (use or lose) – become round ligaments of the bladder.
Internal umbilical vein redundant – becomes round ligament of the liver.
- ductus venosus becomes ligamentum venosum.
- foramen ovale becomes fossa ovalis
- ductus arteriosus becomes ligamentum arteriosum
Recognise the physiological changes that occur at the transition from foetal to neonatal life
Increased oxygenation
Haemoglobin alters fetal to adult
Blood pressure increase
Enteral intake of nutrients
