Fetal Circulation Flashcards
Where is blood for the fetus oxygenated?
In the placenta
In utero, describe whether the following are low or high:
Systemic pressure
Pulmonic pressure
Systemic - low
Pulmonic - high
Name the FOUR additional components/structures in the fetal circulatory system not required in extra uterine life
- Ductus Arteriosus
- Ductus Venosus
- Foramen Ovale
- Two Hypogastric Arteries
Where is the tricuspid valve located?
On the right side of the heart between the atrium and ventricle
Where is the bicuspid or the mitral valve located?
On the left side of the heart between the atrium and ventricle
What percentage of the cardiac output to the fetal lungs receive?
5-10%
How does oxygenated blood travel from the placenta to the fetus?
Via the umbilical vein
What is the average oxygen saturation in the umbilical vein?
80%
Which vein does the umbilical vein join up with once inside the body?
The hepatic portal vein
Which temporary structure connects the portal vein with the inferior vena cava to bypass the liver?
Ductus Venosus
When the oxygenated blood from the umbilical vein mixes with the deoxygenated blood in the inferior vena cava, what is the approximate resulting O2 saturation percent?
67%
Which temporary structure connects the right atrium to the left atrium?
Foramen Ovale
What percentage of blood from the inferior vena cava passes straight through the foramen ovale and what does this achieve?
Approximately 33% which ensures that the coronary arteries and brain receive blood with the highest PaO2
Which side of the foramen ovale can the valve be closed from?
The left side to prevent backflow
Name the temporary structure which allows blood to bypass the lungs and flow from the pulmonary artery to the descending aorta?
Ductus Arteriosus
Why does a small quantity of blood pass through the pulmonary artery?
To nourish the lung tissue
What temporary structures branch off from the iliac arteries and eventually become the umbilical arteries once in the umbilical cord?
Hypogastric areteries
Name the FOUR stages of fetal lung development and when they occur
Embryonic - 0-5 weeks
Pseudoglandular - 5-17 weeks
Canalicular - 17-24weeks
Saccular / Terminal air sac stage - 24/26 to 37+ weeks
Describe what happens during the embryonic stage of lung development
The respiratory bud arises from the ventral surface of the foregut
Describe what happens during the pseudoglandular stage of lung development
Bronchial tree from the trachea to terminal bronchioles are formed, pulmonary, arterial and venous systems develop
Describe what happens during the canalicular stage of lung development
Distal airways develop and epithelial cells subdivide into:
- Type 1 pneuomocytes~ which are necessary for gas exchange
- Type 2 pneumocytes~ which are necessary for surfactant production
What unique function do pneuomocytes possess?
The ability to change function from a secretory cell producing lung fluid in utero, to a cell capable of absorption in post natal life
Describe what happens during the saccular stage of lung development
- Formation of terminal sacs, alveolar ducts and eventually alveoli.
- Accompanied by increasing surfactant production from 23 weeks
What happens to the following when baby takes its first breath:
Pulmonary Vascular Resistance
Pulmonary Blood Flow
Pulmonary Vascular Resistance - decreases
Pulmonary Blood Flow - increases
What happens to the following when the cord is clamped:
Ductus Venosus
Inferior Vena Cava Blood Flow
Systemic Vascular Resistance
Ductus Venosus - begins to close
Blood Flow - decreases
Vascular Resistance - increases
What triggers the foramen ovale to begin to close?
The increased pulmonary blood flow increases the pulmonary venous return which raises the pressure in the left atrium
What triggers the ductus arteriosus to begin to close?
The rise in PaO2 and a fall in circulating prostaglandin levels
How many hours following birth is the ductus arteriosus normally fully closed?
12 hours
Prior to birth approximately how many millileters of fluid is in the lungs of a healthy term fetus?
Approx 80-100ml
How is lung fluid removed from the lungs in utero?
Can be swallowed and excreted by the kidney into the amniotic fluid
How is lung fluid expelled from the baby following birth?
Expelled via the mouth during birth and by initial respirations and absorbed via the lymphatic system
When aerating the lungs at birth, why does the neonate generate an opening pressure of at least 20 cm H2O?
To overcome:
The viscosity of lung fluid
The surface tension within the fluid filled lung
The elastic recoil and resistance of the tissue of the chest wall, lungs and airways
What stimulates the new born baby to cry?
Towards the end of labour the fetus is relatively hypoxic
With placental separation, the O2 content of the blood decreases further and CO2 rises
This causes chemoreceptors in the carotid arteries to set up a reflex stimulus in the respiratory centre
What are the two main functions of surfactant?
To reduce surface tension in the alveoli, allowing them to expand more easily
To prevent atelectasis at the end of each expiration
What is surfactant?
It is composed of 90% lipids and 10% protein and secreted in to lung fluid from around 22 weeks
What other factors can initiate respirations?
Peripheral stimulation/ handling at birth
Relatively cold environment of labour room
Exposure to sensory stimuli~ light, gravity sound.
Cord clamping
