Foetal and Neonatal Cardiorespiratory Physiology

Question 1

By which stage in gestation is the placenta fully formed?

Answer 1

The placenta is formed by week 14 of gestation.

Question 2

How do oxygen, carbon dioxide and nutrients move between the maternal intervillous space and chorionic villi?

Answer 2

Oxygen, carbon dioxide and nutrients move between the maternal intervillous space and chorionic villi by diffusion.

Question 3

How does the composition of the umbilical arteries compare to the composition of the umbilical veins in the placenta?

Answer 3

The umbilical arteries are deoxygenated and lack nutrients, whereas the umbilical veins are oxygenated and contain nutrients.

*The naming is counterintuitive because they’re named according to the direction of blood flow to the placenta rather than to the foetus. It’s like the pulmonary vessels in adults.

Question 4

List the values for PO2 in the uterine artery and vein.

List the values for PO2 in the umbilical artery and vein.

How saturated is the foetal haemoglobin with oxygen at this PO2?

Answer 4

• PO2 in the uterine artery is 12.7kPa.
• PO2 in the uterine vein is 5.6kPa
• PO2 in the umbilical artery is 3.2kPa.
• PO2 in the umbilical vein is 4.2kPa.
• At 4.2kPa, foetal haemoglobin is 75% saturated with O2.

Question 5

List the values for PCO2 in the uterine artery and vein.

List the values for PCO2 in the umbilical artery and vein.

Answer 5

• PCO2 in the uterine artery is 5.3kPa.
• PCO2 in the uterine vein is 6.1kPa.
• PCO2 in the umbilical artery is 6.6kPa.
• PCO2 in the umbilical vein is 5.8kPa.

Question 6

Does the foetal blood achieve equilibrium with the maternal blood in the placenta for oxygen and carbon dioxide?

Why is this?

Answer 6

• The foetal blood does achieve equilibrium (almost) with maternal blood for CO2.
• The foetal blood does not achieve equilibrium with maternal blood for O2 i.e. oxygen exchange is relatively limited in the placenta.

This is because:

1 - The placental barrier is more permeable to CO2 than O2.

2 - Not all of the maternal blood comes in contact with the chorionic villi (makes the gap bigger because of point 1).

3 - The placental tissue is highly active and consumes 20% of O2 in the maternal artery.

Question 7

How does the structure of foetal haemoglobin (HbF) compare with the structure of adult haemoglobin?

Answer 7

• Foetal haemoglobin comprises 2 alpha and 2 gamma subunits.

- Adult haemoglobin comprises 2 alpha and 2 beta subunits.

Question 8

How does the level of foetal haemoglobin change with advancing gestation?

Answer 8

• Foetal haemoglobin reaches peak level at 10 weeks.
• This level is maintained until 30 weeks.
• By term, the level of foetal haemoglobin declines to 80% of the peak level.
• The remaining 20% gradually disappears after birth.

Question 9

How does foetal haemoglobin differ functionally from adult haemoglobin?

What gives rise to this difference?

How is this advantageous?

Answer 9

• Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin (the oxygen saturation curve in foetal Hb is shifted to the left relative to adult Hb).
• This is because foetal haemoglobin doesn’t interact with 2,3-DPG.
• This is advantageous as there are limiting factors in the placenta for O2 exchange (as seen in a previous card).
• Remember 2,3-DPG is the metabolite formed within the RBC from the glycolytic pathway which decreases affinity for oxygen. This is useful for oxygen unloading.

Question 10

In the placenta, how does the foetal blood affect the oxygen affinity of the maternal blood?

How is this advantageous?

Answer 10

• As the maternal blood picks up CO2 from the foetus, the O2 saturation curve of the maternal blood experiences a Bohr shift to the right.
• There is a concomitant Bohr shift to the left in the foetal blood as it loses CO2.
• This is advantageous as this further increases affinity of foetal haemoglobin for oxygen, which is necessary as the placenta is a limiting environment for O2 exchange (as seen in a previous card).
• Remember the Bohr effect is the reduction in affinity for oxygen with increasing concentration of CO2.

Question 11

How does the concentration of foetal haemoglobin in foetal blood compare with the concentration of adult haemoglobin in adult blood?

Answer 11

• Foetal blood has 18g of HbF per dl.

- Adult blood has 15g of Hb per dl.

Question 12

List the 3 shunts in the foetal circulation.

What is the function of these shunts?

Answer 12

1 - Ductus venosus.

2 - Foramen ovale.

3 - Ductus arteriosus.

• These shunts optimise oxygen delivery (particularly to the brain).

Question 13

What is the ductus venosus?

Answer 13

The ductus venosus is a shunt from the umbilical vein through the liver to join the inferior vena cava.

Question 14

What is the foramen ovale?

Answer 14

The foramen ovale is a shunt between the right and left atria.

Question 15

What is the ductus arteriosus?

Answer 15

The ductus arteriosus is a shunt between the pulmonary artery and the aorta.

Question 16

What explains the difference in oxygen saturation in the aorta compared to the descending aorta?

What are the values for oxygen saturation in the aorta and descending aorta?

Why is this advantageous?

Answer 16

• Oxygenated blood comes into the heart from the placenta through the inferior vena cava, which empties into the right atrium.
• When the oxygenated blood and deoxygenated blood enter the right atrium from the superior vena cava, the oxygenated blood stays separate to the deoxygenated blood due to the effects of streaming, which relies on laminar flow.
• When the blood leaves the heart, the streaming of the flow of blood is such that the oxygenated blood stays on the anterior side of the aorta, whereas the deoxygenated blood stays remains on the posterior side of the aorta.
• The oxygenated blood on the anterior side of the aorta is able to preferentially flow through the subclavian and brachiocephalic arteries.
• The total oxygen saturation of all blood in the aorta is 62%, whereas the oxygen saturation of blood in the descending aorta is 58%.
• This is advantageous because this will shunt more of the foetus’ oxygen content to the brain.

Question 17

What is the difference in right ventricle stroke volume and left ventricle stroke volume in the foetus?

What is the cause of this difference?

Answer 17

In the foetus:

• The right ventricle receives 65% of venous return (35% is lost through shunting).
• The left ventricle receives 35% of venous return (a further 30% is lost through shunting).
• The right stroke volume is lower than normal because some blood shunts from the right atrium into the left atrium through the foramen ovale.
• The left stroke volume is lower than normal because some more blood shunts from the right pulmonary artery into the aorta through the ductus arteriosus.
• Basically some blood skips the right ventricle through the foramen ovale and some blood skips the left ventricle through the ductus arteriosus.

Question 18

How is cardiac output calculated in adults?

How is cardiac output calculated in the foetus?

Answer 18

• In adults, cardiac output = flow through one ventricle (either will do as it’s the same through both).
• In the foetus, cardiac output = combined ventricular output (CVO - this is necessary as some blood is lost through shunting).

Question 19

Describe the distribution of blood around the foetus after it is pumped out of the heart.

Answer 19

• 45% of the combined ventricular output is directed to the placental circulation to be oxygenated.
• 8% of the combined ventricular output is directed to the pulmonary circulation.
• The remaining 47% is circulated around the foetus.

Question 20

Describe the hormonal control of the foetal circulation.

Answer 20

• Circulating catecholamines act on alpha and beta adrenoceptors to cause an increase in contractility.
• Peripheral circulation of the foetus is under a tonic adrenergic vasoconstrictor influence, mainly circulating adrenaline.
• Vasopressin and the renin-angiotensin system also have a minor role.

Question 21

How does the heart rate and blood pressure of the foetus change with advancing gestation?

Why is this significant?

Answer 21

• By the 11th week of gestation, the heart rate of the foetus is 160 bpm and the blood pressure is 70/45.
• By the 28th week of gestation, the heart rate slows to 140 bpm and the blood pressure is 80/55.
• This indicates that the autonomic nervous system is developing.

Question 22

What is the foetal response to hypoxia?

Why is this advantageous?

Answer 22

• Foetal hypoxia evokes the primary chemoreceptor reflex, causing bradycardia and peripheral vasoconstriction. This is advantageous because:
• Bradycardia reduces metabolic activity of cardiac muscle.
• Peripheral vasoconstriction reduces the oxygen supply to the periphery, preserving oxygen for the brain.

Question 23

Why does the foetus show breathing movements in utero?

Answer 23

The foetus shows breathing movements in utero in order to strengthen its respiratory muscles in preparation for birth.

Question 24

When do foetal breathing movements begin?

How do the movements change with advancing gestation?

Answer 24

• Foetal breathing movements begin at 11 weeks of gestation.

- The movements become less shallow and less irregular with advancing gestation.

Question 25

How does hypoxia affect foetal breathing movements?

Why?

Answer 25

• Hypoxia causes foetal bradypnoea.
• It is due to the local direct effects of hypoxia on the central respiratory centre.
• This is not because of the chemoreceptor reflex - this hasn’t developed yet.

Question 26

How do foetal breathing movements change close to delivery?

Answer 26

Foetal breathing movements decrease prior to delivery.

*This is used as a predictor of delivery.

Question 27

When is surfactant produced in the lungs of the foetus?

What stimulates secretion of surfactant in the foetus?

Which cells secrete it?

What is the role of surfactant in assisting with the foetus’ first breath after birth?

Answer 27

• Surfactant is produced in the foetus at 30 weeks of gestation.
• Surfactant secretion is stimulated by cortisol.
• Surfactant is secreted by type II cells in the alveoli.
• Surfactant reduces the surface tension force that opposes lung inflation. It therefore assists in the opening of the lungs for breathing.

Question 28

What happens to the amniotic fluid in the foetus’ lungs at birth?

Answer 28

• Some amniotic fluid is squeezed out during birth (this action is lost in a Cesarean section).
• As air moves in when the foetus is born, the rest of the amniotic fluid is forced across the alveoli, and is excreted through the blood.

Question 29

How does the compliance and tidal volume of the lungs change from birth over time?

Answer 29

• At birth, the lungs have a low compliance, large tidal volume and FRC is 0 (FRC is 0 because the lungs are filled with fluid).
• Over time, compliance increases, tidal volume decreases and FRC increases.

Question 30

List the events that cause the foetal cardiovascular shunts to close.

Answer 30

1 - Ductus arteriosus

• The umbilical cord is clamped, causing TPR to increase.
• This makes the systemic pressure greater than the pulmonary pressure, causing the flow to reverse through the ductus arteriosus.
• This causes the ductus arteriosus to close.
• Closure of the ductus arteriosus is also assisted by high PO2 in the blood.

2 - Foramen ovale

• With the first breath, the pulmonary vascular resistance decreases as the lungs expand.
• This means the flow of blood to the pulmonary vessels increases.
• This means the flow of blood to the left atrium increases, causing left atrial pressure to increase.
• Furthermore, as the ductus arteriosus has closed, more blood flows to the left atrium through the pulmonary vein, causing left atrial pressure to increase further.
• This causes the foramen ovale to close.

3 - Ductus venosus.

• When the umbilical cord clamps, the flow of blood to the umbilical vein stops.
• This causes the ductus venosus to constrict, and then close.

Question 31

Describe the mechanism by which an increase in PO2 in the blood assists in the closure of the ductus arteriosus.

Why is this important clinically?

Answer 31

• When oxygen is low, prostaglandins are produced in the vascular smooth muscle.
• This is advantageous because prostaglandins cause vasodilation.
• When PO2 increases, prostaglandin synthesis decreases, causing a relative vasoconstriction, which assists in closure.
• This is important because it means that NSAIDs can be administered to promote closure.
• Bradykinin may also be responsible for closure via the same mechanism (in the lungs, bradykinin is a vasoconstrictor, but peripherally it is a vasodilator).

Question 32

Why does the wall thickness of the pulmonary arteries and right ventricle decrease after birth?

Answer 32

The wall thickness of the pulmonary arteries and right ventricle decreases as pulmonary vascular resistance (PVR) decreases when the lungs expand.

Question 33

Why does the wall thickness of the left ventricle increase after birth?

Answer 33

The wall thickness of the left ventricle increases after birth as TPR increases.

Question 34

Until which age does ABP increase?

Answer 34

ABP increases until 7 years old.

Question 35

Give an example of a possible mechanism for cot death.

Answer 35

• For 2 weeks after birth, the peripheral chemoreceptors must reset to accommodate the adult blood gas range.
• During this period, the chemoreceptor response is weak.
• The direct effects of hypoxia on the central respiratory centre is inhibitory, so if the chemoreceptor reflex is not sufficiently developed to overcome this inhibition, the baby can become vulnerable to hypoxia, which can cause death.

Question 36

List the risks if the foramen ovale fails to close.

Answer 36

1 - A left to right shunt:

• Excess blood will flow back into the right atrium and ventricle.
• This causes an excess of blood to flow into the pulmonary circulation, causing pulmonary remodelling, which results in pulmonary hypertension.
• This in turn causes right ventricular overload, and in turn right ventricular hypertrophy.
• This causes right ventricular pressure to increase.
• When right ventricular pressure exceeds left ventricular pressure, the shunt reverses such that blood shunts from the right atrium into the left atrium, compromising oxygenation.
• This is known as Eisenmenger’s syndrome.

2 - A right to left shunt:

• This results in a mixing of oxygenated and deoxygenated blood.
• This results in Tetralogy of Fallot (blue baby).

Question 37

What is the risk if the ductus arteriosus remains open?

Answer 37

• If the ductus arteriosus remains open, the left ventricular output can be diverted into the pulmonary circulation from the aorta.
• This can cause pulmonary hypertension, which in turn can cause right ventricular overload.

Question 38

Give an example of a possible mechanism for preeclampsia.

List 2 signs of preeclampsia.

Answer 38

• Preeclampsia might be caused by inflammatory mediators released by hypoxic placental tissue.

1 - High maternal ABP.

2 - Maternal proteinuria.

3 - Convulsions.