15. Fetal growth and development

Question 1

In which umbilical vessel is oxygenated blood carried?

Answer 1

• Umbilical arteries bring unoxygenated blood from the fetus to the placenta
• umbilical vein takes oxygenated blood from the placenta back to the fetus

Question 2

what is needed to maintain the concentration gradient of oxygen at the placenta?

Answer 2

• Maternal pO2 increases only marginally

* Therefore to make the gradient work, fetal pO2 must be lower than maternal pO2

Question 3

Describe the features of uteroplacental circulation

Answer 3

placenta as the main exchange unit between the
maternal and fetal circulations.
The chorionic villi allow for greater surface area for exchange, and are bathed in maternal blood from the
uterine arteries.
The uterine arteries develop to allow a high flow, low resistance circulation in the utero-placental unit.

Question 4

what is the pO2 of fetal blood?

Answer 4

• Fetal blood has low pO2

* pO2 approx. 4kPa compared to normal adult pO2 of 11 – 13kPa

Question 5

What adaptations does the fetus have to overcome the low pO2?

Answer 5

• Fetal haemoglobin variant - greater affinity for oxygen transfer
• Fetal haematocrit is increased over that in the adult
• Increased maternal production of 2,3 DPG
• Double Bohr effect

Question 6

What ABG state is the mother in during pregnancy and how does this affect the Hb saturation curve?

Answer 6

• Progesterone causes a physiological hyperventilation, resulting –> physiological respiratory alkalosis.
• Lower CO2 levels to maintain conc gradient at placenta
• Increased pH would ordinarily increase maternal affinity for oxygen, shifting the oxygen dissociation curve to the left.

Question 7

Why isn’t gas exchange compromised despite rise in mothers blood pH?

Answer 7

increased maternal 2,3-BPG is produced, which shifts the curve back to the right to reduce the Hb affinity for oxygen. This will ultimately promote release of oxygen to the fetus

Question 8

What is the predominant form of Hb present in the fetus?

Answer 8

HbF (2 alpha and 2 gamma subunits), from week 12 to term

Question 9

What produces Hb in the fetus?

Answer 9

Yolk sac -> Liver -> Bone

Question 10

What is affinity of HbF for oxygen compared to HbA and why?

Answer 10

Greater affinity for oxygen because it doesn’t bind 2,3-BPG as effectively as HbA

Question 11

What is the double bohr effect and why is it important?

Answer 11

Bohr effect on both the maternal and fetal circulations. This will speed up oxygen transfer

Question 12

Describe how the double bohr effect works.

Answer 12

The mother:
As CO2 passes into the intervillous blood, the pH will decrease, causing a decreased affinity for oxygen for the mother so O2 more easily released for transfer.

The fetus:
The fetus will be giving up CO2 due to the gradient of transfer, causing increased pH, which results in increased affinity of Hb for oxygen.

Question 13

What is the Haldane effect?

Answer 13

Describes the ability of hemoglobin to carry increased amounts of carbon dioxide (CO2) in the deoxygenated state

Question 14

Describe the double haldane effect?

Answer 14

The mother:
As maternal Hb gives up oxygen, it can accept increasing amounts of CO2.
The fetus:
The fetal Hb gives up more CO2 as oxygen is accepted.

Question 15

What is the function of the umbilical arteries and veins in the fetal circulation?

Answer 15

• Receives oxygenated blood from mother via placenta in umbilical vein
• Returns to the placenta via umbilical arteries

Question 16

What is the function of the lungs in fetus circulation?

Answer 16

• Lungs are non-functional

Question 17

What shunts are present in the fetal circulation?

Answer 17

Ductus venosus: bypasses the liver
Foramen ovale: bypasses the right ventricle & lungs
Ductus arteriosus: bypasses the lungs

Question 18

What is the saturation of HbF in the umbilical vein?

Answer 18

70%

Question 19

Why is the ductus venosus needed, what is oxygen saturation after this?

Answer 19

• DV connects umbilical vein carrying oxygenated blood to the IVC
• Blood enters right atrium
• By ensuring shunting of blood around the liver, saturation is mostly maintained
• Drops from 70% to 65

Question 20

Why does the fetal circulation need to bypass the liver?

Answer 20

Very large and highly metabolic so could potentially engulf the whole fetal circulation.
Shunting the blood away from the liver maintains a high level of oxygen in the circulation that will be arriving to the brain and rest of the fetal body

Question 21

Why is the foramen ovale needed, what is oxygen saturation after this?

Answer 21

Bypass the right ventricle and lungs
- drops from 65 to 60%

• Right atrial pressure is greater than that in the left atrium
• Forces leaves of FO apart and blood flows into LA

Question 22

Why does some blood from the right atrium go into the right ventricle?

Answer 22

• Free border of septum secundum forms a “crest” - crista dividens
• Creates two streams of blood flow
• Majority flows to LA
• Minor proportion flows into RV, mixing with blood from SVC

Question 23

What is the saturation of blood in LA and why is this important?

Answer 23

• only Small amount of pulmonary venous return
- deoxygenated
• Blood reaching left atrium has saturation approx. 60%
• Pumped by LV to aorta
• Heart and brain get lion’s share of oxygen

Question 24

Why is the ductus arteriosus necessary, where does it join the aorta?

Answer 24

• Shunts blood from RV and PT to aorta
• joins aorta distal to the supply to the head (and heart)
• Minimising drop in O2 saturation