Fetal physiology

3. Fetal Circulation
 Nutrients for growth
and development are
delivered from the
umbilical vein in the
umbilical cord →
placenta → fetal heart

4. Oxygenated blood from mother
↓ (via umbilical vein)
Liver
Portal sinus Ductus venosus
↓
Inferior vena cava (mixes with
deoxygenated blood)
↓
Right atrium

5. Right atrium
↓ (through Foramen ovale)
Left atrium
↓
Left ventricle
↓ (through Aorta)
Heart and Brain

6. Deoxygenated blood
from lower half of
fetal body
↓
Inferior vena cava
Deoxygenated blood
flowing through
Superior vena cava
Right atrium
↓
Right ventricle

7. Right ventricle
↓
Pulmonary artery
↓ (through Ductus arteriosus)
Descending aorta
↓
Hypogastric arteries
↓
Umbilical arteries
↓
Placenta

8. 1st difference:
 Presence of shunts which allow oxygenated blood
to bypass the right ventricle and pulmonary
circulation, flow directly to the left ventricle, and
for the aorta to supply the heart and brain.

9.  3 shunts:
- Ductus venosus
- Foramen ovale
- Ductus arteriosus

10. 2nd difference:
 Ventricles of the fetal heart work in parallel
compared to the adult heart which works in
sequence.

11.  Fetal cardiac output per unit weight is 3 times
higher than that of an adult at rest.
 This compensated for low O2 content of fetal
blood.
 Is accomplished by ↑ heart rate and ↓ peripheral
resistance

12.  Clamped cord + fetal lung expansion =
constricting and collapsing of umbilical
vessels, ductus arteriosus, foramen ovale,
ductus venosus
 Fetal circulation changes to that of an adult

13. Shunt
Ductus
arteriosus
Functional
closure
Anatomical
closure
Remnant
10 – 96 hrs after 2 – 3 wks after Ligamentum
birth
birth
arteriosum
Formamen Within several
ovale
mins after birth
One year after Fossa ovalis
birth
Ductus
venosus
3 – 7 days
after birth
Within several
mins after birth
Umbilical arteries → Umbilical ligaments
Umbilical vein → Ligamentum teres
Ligamentum
venosum

14.  Maintenance of ductus arteriosus depends
on:
- difference in blood pressure bet. Pulmonary
artery and aorta
- difference in O2 tension of blood passing
through ductus. ↑ p O2 = stops flow. Mediated
through prostaglandins.

15. Hematopoiesis
 First seen in the yolk sac during embryonic period
(mesoblastic period)
 Liver takes over up to bear term (hepatic period)
 Bone marrow: starts hematopoietic function at
around 4 months fetal age; major site of blood
formation in adults (myeloid period)

16. Hematopoiesis
 Erythrocytes progress from nulceated to non-
nucleated
 Blood vol. and Hgb concentration increase
progressively
 Midpregnancy: Hgb 15 gms/dl
 Term: 18 gms/dl

17. Hematopoiesis
 Fetal erythrocytes: 2/3 that of adult’s (due to
large volume and more easily deformable)
 During states of fetal anemia: fetal liver
synthesizes erythropoietin and excretes it into
the amniotic fluid. (for erythropoiesis in utero)

18. Fetal Blood Volume
 Average volume of 80 ml/kg body wt. right after
cord clamping in normal term infants
 Placenta contains 45 ml/kg body weight
 Fetoplacental blood volume at term is approx.
125 ml/kg of fetus

19. Fetal Hemoglobin
Type
Description
Hemoglobin F Fetal Hgb or alkalineresistant Hgb
Chains
2 alpha chains,
2 gamma chains
Hemoglobin A Adult Hgb. Formed starting 2 alpha chains,
at 32-34 wks gestation and 2 beta chains
results from methylation of
gamma globin chains
Hemoglobin
A2
Present in mature fetus in
small amounts that
increase after birth
2 alpha chains,
2 delta chains

20. Fetal Hemoglobin
 Fetal erythrocytes that contain mostly Hgb F bind
more O2 than Hgb A erythrocytes
 Hgb A binds more 2-3 BPG more tightly than Hgb
F (this lowers affinity of Hgb for O2)

21.  Increased O2 affinity of fetal erythrocytes results
from lower concentartion of 2-3 BPG in the fetus
 Affinity of fetal blood for O2 decreases at higher
temp. (maternal hyperthermia)

22.  Sufficient development of synaptic functions are
signified by flexion of fetal neck & trunk
 If fetus is removed from the uterus during the 10 th
wk, spontaneous movements may be
observed although movements in utero aren’t
felt by the mother until 18-20 wks

23. Gestational
age
Fetal development
10 wks
Squinting, opening of mouth, incomplete finger closure, plantar
flexion of toes, swallowing and respiration
12 wks
Taste buds evident histologically
16 wks
Complete finger closure
24 – 26 wks
Ability to suck, hears some sounds
28 wks
Eyes sensitive to light, responsive to variations in taste of ingested
substances

24.  11 wks gestation → peristalsis in small intestine,
transporting glucose actively
 16 wks gestation → able to swallow amniotic fluid,
absorb much water from it, and propel
unabsorbed matter to lowe colon
 Hydrochloric acid & other digestive enzymes
present in very small amounts

25. 
Term fetuses can swallow 450 ml amniotic fluid in 24
hours

This regulates amniotic fluid volume:
- inhibition of swallowing (esophageal atresia) =
Polyhydramnios

Amniotic fluid contributes little to caloric requirements of
fetus, but contributes essential nutrients: 0.8 gms of
soluble protein is ingested daily by the fetus from
amniotic fluids. Half is alubumin.

26. 
Meconium passed after birth

Dark greenish black color of meconium caused by bile
pigments (esp. biliverdin)

Meconium passage during labor due to hypoxia
(stimulates smooth muscle of colon to contract)

27.  Small bowel obstruction may lead to vomiting in
utero
 Fetuses with congenital chloride diarrhea may
have diarrhea in utero. Vomiting and diarrhea in
utero may lead to polyhydramnios and preterm
delivery

28. Liver and Pancreas
 Fetal liver enzymes reduced in amount compared to
adult
 Fetal liver has limited capacity to convert free bilirubin
to conjugated bilirubin
 Fetus produces more bilirubin due to shorter life span
of fetal erythrocytes. Small fraction is conjugated and
excreted and oxidized to biliverdin
 Much bilirubin is transferred to the placenta and to the
maternal liver for conjugation and excretion

29.  Fetal pancreas responds to hyperglycemia
by ↑ insulin
 Insulin containing granules identified in fetal
pancreas at 9-10 wks. Insulin in fetal plasma
detectable at 12 wks.

30.  Insulin levels: ↑ in newborns of diabetic mothers
and LGAs (large for gestational age); ↓in infants
who are SGA (small for gestational age)