Fetal CirculationNutrients for growth and development are delivered from the umbilical vein in the umbilical cord → placenta → fetal heart

Oxygenated blood from mother

↓ (via umbilical vein) Liver

Portal sinus Ductus venosus↓

Inferior vena cava (mixes with deoxygenated blood)

↓Right atrium

Right atrium ↓ (through Foramen

ovale)Left atrium

↓Left ventricle

↓ (through Aorta)Heart and Brain

Deoxygenated blood

from lower half of fetal body ↓Inferior vena cava

Right atrium ↓

Right ventricle

Deoxygenated blood flowing through

Superior vena cava

Right ventricle

↓Pulmonary artery

↓ (through Ductus arteriosus)

Descending aorta↓

Hypogastric arteries ↓

Umbilical arteries↓

Placenta

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.

3 shunts: - Ductus venosus- Foramen ovale - Ductus arteriosus

2nd difference:Ventricles of the fetal heart work in

parallel compared to the adult heart which works in sequence.

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

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

Shunt Functional closure

Anatomical closure

Remnant

Ductus arteriosus

10 – 96 hrs after birth

2 – 3 wks after birth

Ligamentum arteriosum

Formamen ovale

Within several mins after birth

One year after birth

Fossa ovalis

Ductus venosus

Within several mins after birth

3 – 7 days after birth

Ligamentum venosum

Umbilical arteries → Umbilical ligaments

Umbilical vein → Ligamentum teres

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.

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)

Hematopoiesis

Erythrocytes progress from nulceated to non-nucleated

Blood vol. and Hgb concentration increase progressively

Midpregnancy: Hgb 15 gms/dlTerm: 18 gms/dl

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)

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

Type Description Chains

Hemoglobin F Fetal Hgb or alkaline-resistant Hgb

2 alpha chains,

2 gamma chains

Hemoglobin A Adult Hgb. Formed starting at 32-34 wks gestation and results from methylation of gamma globin chains

2 alpha chains,

2 beta chains

Hemoglobin A2

Present in mature fetus in small amounts that increase after birth

2 alpha chains,

2 delta chains

Fetal Hemoglobin

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)

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)

Sufficient development of synaptic functions are signified by flexion of fetal neck & trunk

If fetus is removed from the uterus during the 10th wk, spontaneous movements may be

observed although movements in utero aren’t felt by the mother until 18-20 wks

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

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

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.

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)

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

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

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.

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