Fetal circulation Neonatal circulation Placenta and umbilical vessels

Fetal circulation

Neonatal circulation

Placenta and umbilical vessels • Placenta functions as fetal lungs and the oxygenated blood passes into left umbilical vein which enters the liver. • The umbilical arteries(right and left) carry deoxygenated blood from the fetus to the placenta

Problems of fetal circulation • The lungs do not function in fetal life. • Hence the left heart and the systemic circulation are maintained by number of bypass mechanisms namely; • Foramen ovale • Ductus arteriosus • Ductus venosus

The left umbilical vein • Highly oxygenated, nutrient rich blood comes from the left umbilical vein. • Much of this blood is diverted into ductus venosus, which connects the left umbilical vein to IVC in the liver left umbilical vein

Sphincter mechanism in the liver • This regulates the flow of remaining blood from umbilical vein into IVC through hepatic veins. • It is generally agreed that a physiological sphincter exists and prevents overloading of heart when the venous flow in the left. umb. vein is high(eg. during uterine contractions)

Foramen ovale • After a short course in IVC the blood enters the right atrium and much of it passes into the left atrium through foramen ovale(a gap in the interatrial septum)

Ductus arteriosus • Low oxygenated blood from SVC and some amount of blood from IVC pass into right ventricle and thence into pulmonary artery(trunk). • 90% of this blood is bypassed into the aorta by a channel ductus arteriosus

Ductus arteriosus • Connects the left branch of the pulmonary trunk to arch of aorta(beyond the origin of left subclavian artery) • It protects the lungs from circulatory overloading.

Pulmonary vascular resistance • is high in fetal life and pulmonary blood flow is low as the lungs do not need much blood for their survival but developing brain does.

Umbilical arteries • About 65%of blood in the descending aorta • Passes into umbilical arteries(right and left) • Which are direct branches of fetal internal iliac arteries(hypo gastric arteries) • Remaining 35% of blood supplies the lower half of the body and viscera

Postnatal changes • Once the child takes the first respiration, pulmonary circulation begins and the right and left hearts become completely independent of each other. • All the by-pass channels having served their purpose, obliterate. • Foramen ovale is closed and becomes fossa ovalis in the right atrium

Ligamentum arteriosum • The ductus arteriosus becomes a fibrous band called ligamentum arteriosum(which has left recurrent laryngeal nerve hooking around) • (By 96 hours after birth 100% closure occurs) • Many factors contribute to this process of obliteration called involution(mainly oxygen, and transforming growth factor)

Ligamentum venosum • Ductus venosus becomes a fibrous band called ligamentum venosum which is seen in continuation with ligamentum teres(obliterated left umbilical vein)

Other changes and clinical aspects • The umbilical arteries become umbilical ligaments attached to the internal iliac arteries upto superior vesical arteries. • Any failure of all these closures lead to • Patent foramen ovale, ASD, VSD, PDA and coarctation of aorta. • The left umbilical vein remains patent for considerable time and can be used for exchanging transfusions.