FETAL NUTRITION AND CIRCULATION
Picture: Fetal circulation: The upper end of the inferior vena cava opens directly into the left atrium through the oval foramen [see inset] aswell as into the right atrium.R.A.is right atrium;L.A.is left atrium;R.V.is right ventricle;L.V.is left ventricle;D.A.is ductus arteriosus;D.V.is ductus venosus;F.O.is foramen ovale [oval foramen];S.V.C. is superior vena cava;I.V.C. is inferior vena cava.
There are 3 stages of fetal nutrition:
1. Absorption. Minimal quantities of tube and uterine fluid are taken in by the fertilized ovum during the 3-4 days prior to nidation [implantation].
Source: George H.Bell et al.Physiology and Biochemistry.London,1956, p 984-985.
2. Histotrophic transfer. Strategic and waste materials are passed between the early embryo and decidua for 3 months before the establishment of an effective fetal circulation
3. Hematotrophic transfer. Anabolic and catabolic products traverse the placental barrier between the fetal and maternal circulations by both active and passive processes. If the mother does not receive sufficient food, the fetus draws on the maternal tissues. The birth weight of the infant is not, therefore, governed by the mother’s diet unless is severe under-nutrition.
Hematopoiesis: [the forming and development of blood cells] begins in the liver, spleen, and mesonephros about the second month, although clumps of blood cells may be seen in the yolk sac during the first 1-2 months of fetal life. Fetal blood is slightly more saturated with oxygen than maternal blood.
Environmental changes occurring in the abrupt transition from intrauterine life to an independent existence necessitate certain circulatory adaptations in the newborn. These include diversion of blood flow through the lungs, closure of the arterial duct [ductus arteriosus] and oval foramen [foramen ovale] , and obliteration of the vein duct [ductus venosus] and umbilical vessels.
Infant circulation has 3 phases
The pre-delivery phase, in which the fetus depends upon the placenta;
The intermediate phase , which begins immediately after delivery with the
infant‘s first breath;
The adult phase, which is normally completed during the first few months of life.The ductus arteriosus usually is obliterated in the early postnatal period.Obliteration of the oval foramen is usually complete in 6-8 weeks, with fusion of its valve to the left inter-atrial septum. The foramen may remain patent in some individuals, however, with few or no symptoms.The obliterated ductus venosus from the liver to the vena cava becomes the ligamentum venosum.The occluded umblical vein becomes the ligamentum teres of the liver