|
Birth involves far more immediate, dramatic physiological change in the fetus than merely where the nutrients and oxygen come from. These profound changes are a reason that I consider birth to be an 'initiation' to air-breathing 'personhood.' Do you have any idea of the massive changes necessary to accomplish receiving oxygen from the atmosphere? Let me just address circulation a bit, and leave the even more profound respiratory and digestive changes for another time... I think you'll regard birth as even more miraculous when you understand what an amazing physiological event it is, and what awesome changes happen at that moment! At birth, two major events happen that radically alter fetal hemodynamics;
With the first few breaths, the intrathoracic (internal chest cavity) pressure remains low; after distention of the airways, assuming sufficient surfactant, the pressure quickly rises to that of an adult..(-7 to -8 mmHg). Pressure in the pulmonary artery falls by 50%, but pressure in the atrium immediately doubles or even triples. In the fetus, the high resistance of the pulmonary bed (the capillaries that exchange oxygen in the working lung) causes most of the deoxygenated blood in the pulmonary artery to rush into the descending aorta via a vessel present only in the fetus called the ductus arteriosus. At birth, the first expansion of the lungs forces all the blood in the right ventricle into the pulmonary artery for the first time. Furthermore, increased systemic arterial pressure actually reverses the flow through the ductus arteriosus! Now, neonatal blood flows from the high-pressure aorta to the low pressure pulmonary artery. The massive increase in the left atrial pressure would, before birth, result in a fatal backflow of blood into the right heart through the patent (open) foramen ovale. (An oval opening in the atrial septum that we all have before birth.) However, the anatomical configuration of the foramen is such that a valvelike fold in the left atrial wall automatically closes the foramen (hopefully) on the first pulse of reversed blood. That always amazes me. The neonatal circulation changes at birth complete with closure of the ductus arteriosus and foramen ovale, but some minor adjustments continue for 1-2 months, until the adult phase begins. Fetal circulatory adaptions that disappear at birth....
Immediately following birth, the umbilical vessels constrict. The arteries close first, and if the umbilical cord is not clamped or severed for a minute or so, blood continues to flow from the placenta to the newborn through the umbilical vein, adding to the newborn's blood volume. The proximal portions of the umbilical arteries persist in the adult as the superior vesical arteries that supply blood to the urinary bladder. The more distal portions become solid cords (lateral umbilical ligaments.) The umbilical vein becomes the cordlike ligamentum teres that extends from the umbilicus to the liver in an adult. Similarly, the ductus venosus constricts shortly after birth and is represented in the adult as a fibrous cord (ligamentum venosum), which is superficially embedded in the wall of the liver. So, to summarize, the hemodynamics of the immediate newborn and term fetus differ in these major ways, and many more minor ones...ALL abruptly changing at the moment of birth:
The 'pipework' is still mostly there, but what enormous changes have taken place in a few short seconds! So, I'd appreciate if we didn't say that the immediate newborn and term fetus are almost identical, because they just aren't. The digestive changes alone would be ten times the length of this very basic circulatory primer, and the respiratory chemistry changes at the instant of birth could fill a book. My sources:
|
| FAIR
USE NOTICE:
Warning: This site contains some original texts that are protected by U.S. or international copyrights. They have sometimes been reprinted without specific authorization by the copyright owner. Such material(s) are made available in an efforts to provide an important reference resource. It is believed this constitutes a "fair use" of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this site is distributed without remuneration of any form, or profit, for research and educational purposes. Any further reproduction, copying or distribution by any means is done at the user's own risk and is not sanctioned by the owner or editor of this site. If any copyright-holder would like his/her work removed from this site, please send a message to EileenE#GMX.net.
|