MadSci Network: Development

Re: Mammalian birth

Date: Wed Mar 11 10:30:48 1998
Posted By: Carolyn Pettibone, grad student, Genetics, Harvard Medical School
Area of science: Development
ID: 887655205.Dv

Question:  When a mammal is born, what signals the baby to 
cut off the blood supply to the umbilical cord and begin 
breathing on its own?
Hello Matthew,

You've asked a really tough question, because not much of 
this is understood in any detail.  There are lots of theories 
with some supporting evidence, but there is still a lot left 
to be learned.  Most of the related research has been carried 
out using the sheep fetus in utero as a model.  It makes 
sense that this would be a difficult thing to study because 
the sheep fetus is so well protected by the ewe's body that 
it is difficult to observe or manipulate it.

There are three umbilical vessels: two arteries and one 
vein.  The umbilical arteries are responsible for taking 
unoxygenated blood and wastes from the fetus to the placenta, 
and the umbilical vein brings clean oxygenated blood back to 
the fetus.  These vessels have thick muscular walls, and many 
different stimuli will cause them to either constrict or 
dilate.  When human babies are born, the umbilical cord is 
cut and tied off, so blood flow is physically stopped.  But 
if the cord isn1t cut, the vessels themselves will constrict 
and cut off blood flow.  The cord is probably acting in 
response to several signals, which may include trauma, 
tension, temperature drop, and a change in umbilical cord 
blood oxygen content.  These signals induce the production of 
specific chemicals in the vessel walls or the placenta.  
These chemicals can then act directly on the muscle 
surrounding the vessels and cause constriction.  Various 
molecules that have vasoconstrictive affects on the umbilical 
vessels include endothelin-1, thromboxane, and histamine, but 
it is not know which (if any!) of these are really important 
at birth.

As soon as the umbilical blood supply stops, the baby's own 
organs must quickly assume control of blood oxygenation and 
removal of wastes.  This is an amazing process in which the 
pattern of blood flow through the body changes very 
dramatically!  As breathing starts, it almost instantaneously 
switches from a fetal pattern, which largely bypasses the 
lungs and liver, to an air-breathing pattern like ours.  For 
more information on the changes in fetal circulation at 
birth, consult a developmental biology textbook or physiology 
book (see suggested references at end).

The onset of breathing is another controversial issue.  
Incidentally, the fetus actually breathes (!) during the 
first two trimesters.  For some (not well understood) reason 
it stops these breathing motions by the beginning of the 
third trimester.  It has been suggested that the fetus uses 
these motions to help exercise and strengthen the muscles of 
respiration that will be crucial for breathing after birth.   
Thus it is often said that the issue is not what stimulates 
the onset of breathing, but what is it that stimulates the 
onset of _continuous_ breathing at birth?  

As with umbilical vessel constriction, the signals and 
stimulus for breathing at birth are not thoroughly 
understood.  The most common model is that labor and delivery 
cause the baby1s oxygen supply to be compromised, with the 
resulting lack of oxygen stimulating the baby1s respiratory 
center to induce the first breath.  Sensory stimuli including 
cold and touch probably also help sustain breathing.  
However, more recent research challenges this view.  The work 
of Dr. Henrique Rigatto at the University of Manitoba, 
Canada, strongly suggests that the cessation of fetal blood 
flow through the placenta is involved.  His group has found a 
placental peptide (small protein) that inhibits breathing in 
utero.  When blood flow through the placenta is stopped (via 
cutting the cord or vessel constriction) perhaps the sudden 
absence of this peptide is what helps stimulate breathing.

The lungs before birth are completely collapsed, and so the 
first breaths must inflate the lungs and open up the small 
air sacs called alveoli, which are the actual site of oxygen 
transfer to the blood.  During the last trimester the fetus 
starts secreting a fluid called surfactant into the alveoli.  
The surfactant lowers the surface tension in the alveoli and 
makes it easier to inflate them.  Inadequate surfactant 
production is one reason why premature infants often have a 
much harder time breathing, and they may be given artificial 
surfactant or cow surfactant to help them until their own 
lungs start producing it.  Premature infants also often have 
another problem with breathing called apnea -- they 
occasionally forget to breathe.  For unclear reasons their 
respiratory centers are not yet mature enough to control 
consistent and continuous breathing.

To summarize, both umbilical vessel constriction and the 
onset of continuous breathing are probably affected by 
various physical stimuli, including cold, touch, temperature, 
and oxygen supply.  A placental peptide may also inhibit 
breathing in utero, and its absence may then allow breathing 
at birth.  The presence of surfactant and the degree of 
maturation of the baby's respiratory center is also important 
for maintaining continuous breathing after birth.

I hope this information helps to answer your question.  
Research is ongoing and every new edition of textbooks on 
neonatal physiology will include more detail and better 
models.  Someday in the future a complete answer will be 
available, so stay tuned!


Selected references:

1.  Guyton and Hall (1996)  Textbook of Medical Physiology.  				         			
Saunders, Philadelphia. 

Chapter 83, Fetal and Neonatal Physiology, is especially     

2.  Larsen, WJ. 1993. Human Embryology. Churchill Livingston,      			 
New York.

3.  Polin, RA and WW Fox (eds). 1998.  Fetal and Neonatal 
Physiology, 2nd ed., vol 1.  Saunders, Philadelphia.

If you want very detailed information and have some 
background in physiology, this book is very useful.  Also, 
the end of each chapter has several pages of references to 
primary literature.  The following chapters are especially 
	Chapter 90  Regulation of Umbilical Blood Flow
	Chapter 103 Control of Breathing in Fetal Life and Onset 
	and Control of Breathing in the Neonate
	Chapter 81  Physiologic Development of the Cardiovascular 
	System in the Fetus


Current Queue | Current Queue for Development | Development archives

Try the links in the MadSci Library for more information on Development.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-1998. All rights reserved.