Closure of the umbilical arteries by oxygen
McGrath et al (1986) measured contraction of
muscle segments from umbilical arteries and veins of
umbilical cords collected after birth [1]. In a series of
experiments with varying mixtures of oxygen, carbon
dioxide, nitrogen and a few drugs of interest, oxygen
was found to be a strong stimulus for contraction of
umbilical artery muscle. Muscle from the umbilical
vein, on the other hand, did not contract with
increasing oxygen. In discussing this finding they
made the following comments:
"We have confirmed that the human umbilical
artery is contracted by 02. The threshold for
this was found to be 36 mmHg at pH 7.28, i.e. at
the pH of umbilical blood in utero (Pearson
1976). This threshold lies between the normal
fetal (15-25 mmHg) and neonatal (100 mmHg)
levels and thus potentially represents a major
physiological mechanism capable of initiating
the change from fetal to neonatal circulations at
birth. However, modern obstetric practice of
clamping the cord immediately after birth
obviates the need for such a mechanism...
...The vein, however, lacks this 02-induced
mechanism...
...Since 02 contracts the artery but not the vein,
the selective closure of the artery when the Po2
rises after birth could allow the considerable
amount of blood contained in the placenta to be
transfused to the fetus at birth, a well known
observation in the new-born when the cord is
not ligated (Windle, 1940). This will not occur if
the cord is quickly clamped." [1, p518]
These experiments by McGrath et al. were done in
conjunction with investigations of prostaglandin
mediated adjustments of circulation at birth,
including closure of the ductus arteriosus and
opening (vasodilation) of the pulmonary arteries.
McGrath et al. also demonstrated that the delicate
balance of oxygen tension and pH can be altered by
drugs affecting the prostaglandin system. It should
be kept in mind that other unknown physiological
mechanisms may also be affected by human
interventions.
- McGrath JC et al. (1986)
Contraction of human
umbilical artery, but not vein,
by oxygen.
- Pearson, JF (1976). Maternal
and fetal acid-base balance.
- Windle, W (1940). In
Physiology of the Fetus.
Philadelphia and London: W.
B. Saunders.
- McGrath JC, MacLennan SJ, Mann AC, Stuart-Smith K, Whittle MJ.
Contraction of human umbilical artery, but not vein, by oxygen. J Physiol.
1986 Nov;380:513-9.
- Pearson, JF (1976). Maternal and fetal acid-base balance. In Fetal
Physiology and Medicine, ed. BEARD, R. W. & NATHANIELSZ, P. W., pp.
492-509. London: W. B. Saunders.
- Windle, W (1940). In Physiology of the Fetus. Philadelphia and London: W.
B. Saunders.