Closure of the human umbilical artery: a physiological demonstration of Burton's theory.

30 umbilical cords of normal full-term infants were examined immediately after birth for closure of the arteries. Multiple constrictions were observed in the arteries at birth, which increased in number and degree with time. Between the constrictions there were segments of dilatations with trapped blood. Cutting through the constrictions in the cords clamped later than 1 min after birth did not result in leakage of blood from the adjoining dilated segments which contained trapped uncoagulated blood indicating complete closure. This behavior of the vessels is a physiologic example of critical closure of small arteries according to Burton's theory.