Vascular regulation of blood flow to organs in the preterm and term neonate

This chapter describes some of the multiple molecular mechanisms that regulate the contraction of the vascular smooth muscle cell and thus the arterial vascular tone. This tone is central to the regulation of blood flow to tissue and thereby the distribution of cardiac output. In the newborn infant, the large distributary arteries take an active part in this. In the normal newborn infant, cardiac output is distributed equally to the upper and lower part of the body and the perfusion rate (in mL/100 g/min) is higher in liver and kidneys, as compared to the brain. Blood flow to the brain has been most actively studied, and autoregulation is already developed at the limit of viability, although autoregulation may be impaired and best be described as a degree of capacity rather than an on-off phenomenon. Vascular reactivity to Pco₂ and to hypoxia is robust. The sympathetic nervous system plays a particular role in newborn infants and may be responsible for the decrease in blood flow to the cerebral hemispheres during the diving reflex as elicited by hypoxia.