i6 3 8 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



low oxygen tension mixtures to the mother in both 

 the sheep and the guinea pig (39, 133)- It appears 

 that there is no species difference for the sensitivity 

 of the fetal heart to hypoxia in utero. Hon has also 

 described intermittent fetal bradycardia during de- 

 livery which he considered to be unrelated to altera- 

 tion in placental blood flow and due to either com- 

 pression of the cord (109) or medullary asphyxia. 

 The bradycardia during cord compression had a 

 long time course, but swift physiological brady- 

 cardia was frequently observed in vertex presenta- 

 tions and could be related to the degree of cervical 

 dilatation and was possibly caused by the rise in 

 intracranial pressure. This is an old clinical observa- 

 tion and Harvey Gushing also observed bradycardia 

 in adult animals during experimental asphyxia of 

 the medulla (59). 



The great ability of the fetus to survive asphyxia 

 is still not understood (139, 171) and it is not known 

 whether the ultimate damage to the tissues is mainly 

 due to the absence of oxygen and, therefore, the 

 supply of energy, or to the fall in pH as the lactic 

 acid accumulates. Whittam (188) has shown that 

 anoxic fetal kidney slices maintain their potassium 

 content better than adult tissue and, if this is true 

 for both the heart and the brain, it possibly explains 

 the maintenance of their excitability and activity 

 for long periods during asphyxia. Mott stresses the 

 importance of the maintenance of a circulation during 

 anoxia so that glucose, from the liver glycogen, may 

 be supplied to all the tissues, and lactic acid removed 

 (139): liver glycogen is partially mobilized during 

 anoxia and the brain and heart both suffer a large 

 reduction in glycogen content; in the young fetus 

 total lactate production can be accounted for by the 



loss of carbohydrate from the heart. The survival 

 time of the fetal heart is directly related to its carbo- 

 hydrate stores which are larger than those of the adult 

 (fig. 17); these reserves may be depleted by repeated 

 episodes of hypoxia which may have a cumulative 

 effect. 



CHANGES IN THE FETAL CIRCULATION AT BIRTH 

 AND IN THE NEONATAL PERIOD 



Umbilical Curd: Ductus Venosus 



The detailed structure of the umbilical cord varies 

 widely among the species, but all the arteries and 

 veins have thick muscular walls and lack a nerve 

 supply (27); the horse and rabbit have separate 

 sphincters in the region of the umbilical ring (194). 

 The isolated umbilical and placental vessels are very 

 reactive: constriction occurs in response to cooling, 

 stretching, or handling, the presence of the sympa- 

 thetic autonomic drugs in the perfusion fluid and 

 high oxygen tensions; relaxation occurs in the pres- 

 ence of low oxygen tensions and high CO-; tensions. 

 Rogers (163) observed the phenomenon of "pressure 

 spasm," a complete but temporary occlusion follow- 

 ing an increased and sustained perfusion pressure; 

 this response to pressure is also observed in dener- 

 vated systemic vessels (33). Following a natural birth 

 there will, therefore, be many factors combining to 

 ensure an effective closure of the umbilical vessels. 

 Intrauterine asphyxia might be expected to impair 

 the effectiveness of these stimuli and, recently, it 

 has been observed that the cord continues to pulsate 

 for long periods in infants following a difficult de- 

 livery (80). 



40 



30 



20 



IO 



„ monkey 



puineapig 



sheep 



STAGE OF GESTATION 



20 



fig. 17. Cardiac glycogen in different 

 species before and after birth. [From Sheliev 



(171).] 



