1644 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



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Gestation age in days 



fig. 24. Increase in distensibility of fetal lamb lungs with 

 age. A: tidal air/peak intratracheal pressure = distensibility; 

 B: distensibility per kg body weight plotted against age. [From 

 Dawes (66).] 



mechanisms are not very active in the newborn infant 

 (197). The physiological activity of the vasomotor 

 sympathetic mechanism to the skin blood vessels is, 

 however, well developed at birth and quite compa- 

 rable with that of the adult — a fact which was dem- 

 onstrated clearly by Day (79) who showed, by 

 conductivity measurements, that the circulatory re- 

 sponses to changes in environmental temperature were 

 as effective as in the adult in maintaining body tem- 

 perature. These observations have recently been am- 

 plified by Briick (47). 



Renal blood flow appears to be low in the sheep 

 fetus (5) and the newborn infant (173) when com- 

 pared with the adult on a body weight basis; PAH 

 clearance was used in these measurements but nothing 

 is known of the secretory capacity of the tubules for 

 this substance. Unilateral renal artery stenosis, with 

 fatal arterial hypertension of 180 mm Hg, has been 

 observed in a newborn infant ( 1 30) suggesting that 

 the renin-hypertensinogen mechanism is active early 

 in life in man and may account for the hypertension 

 above the lesion with coarctation of the aorta. 



Viability 



tachycardia followed the hypotension occurring with 

 comparable doses in the adult. 



In the newborn monkey, the carotid sinus reflexes 

 are functional and occlusion of the carotid arteries 

 has been shown to cause a rise in the arterial pressure, 

 which is abolished by cutting the carotid sinus nerves 

 (71) : but acute anoxia causes a fall in arterial pressure 

 suggesting that the vasomotor center itself is not very 

 active: there was, however, a rise in arterial pressure 

 in the fetus in response to asphyxia. In the newborn 

 baby the mean arterial pressure is about 40 mm Hg 

 below the mean pressure of the adult and the avail- 

 able evidence shows that, once the immediate read- 

 justment of birth are complete, there is a low periph- 

 eral resistance (196); the cardiac output per kg of 

 body weight is about double that of the adult, the 

 blood flow to the extremities is likewise double and 

 the cerebral blood flow is high (123). Low tonic 

 activity of both the chemical and reflex regulating 

 mechanisms are probably concerned, and the develop- 

 ment of these will probably contribute relatively 

 more to the gradual rise in arterial pressure during 

 growth than the cardiac output, which declines in 

 relation to body weight. Recently, records of arterial 

 blood pressure changes during replacement trans- 

 fusions, when the blood volume was reduced rapidly 

 by 10 per cent, demonstrated that the baroceptor 



Viability, in its narrowest sense, may be considered 

 as the capacity of the newborn to establish correct 



200 



150 



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1 38 days gestation age 



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Pulmonary arterial perfusion pressure (mm Hg) 

 I 1 ■ ■ 1 1 



10 20 30 40 50 



fig. 25. Perfusion of isolated lungs of two fetal lambs, 



mature, above; nonviable, below. Pressure flow diagrams were 



constructed before ventilation (o) and about 15 min later (•). 



Following ventilation, there is a large decrease of pulmonary 



vascular resistance in the mature lamb and almost no change 



in the premature. [From Dawes (66).] 



