THE HEART AND CIRCULATORY SYSTEM 



121 



When the cord is tied the source of oxygenation is cut off 

 and this results in a fall of the oxygen tension in the circulating 

 blood. This leads to stimulation of the respiratory centre in the 

 medulla and the lungs become filled with air. A further result 

 of cutting off the placenta is to decrease the volume of blood 



/Brachioc ephalic 

 artery 



— 58% 



Fig. 36. 



A diagram to show the main parts of the circulatory system in a 

 foetal lamb. The percentage saturation of the blood with oxygen is 

 shown for different parts of the circulation. The size of the vessels 

 indicates the relative volumes of blood flow. (Data of Born, Dawes 

 Mott and Widdicombe, 1954.) 



returning to the right side of the heart. Contrasting with this, 

 there is an increase in the volume of blood returning to the left 

 auricle because expansion of the lungs leads to a decrease in the 

 resistance to flow of blood in the pulmonary circuit. This dilTer- 



