Effect of O2 and CO2 in N2, He, and SF6 on chick embryo blood pressure and heart rate.

Arterial pressure of chick embryos was measured electromanometrically to investigate the effect of altered gaseous environments on blood pressure (BP) and heart rate (HR). The experiments were made in eggs incubated for 14-16 days at 38 degrees C without impeding the diffusive respiratory gas exchange through the shell and chorioallantois. In air, the HR was counted 260-270 beats/min and the BP increased from 14/7 Torr at day 14 to 21/12 Torr at day 16. Both the BP and HR decreased with hypoxia, whereas hyperoxia affected a slight increase in BP and little change in HR. Hypercapnia decreased the HR and tended to enhance a systolic maximum pressure. The effect of hypoxia was augmented markedly in the presence of hypercapnia and vice versa. When N2 was replaced with helium (He), the effect of hypoxia was mitigated significantly. On the contrary, replacement of N2 with sulfur hexafluoride (SF6) augmented the effect of hypoxia. Because the respiratory gas exchange of the egg takes place by diffusion through the shell and chorioallantoic capillaries, the effect of He and SF6 atmospheres on BP and HR is attributed to an altered diffusivity of O2 and CO2 in these inert gases.