Pressure and heart rate responses to raised carotid pressure in the toad.

Neither transient nor steady occlusion of the bilateral carotid arterial trunks had noticeable effects on blood pressure or heart rate of the toad Bufo arenarum Hensel. Distension of isolated carotid labyrinths had no circulatory effects, whereas distension of unisolated carotid trunks by fluid infusion caused an increase in blood pressure proportional to the applied pressure. A simultaneous tachycardia was recorded. These circulatory effects seem to depend on the venous return to the heart because carotid distension was followed by a proportional rise in right atrial pressure. Consequently infusion of small volumes of saline into the right atrium also promoted tachycardia and hypertension. These responses seem not to depend on central nervous mechanisms because they persisted after complete deafferentation or removal of the bulbomesencephalic centers.