The ventilatory and cardiovascular effects of a decreased O2-carrying capacity of the blood were evaluated in the toad Bufo paracnemis. Pulmonary ventilation was monitored using a pneumotachographic technique. Chronic arterial catheters served to record both cardiac frequency and blood pressure and enabled the withdrawal of blood samples for analysis of pH and partial pressure of O2 (PO2). Haemoglobin concentrations were determined by the cyanmethaemoglobin method. The ventilatory response to hypoxia was not affected by the reduction in blood O2-carrying capacity, which suggests that PO2 rather than O2 content is the regulated variable. The reduction in capacity is accompanied by an increased normoxic heart rate and by a reversal of the hypoxic tachycardia normally observed.

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