Abstract
1. The effect of haemorrhage on minute ventilation, arterial blood pH and end-tidal CO(2) has been investigated in cats anaesthetized with Nembutal. There was no change in minute ventilation with alteration of blood pressure above 100 mm Hg. Below this level ventilation progressively increased, the minute ventilation at 60 mm Hg being 33% above that at 100 mm Hg. The increase in minute ventilation was always associated with a fall in end-tidal CO(2) and a decrease in arterial blood hydrogen ion concentration.2. The effects were not influenced by vagotomy, but were abolished by carotid sinus denervation or carotid body destruction. After vagotomy and sinus denervation or aortic and carotid chemoreceptor destruction, haemorrhage resulted in a depression of respiration.3. The changes in arterial blood pH and end-tidal CO(2) did not occur without a change in minute ventilation.4. When the changes in arterial blood pH during haemorrhage were prevented by administration of CO(2) there was a marked increase in minute ventilation. It is concluded that the respiratory drive to respiration in the cat during haemorrhage is controlled mainly by carotid body chemoreceptors and that the resultant alkalaemia decreases the respiratory response.