A Respiratory Reflex Elicited by Hemorrhage and Mediated by Vagal Afferents

Ventilation, heart rate and arterial pressure were recorded in rats on blood losses from 1 to 6 ml (6–36 % of blood volume). About 15 s after the onset of the blood drawing and the coincident decrease of arterial pressure, the tidal volume increased on a loss of 1–3 ml. This was regularly observed in 10 rats. On larger hemorrhages the tidal volume decreased. The respiratory rate decreased in some animals already at a blood loss of 1 ml, in others at losses above 2 ml. The increase of the tidal volume was totally inhibited in 5 rats by exclusion of afferents in the recurrent nerves by cold block or section. In three other rats the increase of the tidal volume was inhibited on cutting also of the right or both vagal nerves. The decrease of tidal volume and of respiratory frequency on large blood losses was not inhibited by vagotomy. It is presumed that the increase of the tidal volume on a small to moderate blood loss is provoked by a vagal reflex elicited by receptors on the low pressure side and that the decreased ventilation on larger hemorrhages is elicited from the respiratory centres by the change of the cerebral blood flow.