The effects of alteration of blood-volume on the concentration of circulating angiotensin in anaesthetized dogs.

1. The blood-bathed organ technique was used to assay the concentration of angiotensin in the blood of anaesthetized dogs.2. Alterations of blood volume caused inverse changes of angiotensin concentration owing to changes in the rate of generation of angiotensin which are probably due to changes of the rate of renin secretion.3. Haemorrhage of 14-26 ml. blood/kg caused an increase of 0.25-1.5 mug/min in the rate of generation and an increase of 0.1-0.33 ng/ml. in the blood concentration of angiotensin.4. The changes of angiotensin generation rate were not due to changes of renal arterial or venous pressure. They were abolished by blocking the renal nerves with lignocaine; they showed a consistent inverse correlation with central venous pressure but not with systemic arterial pressure.5. It is concluded that changes of blood volume bring about changes of the rate of generation of angiotensin by a reflex mechanism the efferent limb of which involves the renal nerves. The afferent pathway remains to be elucidated, but the systemic baroceptors do not appear to be of primary importance.6. The renin-angiotensin system is important in the homeostatic response to changes of blood volume.