1.We studied cardiovascular variability, baroreflex and blood volume regulating hormones to determine the relative roles of autonomic regulation and hormones during blood donation.

2.The sympathetic response was studied by measuring the R–R interval and systolic blood pressure variability using coarse graining spectral analysis in eight blood donors. Beat-by-beat R–R intervals and blood pressure were recorded for 20 ;min before and 5 ;min after a whole-blood donation of 480±10 ;ml (about 7 ;ml/kg of blood volume, over 4 ;min). Plasma catecholamines, vasopressin, atrial natriuretic peptide, endothelin, active renin, osmolality, Na+, K+, haemoglobin and haematocrit were measured just before and after blood withdrawal.

3.Blood donation led to increases in the plasma catecholamines (adrenaline, 21±2 versus 35±3 ;pg/ml; noradrenaline, 229±26 versus 323±37 ;pg/ml; dopamine, 34±3 versus 66±9 ;pg/ml) and in systolic blood pressure (130±6 versus 140±5 ;mmHg). These changes were independent of ionic or slow endocrine mechanisms. Heart rate, cardiovascular variability and the spontaneous baroreflex sensitivity did not change despite the increase in blood pressure and catecholamines. Thus the peripheral vascular control was probably involved.

4.We conclude that the absence of any change in heart rate usually observed during non-hypotensive hypovolaemic stress is probably due to the sympathetic activation being counterbalanced by the high supine vagal tone at the heart and not to the heterogeneous nature of the sympathetic neural response or to changes in sympathetic and parasympathetic activity without any change in autonomic balance.

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