Mathematical tools applied on physiological parameter have shown their ability to help in our understanding in physiological processes such as autonomic regulations. However, such methods are not adapted to quantify close-loop interactions between respiratory, blood pressure and heart rhythm. To overcome this limitation, classical Granger causality (cGC) has shown efficiency for assessing directional interactions but cGC cannot assess instantaneous effects. Therefore extended Granger causality (eGC) has been proposed including instantaneous effects through zero-lag coefficient Bo. Here we studied the ability of cGC and eGC to approach interactions between respiratory, blood pressure and RR intervals in healthy subjects during orthostatic challenge. Methods: Electrocardiogram, arterial blood pressure and nasal cannula have been continuously collected in supine then upright conditions in twelve healthy subjects. cGC and eGC causal direction (CD) were calculated to detect instantaneous interaction between RR intervals (RR), respiratory (RE) and systolic blood pressure (SBP). During measures respiratory frequency was normal. Results: As we expected RRI decrease and SBP increase (p<0.05) in upright position. Instantaneous effects have been observed with zero-lag coefficient Bo non-zero in eGC both in upright and supine where CD(RR->SBP,eGC) > CD(RR->SBP,cGC), CD(RE->SBP,eGC) > CD(RE->SBP,cGC) (p<0.05). Moreover zero-lag coefficients Bo(RE->SBP,supine) = 0.2 +/- 0.03 and Bo(RE->RR,supine) = 0.27 +/- 0.06 with Bo(supine) > Bo(upright) showing that respiratory causes instanteneous effects with RR and SBP. Conclusion: Granger causality analysis helps to better analyses physiological signals and provides complementary results to traditional methods, especially extended Granger, showing through coefficients Bo instantaneous effects between RE and RR (resp.SBP).