['However', ',', 'in', 'order', 'to', 'understand', 'the', 'variability', 'of', 'the', 'cardiovascular', 'system', ',', 'discussion', 'of', 'a', 'single', 'source', 'is', 'insufficient', 'because', 'the', 'cardiovascular', 'system', 'is', 'composed', 'of', 'many', 'different', 'physiological', 'components', '(', 'subsystems', ')', 'and', 'it', 'is', 'the', 'effects', 'of', 'their', 'mutual', 'interaction', 'that', 'combine', 'to', 'produce', 'HRV', '.']