Cardiorespiratory Dynamics in the Brain: Review on the Significance of Cardiovascular and Respiratory Correlates in functional MRI signal

•Cardiorespiratory dynamics significantly impact brain hemodynamics.•Heart rate variability (HRV) and respiratory sinus arrhythmia (RSA) reflect vital physiological states in the brain.•CO2 and O2 levels modulate cerebral blood flow and BOLD signal.•Glymphatic system dynamics influenced by CSF flow can be indirectly detected in fMRI.•Recognizing physiological signals can enhance insights into brain function and health. Cardiorespiratory signals have long been treated as "noise" in functional magnetic resonance imaging (fMRI) research, with the goal of minimizing their impact to isolate neural activity. However, there is a growing recognition that these signals, once seen as confounding variables, provide valuable insights into brain function and overall health. This shift reflects the dynamic interaction between the cardiovascular, respiratory, and neural systems, which together support brain activity. In this review, we explore the role of cardiorespiratory dynamics—such as heart rate variability (HRV), respiratory sinus arrhythmia (RSA), and changes in blood flow, oxygenation, and carbon dioxide levels—embedded within fMRI signals. These physiological signals reflect critical aspects of neurovascular coupling and are influenced by factors such as physiological stress, breathing patterns, and age-related changes. We also discuss the complexities of distinguishing these signals from neuronal activity in fMRI data, given their significant contribution to signal variability and interactions with cerebrospinal fluid (CSF). Recognizing the influence of these cardiorespiratory dynamics is crucial for improving the interpretation of fMRI data, shedding light on heart-brain and respiratory-brain connections, and enhancing our understanding of circulation, oxygen delivery, and waste elimination within the brain.