Fig. 2: Uncoupling of arteriole diameter, blood velocity and blood flow during briefCO_2.

a Top, red blood cell (RBC) velocity and diameter were measured simultaneously with a broken line-scan drawn along and across the pial arteriole (white line). b Upon briefCO₂, velocity (blue trace) decreases before the delayed arteriole dilation (pink trace). Calculated blood flow (velocity * π.radius²) (purple trace) shows a transient decrease followed by an increase due to dilation. Right inset shows data presented in percent change from baseline (n = 7 vessels, 6 mice). c A sagittal section of a mouse brain was recorded with ultrasound localization microscopy (ULM) upon microbubbles injection (i.v.). This approach allowed to position CBV measurements in the internal carotid artery (ICA). Note that it also reported resting blood velocity, the color map representing the horizontal component of the RBC speed. d Power Doppler image of the same sagittal section acquired with standard functional ultrasound imaging (fUS). A high-pass filter selecting the CBV flowing with an axial velocity >8 mm/s was applied. e Left, briefCO₂ generated a delayed increase in CBV in the cortex preceded by a small early drop, concomitant with an early CBV decrease in the ICA (average of 5 stimulations). Inset, comparison of the dynamics of CBV responses in the ICA (top, n = 6 experiments, 3–5 stimulations, 5 mice) and RBC velocity responses in pial arterioles (bottom, same trace as in panel b). The arrows indicate the onset time to reach 10% of the fit of the curves. Data are represented as mean ± SD (shadings).