Fluid flows in the narrow gaps between brain cells. It doesn't stay still: each heartbeat pushes it forward, and breathing adds its own waves. This was seen in images ten years ago, but only now have researchers calculated how this pulsing accelerates molecules.
If you drop ink into a stream, it drifts with the current. But if the stream speeds up and slows down, the ink spreads wider. Same here: a molecule in the brain bounces back and forth with the fluid, and during a single heartbeat it travels farther than it would if the fluid were calm. The scientists built a model of this 'jumping' motion and calculated how much stronger the spreading becomes.
The result matched experiments in humans: when a dye was injected into the cerebrospinal fluid, it spread through the brain faster than simple diffusion predicted. So the heart and lungs act as pumps that mix the fluid in the brain. If we can learn to strengthen or weaken this pump, we could deliver drugs more precisely to target areas.
The pulse that drives blood through the body also ventilates the brain. Maybe that's why a walk clears your head: your heart beats faster, and the gaps between cells get flushed better.
In a day, as much fluid passes through the gaps between brain cells as would fill a small glass, yet it manages to wash every corner.
In Greg Egan's novel 'Permutation City,' people control blood flow and breathing to influence brain function: this model adds a scientific basis to such a fantasy.