<unk> s is a medical term for the dynamic regulation of the blood flow in the brain. It is the principle on which functional magnetic resonance imaging is based.
Neurons, like all other cells, require energy to function.  This energy is supplied in the form of glucose and oxygen (the oxygen being carried in haemoglobin).  The blood supply of the brain is dynamically regulated to give active neural assemblies more energy whilst inactive assemblies receive less energy.
Hemodynamic response
Vagovagal reflex
Fight-or-flight response
Muscle tone
Capillary refill
Cough reflex
Acoustic reflex
Pulse