Interruption of blood supply to the brain for as short a period as 10 seconds may result in the loss of consciousness. Interruption lasting for a few minutes may cause irreversible damage to the brain because, unlike other cells in the body, neurons are capable of storing only a small amounts of oxygen and glucose. Interestingly, the demand for blood supply does not vary depending on what the brain is doing. It needs the same amount of blood whether one is sleeping, talking, reading, or thinking. However, regional differences in blood flow exist within the brain depending on the activity.
More blood flows through the Broca's area when one is talking.
Similarly, the part of the motor strip that controls hand movement and the part of the somatosensory area that processes tactile and proprioceptive sensations from the hand receive an increased supply of blood when hand movements are occuring. In other words, although the total amount of blood reaching the brain is the same at all times, it is distributed within the brain based on the the metabolic activity in different parts of the brain. The paired internal carotid arteries and the vertebral arteries supply blood to the brain. The two internal jugular veins return the blood from the brain to the heart. The two vertebral arteries branch off from the respective subclavian arteries near the arch of Aorta and course rostrally to enter the cranial cavity through the foramen magnum , a large opening on the underside of the skull.