The vessels supplying the brain tissue are separated from the CSF by a blood-brain barrier. Blood flow in these vessels is autoregulated through intrinsic mechanisms (autoregulation). The brain does have sympathetic nerves innervating its blood vessels but these nerves do not play any role in regulating the blood flow. Intrinsic mechanisms that are involved in autoregulation of cerebral blood flow are theorized as follows:
- Myogenic theory: This theory states that increase in blood flow in cerebral vasculature is counteracted by contraction of smooth muscles surrounding the blood vessels. This action helps to nullify the increase in blood flow and pressure.
- Metabolic theory: According to the metabolic theory, the brain has the ability to autoregulate blood flow in response to changes in pH of the blood. The pH of blood circulating in the cerebral vasculature is different from that in the CSF. In other words, it is not necessary that the CSF will undergo changes in its pH if the blood being delivered to the brain has a different pH. This is explained by the fact that H+ ions, being positively charged, cannot travel across the blood-brain barrier. The pH of blood changes regularly with changes in the body. Hypoventilation causes respiratory acidosis which causes the pH to decrease. Similarly, ingesting drugs like aspirin, sulfonamides etc also cause marked changes in pH of blood. Therefore, if the H+ ions were allowed to pass uninterruptedly across the blood-brain barrier, the consequent changes in pH would have caused devastating effects on the brain tissue. On the other hand, gases such as carbon dioxide and oxygen can cross the blood-brain barrier as they don’t carry a charge. The relation of blood flow with carbon dioxide is linear, i.e. increase in carbon dioxide concentration will directly cause increase in blood flow. The effects of oxygen on the cerebral blood flow are negligible; however, pathologically increasing oxygen concentration will show a prominent decrease in blood flow.
Hypoventilation causes carbon dioxide levels to increase in the blood. After crossing the blood brain barrier, carbon dioxide enters the CSF where it reacts with water and forms hydrogen carbonate. The hydrogen carbonate formed disassociates into hydrogen ion and hydrogen bicarbonate. The H+ ions act on the ventrolateral part of medulla which is the central chemoreceptor of the body. The chemoreceptor zone detects changes in the chemical content of the body. The role of central chemoreceptors is different from that of the peripheral chemoreceptors in maintaining the homeostasis in the body. When triggered, it sends impulses to the nuclei present on the lower 1/3rd of Pons and upper parts of Medulla. These areas of brain are involved in cardiovascular and respiratory changes that are carried out through sympathetic and parasympathetic nerves. As a result, the vessels in the body as well as in the brain vasodilate and the blood flow increases. Moreover, intrinsic metabolites which are released by the cerebral tissues cross the blood-brain barrier and act on the vascular smooth muscles. This causes further vasodilation of the cerebral vessels. These vasodilator metabolites include the following:
- K+ ions
- H+ ions
- Nitric Oxide
The Monroe Kelly Doctrine: The principle says that the combined volume of blood and CSF in the cranial vault is maintained at a constant value in all circumstances. If a change in volume occurs in either of the constituents, the volume of the other fluid is shifted out or into the cranium in order to compensate for the change.