The lateral ventricles are found within the two cerebral hemispheres and connect with the third ventricle through the interventricular foramina (of Monro). The third ventricle is situated in the midline between the diencephalon on both sides. The cerebral aqueduct (of Sylvius) passes through the midbrain and links the third ventricle with the fourth ventricle. The fourth ventricle is a space defined by the pons and medulla below and the cerebellum above. The central canal of the spinal cord continues from the fourth ventricle, although in adult humans, it tends to close with age as it is not fully patent.

Cerebrospinal fluid (CSF) exits the ventricular system through three apertures or foramina (a medial foramen of Magendie and two lateral foramina of Luschka) located in the roof of the fourth ventricle. Once it leaves the ventricular system, CSF circulates through the subarachnoid space surrounding the brain and spinal cord. Expanded regions of these spaces are referred to as subarachnoid cisterns. An example of a subarachnoid cistern is the lumbar cistern, which surrounds the lumbar and sacral spinal roots below the level where the spinal cord ends. The lumbar cistern is the target area for lumbar puncture, a clinical procedure used to sample CSF. The majority of CSF is removed by bulk flow through the valvular arachnoid granulations into the dural venous sinuses within the cranium.

The composition of the CSF serves as an indicator of the extracellular environment of neurons within the brain and spinal cord since the extracellular fluid within the central nervous system (CNS) communicates with the CSF. In the lumbar cistern, the main constituents of CSF are listed in Table 4.2 of the medical textbook. To provide a comparison, the concentrations of these same constituents in the blood are also included. CSF has lower levels of K+, glucose, and protein but higher levels of Na+ and Cl- compared to blood. Additionally, CSF contains virtually no blood cells. The increased concentration of Na+ and Cl- ensures that CSF is isotonic to blood.

When a person is in a recumbent position, the pressure within the CSF column is approximately 120 to 180 mm H2O. The rate of CSF formation remains relatively independent of the pressure in the ventricles and subarachnoid space, as well as systemic blood pressure. However, the rate of CSF absorption is directly influenced by CSF pressure.

An obstruction in the circulation of CSF results in increased CSF pressure and hydrocephalus, which is an abnormal accumulation of fluid within the cranium. In hydrocephalus, the ventricles become distended, and if the pressure increase persists, brain tissue can be lost. When the obstruction occurs within the ventricular system or the foramina of the fourth ventricle, it is referred to as noncommunicating hydrocephalus. If the obstruction is within the subarachnoid space or the arachnoid villi, it is known as communicating hydrocephalus.

The Blood-Brain Barrier
Fig. 4.9 ). The lateral ventricles are situated within the two cerebral hemispheres. They each connect with the third ventricle through one of the inter-ventricular foramina (of Monro). The third ventricle lies in the midline between the diencephalon on the two sides. The cerebral aqueduct (of Sylvius) traverses the midbrain and connects the third ventricle with the fourth ventricle. The fourth ventricle is a space defined by the pons and medulla below and the cerebellum above. The central canal of the spinal cord continues caudally from the fourth ventricle, although in adult humans the canal is not fully patent and continues to close with age.
CSF escapes from the ventricular system through three apertures or foramina (a medial foramen of Magendie and two lateral foramina of Luschka) located in the roof of the fourth ventricle. After leaving the ventricular system, CSF circulates through the subarachnoid space that surrounds the brain and spinal cord. Regions where these spaces are expanded are called subarachnoid cisterns. An example is the lumbar cistern, which surrounds the lumbar and sacral spinal roots below the level of termination of the spinal cord. The lumbar cistern is the target for lumbar puncture, a clinical procedure to sample CSF. A large part of CSF is removed by bulk flow through the valvular arachnoid granulations into the dural venous sinuses in the cranium.
Because the extracellular fluid within the CNS communicates with the CSF, the composition of the CSF is a useful indicator of the composition of the extracellular environment of neurons in the brain and spinal cord. The main constituents of CSF in the lumbar cistern are listed in
Table 4.2.
For comparison the concentrations of the same constituents in blood are also given. CSF has a lower concentration of K+ , glucose, and protein but a greater concentration of Na+ and Cl− than blood does. Furthermore, CSF contains practically no blood cells. The increased concentration of Na+ and Cl− enables CSF to be isotonic to blood.
The pressure in the CSF column is about 120 to 180 mm H2O when a person is recumbent. The rate at which CSF is formed is relatively independent of the pressure in the
Na+ (mEq/L)148136–145K+ (mEq/L)2.93.5–5Cl− (mEq/L)120–130100–106Glucose(mg/dL)50–7570–100Protein(mg/dL)15–456.8× 103pH7.3
FromWillisWD,GrossmanRG.Medical Neurobiology. 3rded.StLouis:Mosby;1981.
ventricles and subarachnoid space, as well as systemic blood pressure. However, the absorption rate of CSF is a direct function of CSF pressure.
ObstructionofthecirculationofCSFleadstoincreasedCSFpressureandhydrocephalus,anabnormalaccumulationoffluidinthecranium.Inhydrocephalustheventriclesbecomedistended,andiftheincreaseinpressureissustained,brainsubstanceislost.Whentheobstructioniswithintheventricularsystemorintheforaminaofthefourthventricle,theconditioniscalledanoncommunicating hydrocephalus. Iftheobstructionisinthesubarachnoidspaceorthearachnoidvilli,itisknownasacommunicating hydrocephalus.
The Blood-Brain Barrier