There are four interconnecting ventricles, or cavities, within the brain. Each ventricle is lined by ependymal cells and is filled with cerebrospinal fluid (CSF). The largest ventricles are the two lateral ventricles (first and second ventricles), which are located within the cerebral hemispheres. The ihird ventricle is a narrow space that lies on the midline between the lateral masses of the thalamus and superior to the hypothalamus. The fourth ventricle is located on the midline in the posterior portion of the brainstem just anterior to the cerebellum. It is continuous with the central canal of the spinal cord.

Each ventricle contains a choroid (ko i-royd) plexus, a mass of special capillaries and ependymal cells that secrete CSF, but most of the CSF is produced in the lateral ventricles. From the lateral ventricles, the CSF flows through the interventricular foramina into the third ventricle and then through the cerebral aqueduct into the fourth ventricle. From the fourth ventricle, some of the fluid flows inferiorly through the central canal of the spinal cord, but most of it passes into the subarachnoid space of the meninges. Within the subarachnoid space, the CSF flows in two directions. Some flows superiorly around the brain. The remainder flows inferiorly along the posterior of the spinal cord, returns superiorly along its anterior surface, and continues superiorly around the brain in the subarachnoid space. CSF is reabsorbed into the blood-filled dural venous sinus that is located along the superior midline within the dura mater. The secretion and absorption of CSF normally occur at equal rates, which results in a rather constant hydrostatic pressure within the ventricles and subarachnoid space.

As mentioned previously, CSF acts as a protective shock absorber that surrounds the brain and spinal cord. Because it is circulated throughout the CNS, cerebrospinal fluid is used for the transportation of ions, nutrients, and waste products. It also provides the brain with buoyancy, which “floats” the brain within the skull and prevents damaging contact with the cranial floor.