Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 128 of 367 Neural tissue in the CNS is isolated from the general circulation by the blood–brain barrier (BBB). The barrier exists because the endothelial cells that line the capillaries of the CNS interconnect by tight junctions that inhibit diffusion of manymaterials. In general, only lipid-soluble compounds (including carbon dioxide; oxygen; ammonia; lipids, such as steroids or prostaglandins; and small alcohols) can diffuse across the membranes of endothelial cells into the interstitial fluid of the brain and spinal cord. Endothelial cells and astrocytes are not in contact; therefore, the choroid plexus is not part of the neural tissue of the brain. A blood–CSF barrier is created by specialized ependymal cells and substances cannot cross freely into the CNS. These cells, interconnected by tight junctions, surround the capillaries of the choroid plexus. The blood–brain barrier remains intact throughout most of the CNS. Circulation of CSF The choroid plexus produces CSF at a rate of about 500mL/day. The total volume of CSF at any moment is approximately 150 mL; thus, every eight hours the entire amount of CSF is replenished. The rate of removal equals the rate of production. Even with this rapid turnover, the composition of CSF is carefully controlled, and the rate of elimination usually keeps pace with the rate of production. The CSF circulates from the choroid plexus through two lateral apertures and a single median aperture, with openings in the roof of the fourth ventricle. Cerebrospinal fluid then flows through the subarachnoid space surrounding the brain, spinal cord, and cauda equine. Fingerlike extensions of the arachnoid membrane, called the arachnoid villi, penetrate the meningeal layer of the dura mater and extend into the superior sagittal sinus. In adults, clusters of villi from large arachnoid granulations . Cerebrospinal fluid is immersed into the venous circulation at the arachnoid granulations. If the regular circulation or reabsorption of CSF is disturbed, an assortment of clinical issues may appear. For example, if reabsorption of CSF is inadequate in infancy, it will back up in the ventricle and is responsible for hydrocephalus, or “water on the brain.” An infant with this condition has a large volume of CSF that expands the ventricles and the skull, and potentially leads to brain damage if untreated. In adults, the brain can be distorted and damaged by failure of reabsorption or a blockage of CSF circulation. 12.5 The Blood Supply to the Brain The brain has an extensive circulation to supply this extremely active organ with an endless demand for oxygen and nutrients. Arterial blood extends to the brain through the internal carotid arteries and the vertebral arteries. Much of the venous blood from the brain exits the cranium in the internal jugular veins . Cardiovascular disorders that disrupt adequate blood supply to the brain cause cerebrovascular diseases. A cerebrovascular accident (CVA), or stroke, occurs when the blood supply to a portion of the brain is cut off and affected neurons begin to die. The Blood–Brain Ba r er