Nursing Entrance Exam

7.1.3 Ventricles of the Brain During embryonic development, the hollow cavity within the cerebral hemispheres, diencephalon, metencephalon, and medulla oblongata – called the neurocoel – expands to form chambers, or ventricles. Each cerebral hemisphere contains a large lateral ventricle. These ventricles are separated by a thin medial partition known as the septum pellucidum. Each ventricle communicates with the third ventricle of the diencephalon through an interventricular foramen. The mesencephalon has a slender canal known as the aqueduct of the midbrain (also called the mesencephalic aqueduct, aqueduct of Sylvius, or cerebral aqueduct). This passageway connects the third and fourth ventricles. The fourth ventricle continues into the medulla oblongata; it then narrows and becomes continuous with the central canal of the spinal cord. The ventricles are filled with cerebrospinal fluid, or CSF. The CSF continuously circulates through the CNS from the ventricles and central canal into the subarachnoid space of the surrounding cranial meninges. 7.1.4 Protection and Support of the Brain The cranial meninges, cerebrospinal fluid, and the blood-brain barrier protect and support the brain. The cranium, cranial meninges, and CSF combine to protect the brain’s delicate tissues from mechanical forces. 7.1.5 The Blood Supply to the Brain The brain has an extensive circulation to supply it with continuous demand for oxygen and nutrients. Arterial blood reaches the brain through the internal carotid arteries and the vertebral arteries. Most of the venous blood from the brain leaves the cranium through 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, leaving affected neurons to die. 7.1.6 The Blood-Brain Barrier The blood-brain barrier (BBB) isolates the neural tissue in the CNS from the general circulation cycle. The barrier exists because the endothelial cells that line the capillaries of the CNS interconnect by tight junctions that inhibit the diffusion of many materials. 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 intothe 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 surround the capillaries of the choroid plexus. The blood-brain barrier remains intact throughout most of theCNS.

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