The blood-brain barrier is formed by the contacts of special glial cells, called astrocytes, with blood vessels. It is essential for maintaining homeostasis in the brain. In general, only lipid-soluble molecules, such as carbon dioxide, oxygen, steroids, and alcohols, can pass through the blood-brain barrier easily. Water-soluble molecules, such as sodium, potassium, and chloride ions can pass through the blood-brain barrier only with the assistance of specific carrier molecules. Some substances cannot pass through the barrier at all.