Final answer:
After a head injury, the brain typically retains more water, a condition known as cerebral edema, which can lead to increased intracranial pressure and potential brain damage.
Step-by-step explanation:
After a head injury, the retention of water by the brain typically increases. This is due to a condition known as cerebral edema, where an excess of water causes swelling in the brain cells. A relative decrease in blood sodium (hyponatremia) can lead to diluted extracellular fluid (ECF), prompting water to enter cells by osmosis and cause swelling. A serious outcome of water retention in the brain can include increased intracranial pressure, leading to potential brain damage or even death.
The mechanisms that control the retention and passage of water in the body are complex. Factors such as capillary pressure, blood pressure, plasma protein levels, and the osmotic pressure exerted by solutes within the cells and in the plasma contribute to fluid regulation. When the delicate balance of these factors is disrupted by an injury, the body may respond by retaining more water, especially in the brain, where cerebrospinal fluid (CSF) production and flow are critical to maintaining optimal pressure and function.