Final answer:
Rising slowly helps prevent orthostatic hypotension by allowing time for the sympathetic reflex to adjust heart rate and blood vessel constriction, maintaining blood flow to the brain.
Step-by-step explanation:
The nurse is teaching a client experiencing dizziness to rise slowly from a sitting or lying position because gradual changes in position provide time for the sympathetic reflex to maintain blood delivery to the brain. This reflex involves the sympathetic nervous system causing the heart to beat faster and the blood vessels to constrict in response to postural changes. As a person changes position, proprioceptors detect this change and signal the CNS, which in turn signals the sympathetic system to increase cardiac output and maintain blood pressure, preventing orthostatic hypotension and ensuring the brain stays well oxygenated.