Final answer:
The quick recovery from lightheadedness when standing is mainly due to the stimulation of baroreceptors, which triggers a reflexive increase in heart rate and vasoconstriction to elevate blood pressure rapidly.
Step-by-step explanation:
When a person feels lightheaded after jumping up quickly from the couch, the feeling passes quickly due to the stimulation of the baroreceptors in the body. These baroreceptors detect changes in blood pressure; when it drops, they send signals to the cardiovascular center in the medulla oblongata leading to an increased heart rate and vasoconstriction to restore normal blood pressure. This is a rapid physiologic mechanism.
When blood pressure falls suddenly, as it might when someone stands up too quickly, the baroreceptors that monitor blood pressure initiate action potentials at a slower rate. This information is then sent to the cardiovascular center, which reacts by increasing sympathetic stimulation. This elevates both the release of epinephrine and norepinephrine from the adrenal glands, which in turn increases cardiac output and vasoconstriction to quickly raise the blood pressure back to normal.