Gravity causes blood pooling in the legs on standing. Compensatory mechanisms in the cardiovascular system typically prevent a drop in blood pressure associated with the increased blood volume in the legs. Certain conditions, however, may overwhelm or interfere with the normal compensatory mechanisms, resulting in dizziness upon standing.
Hemorrhage
The National Heart, Lung and Blood Institute explains that rapid blood loss due to injury or internal bleeding depletes the circulating blood volume, thereby dropping blood pressure. Causes of acute hemorrhage include the traumatic severing of an artery, blunt trauma to the liver or spleen, the rupturing of dilated veins, or varices, in the esophagus and stomach, and the rupturing of bulging areas, or aneurysms, in the arteries.
Heart Failure
A weakened heart may be unable to generate sufficient force to propel blood up to the brain while in a standing position. In a review article published in "American Family Physician," Drs. John Bradley and Kathy Davis note that an acute heart attack or decompensated congestive heart failure can provoke new onset of dizziness upon standing.
Dehydration
Dehydration is an abnormally low volume of total body water, which can substantially reduce blood pressure. Dehydration-induced low blood pressure frequently leads to dizziness upon standing, notes MayoClinic.com. Possible causes of dehydration include heat exhaustion, severe burns, massive diarrhea or vomiting, high fever and excessive urination due to undiagnosed diabetes mellitus.
Sepsis
Sepsis is a body-wide response to overwhelming bacterial infection. Low blood pressure remains a hallmark sign of sepsis. The blood pressure plummets due to the effects of chemicals released by the immune system. Some bacteria produce toxins that exacerbate the blood pressure-lowering effects of sepsis. Dizziness upon standing occurs frequently among people with sepsis. Merck Manuals reports that other symptoms of sepsis typically include high fever, shaking chills, confusion and decreased urine output. Without aggressive treatment, sepsis may lead to kidney, liver and respiratory failure.
Medications
Dizziness upon standing often occurs as a side effect of medications. The University of Pittsburgh Medical Center notes that medications commonly associated with low blood pressure include diuretics, barbiturates, monoamine oxidase inhibitors, tricyclic and tetracyclic antidepressants, antipsychotic medications and certain drugs used to treat high blood pressure. Dosage adjustments or a change to another medication may prove necessary in cases of unremitting dizziness.
Nerve Failure
Blood vessels normally contract upon standing to prevent a decrease in blood pressure due to blood pooling in the legs. The signals to stimulate blood vessel contraction travel via nerves of the autonomic system, which controls the involuntary responses of body tissues. Failure of the autonomic nervous system response upon standing can lead to dizziness. As noted by Bradley and Davis in their article in "American Family Physician," multiple sclerosis, Guillain-Barré syndrome, HIV/AIDS, tertiary syphilis and alcoholic neuropathy can cause autonomic failure resulting in dizziness upon standing.
