When standing from a sitting or lying position, gravity causes blood to collect in the legs and belly. Blood pressure drops because there's less blood flowing back to the heart. Usually, special cells (baroreceptors) near the heart and neck arteries sense this lower blood pressure. The baroreceptors send signals to the brain. This tells the heart to beat faster and pump more blood, which evens out blood pressure. These cells also narrow the blood vessels and increase blood pressure. Orthostatic hypotension occurs when something interrupts the body's process of dealing with the low blood pressure. Many conditions can cause orthostatic hypotension, including: Dehydration. Fever, vomiting, not drinking enough fluids, severe diarrhea and strenuous exercise with a lot of sweating can all lead to dehydration. Dehydration decreases blood volume. Mild dehydration can cause symptoms of orthostatic hypotension, such as weakness, dizziness and fatigue. Heart problems. Some heart conditions that can lead to low blood pressure include extremely low heart rate (bradycardia), heart valve problems, heart attack and heart failure. These conditions prevent the body from quickly pumping more blood when standing up. Endocrine problems. Thyroid conditions, adrenal insufficiency (Addison's disease) and low blood sugar (hypoglycemia) can cause orthostatic hypotension. So can diabetes, which can damage the nerves that help send signals that control blood pressure. Nervous system disorders. Some nervous system disorders, such as Parkinson's disease, multiple system atrophy, Lewy body dementia, pure autonomic failure and amyloidosis, can disrupt the body's ability to control blood pressure. Eating meals. Some people have low blood pressure after eating meals (postprandial hypotension). This condition is more common in older adults.