The normal response for a change in body position, results in a stabilization to the upright position in approximately sixty seconds. During this process, the normal change in heart rate would include an increase in heart rate of 10 to 15 beats per minute, and an increase in diastolic pressure of 10 mm Hg, with only a slight change in systolic pressure.
For those who are afflicted with Orthostatic Intolerance, there is an excessive increase in heart rate upon standing, resulting in the cardiovascular system working harder to maintain blood pressure and blood flow to the brain.
Upright posture also brings about a neurohumoral response, involving a change in the levels of vasopressin, renin, angiotensin and aldosterone levels - all of which are involved in the regulation of blood pressure.
Additionally, arterial baroreceptors, particularly those in the carotid sinus area, play an important role in the regulation of blood pressure and the response to positional changes. As the heart pumps blood to the body, the left atrium is passively filled with blood as a result of the force exerted by venous blood pressure. The baroreceptors in the left atrium respond, proportionately, to the pressure exerted by this venous blood pressure. Thus, a drop in venous blood pressure will trigger a compensatory response to increase blood pressure.
Any disruption in any of these processes, or their coordination, can result in an inappropriate response to an upright position, and can lead to a series of symptoms.
The symptoms for these conditions may include the following:
Excessive Fatigue
Exercise Intolerance
Recurrent Syncope or Near Syncope
Dizziness
Nausea
Tachycardia
Palpitations
Visual Disturbances
Tremulusness
Weakness - most noticeable in the legs
Chest Discomfort
Shortness of Breath
Mood Swings
Migraines and Other Headaches
Gastrointestinal Problems
National Dysautonomia Research Foundation
http://ndrf.org/orthostat.htm