Editing Blood pressure (section)

====Orthostatic hypotension====
{{Main|Orthostatic hypotension}}
A large fall in blood pressure upon standing (typically a systolic/diastolic blood pressure decrease of >20/10&nbsp;mmHg) is termed [[orthostatic hypotension]] (postural hypotension) and represents a failure of the body to compensate for the effect of [[gravity]] on the circulation. Standing results in an increased [[Hydrostatics|hydrostatic]] pressure in the blood vessels of the lower limbs. The consequent distension of the veins below the [[Thoracic diaphragm|diaphragm]] (venous pooling) causes ~500 ml of blood to be relocated from the chest and upper body. This results in a rapid decrease in central blood volume and a reduction of ventricular [[Preload (cardiology)|preload]] which in turn reduces stroke volume, and mean arterial pressure. Normally this is compensated for by multiple mechanisms, including activation of the [[autonomic nervous system]] which increases [[heart rate]], [[myocardial contractility]] and systemic arterial [[vasoconstriction]] to preserve blood pressure and elicits [[Vein|venous]] vasoconstriction to decrease venous [[Compliance (physiology)|compliance]]. Decreased venous compliance also results from an intrinsic [[Myogenic mechanism|myogenic]] increase in venous [[smooth muscle]] tone in response to the elevated pressure in the veins of the lower body.

Other compensatory mechanisms include the veno-arteriolar [[axon reflex]], the '[[Skeletal-muscle pump|skeletal muscle pump]]' and '[[respiratory pump]]'. Together these mechanisms normally stabilize blood pressure within a minute or less.<ref name=":4">{{cite journal | vauthors = Ricci F, De Caterina R, Fedorowski A | title = Orthostatic Hypotension: Epidemiology, Prognosis, and Treatment | journal = Journal of the American College of Cardiology | volume = 66 | issue = 7 | pages = 848–860 | date = August 2015 | pmid = 26271068 | doi = 10.1016/j.jacc.2015.06.1084 | doi-access = free }}</ref> If these compensatory mechanisms fail and arterial pressure and blood [[rate of fluid flow|flow]] decrease beyond a certain point, the [[perfusion]] of the brain becomes critically compromised (i.e., the blood supply is not sufficient), causing [[lightheadedness]], [[dizziness]], weakness or [[Syncope (medicine)|fainting]].<ref>{{cite journal | vauthors = Franco Folino A | title = Cerebral autoregulation and syncope | journal = Progress in Cardiovascular Diseases | volume = 50 | issue = 1 | pages = 49–80 | year = 2007 | pmid = 17631437 | doi = 10.1016/j.pcad.2007.01.001 }}</ref> Usually this failure of compensation is due to disease, or drugs that affect the [[sympathetic nervous system]].<ref name=":4" /> A similar effect is observed following the experience of excessive gravitational forces (G-loading), such as routinely experienced by aerobatic or combat pilots '[[G-force|pulling Gs]]' where the extreme hydrostatic pressures exceed the ability of the body's compensatory mechanisms.