Editing Homeostasis (section)

===Arterial blood pressure===
{{Main |Baroreflex |Renin–angiotensin system}}
The brain can regulate blood flow over a range of blood pressure values by [[vasoconstriction]] and [[vasodilation]] of the arteries.<ref>{{cite web|url=https://www.orlandoregional.org/pdf%20folder/overview%20adult%20brain%20injury.pdf|archive-url=https://web.archive.org/web/20080227162001/https://www.orlandoregional.org/pdf%20folder/overview%20adult%20brain%20injury.pdf|url-status=dead|archive-date=27 February 2008|date=2004 |title=Overview of Adult Traumatic Brain Injuries: Self-Learning Packet |publisher=Orlando Regional Healthcare}}</ref>

High pressure receptors called [[baroreceptor]]s in the walls of the [[Aorta|aortic arch]] and [[carotid sinus]] (at the beginning of the [[internal carotid artery]]) monitor the arterial [[blood pressure]].<ref>{{cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|title=Human physiology : the basis of medicine|date=2006|publisher=Oxford University Press |isbn=978-0-19-856878-0|page=4|edition= 3rd}}</ref> Rising pressure is detected when the walls of the arteries stretch due to an increase in [[blood volume]]. This causes [[cardiomyocytes|heart muscle cells]] to secrete the hormone [[atrial natriuretic peptide]] (ANP) into the blood. This acts on the kidneys to inhibit the secretion of renin and aldosterone causing the release of sodium, and accompanying water into the urine, thereby reducing the blood volume.<ref name=tortora7>{{cite book |last1= Tortora |first1= Gerard J. |last2=Anagnostakos|first2=Nicholas P.| title=Principles of anatomy and physiology |url= https://archive.org/details/principlesofanat05tort |url-access= registration |page=[https://archive.org/details/principlesofanat05tort/page/430 430]|edition= 5th |publisher= Harper & Row |date= 1987 |isbn= 978-0-06-350729-6 }}</ref>
This information is then conveyed, via [[afferent nerve fiber]]s, to the [[solitary nucleus]] in the [[medulla oblongata]].<ref name="OUP">{{cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|title=Human physiology : the basis of medicine|date=2006|publisher=Oxford University Press |isbn=978-0-19-856878-0|pages=299–302|edition= 3rd}}</ref> From here [[motor nerves]] belonging to the [[autonomic nervous system]] are stimulated to influence the activity of chiefly the heart and the smallest diameter arteries, called [[arterioles]]. The arterioles are the main resistance vessels in the [[arterial tree]], and small changes in diameter cause large changes in the resistance to flow through them. When the arterial blood pressure rises the arterioles are stimulated to [[vasodilation|dilate]] making it easier for blood to leave the arteries, thus deflating them, and bringing the blood pressure down, back to normal. At the same time, the heart is stimulated via [[cholinergic]] [[Parasympathetic nervous system|parasympathetic nerves]] to beat more slowly (called [[bradycardia]]), ensuring that the inflow of blood into the arteries is reduced, thus adding to the reduction in pressure, and correcting the original error.

Low pressure in the arteries, causes the opposite reflex of constriction of the arterioles, and a speeding up of the heart rate (called [[tachycardia]]). If the drop in blood pressure is very rapid or excessive, the medulla oblongata stimulates the [[adrenal medulla]], via "preganglionic" [[sympathetic nerves]], to secrete [[epinephrine]] (adrenaline) into the blood. This hormone enhances the tachycardia and causes severe [[vasoconstriction]] of the arterioles to all but the essential organs in the body (especially the heart, lungs, and brain). These reactions usually correct the low arterial blood pressure ([[hypotension]]) very effectively.