Editing Heart (section)

===Heart rate===
{{main|Heart rate}}{{listen|filename=Emily's racing heartbeat.wav|title=A racing heartbeat|description=Heart sounds of a 16 year old girl immediately after running, with a heart rate of 186 BPM.|format=[[wav]]}}[[File:2020 SA Node Tracing.jpg|thumb|The prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarisation and repolarisation. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarisation and contraction of the cell; there is no resting potential.<ref name="CNX2014"/>]]

The normal [[resting heart rate]] is called the [[sinus rhythm]], created and sustained by the [[sinoatrial node]], a group of pacemaking cells found in the wall of the right atrium. Cells in the sinoatrial node do this by creating an [[action potential]]. The [[cardiac action potential]] is created by the movement of specific [[electrolyte]]s into and out of the pacemaker cells. The action potential then spreads to nearby cells.{{sfn|Guyton & Hall|2011|pp=115–120}}

When the sinoatrial cells are resting, they have a negative charge on their membranes. A rapid influx of [[sodium]] ions causes the membrane's charge to become positive; this is called [[depolarisation]] and occurs spontaneously.<ref name="CNX2014"/> Once the cell has a sufficiently high charge, the sodium channels close and [[calcium]] ions then begin to enter the cell, shortly after which [[potassium]] begins to leave it. All the ions travel through [[ion channels]] in the membrane of the sinoatrial cells. The potassium and calcium start to move out of and into the cell only once it has a sufficiently high charge, and so are called [[voltage-gated calcium channel|voltage-gated]]. Shortly after this, the calcium channels close and [[potassium channels]] open, allowing potassium to leave the cell. This causes the cell to have a negative resting charge and is called [[repolarization|repolarisation]]. When the membrane potential reaches approximately −60 mV, the potassium channels close and the process may begin again.<ref name="CNX2014"/>

The ions move from areas where they are concentrated to where they are not. For this reason sodium moves into the cell from outside, and potassium moves from within the cell to outside the cell. Calcium also plays a critical role. Their influx through slow channels means that the sinoatrial cells have a prolonged "plateau" phase when they have a positive charge. A part of this is called the [[absolute refractory period]]. Calcium ions also combine with the regulatory protein [[troponin C]] in the [[troponin complex]] to enable [[muscle contraction|contraction]] of the cardiac muscle, and separate from the protein to allow relaxation.<ref name="davis">{{cite journal| title=Ca<sup>2+</sup> exchange with troponin C and cardiac muscle dynamics |last1=Davis |first1=J.P. |last2=Tikunova |first2=S.B. | journal=Cardiovascular Research | year=2008 | volume=77 | issue=4 | pages=619–626 | doi=10.1093/cvr/cvm098 | pmid=18079104 | doi-access=free }}</ref>

The adult resting heart rate ranges from 60 to 100 bpm. The resting heart rate of a [[neonate|newborn]] can be 129 beats per minute (bpm) and this gradually decreases until maturity.<ref>{{cite journal|title=Resting pulse rate reference data for children, adolescents and adults, United States 1999–2008|journal=National Health Statistics Reports|issue=41|pages=1–16|url=https://www.cdc.gov/nchs/data/nhsr/nhsr041.pdf|pmid=21905522|year=2011|last1=Ostchega|first1=Y|last2=Porter|first2=K.S.|last3=Hughes|first3=J|last4=Dillon|first4=C.F.|last5=Nwankwo|first5=T|url-status=live|archive-url=https://web.archive.org/web/20170623030620/https://www.cdc.gov/nchs/data/nhsr/nhsr041.pdf|archive-date=23 June 2017}}</ref> An athlete's heart rate can be lower than 60 bpm. During exercise the rate can be 150 bpm with maximum rates reaching from 200 to 220 bpm.<ref name="CNX2014"/>

====Influences====
<!-- Nerves-->The normal [[sinus rhythm]] of the heart, giving the resting heart rate, is influenced by a number of factors. The [[cardiovascular centre]]s in the brainstem control the sympathetic and parasympathetic influences to the heart through the vagus nerve and sympathetic trunk.<ref name=GUYTONHALL2005>{{cite book|last2=Hall|first1=Arthur C. |last1=Guyton |first2=John E.|title=Textbook of medical physiology|date=2005|publisher=W.B. Saunders|location=Philadelphia|isbn=978-0-7216-0240-0|pages=116–122|edition=11th}}</ref> These cardiovascular centres receive input from a series of receptors including [[baroreceptor]]s, sensing the stretching of blood vessels and [[chemoreceptor]]s, sensing the amount of oxygen and carbon dioxide in the blood and its pH. Through a series of reflexes these help regulate and sustain blood flow.<ref name="CNX2014"/>

<!--Baroreceptors-->Baroreceptors are stretch receptors located in the [[aortic sinus]], [[carotid body|carotid bodies]], the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself. Baroreceptors fire at a rate determined by how much they are stretched,{{sfn|Guyton & Hall|2011|p=208}} which is influenced by blood pressure, level of physical activity, and the relative distribution of blood. With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation. As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation.<ref name="CNX2014"/> There is a similar reflex, called the atrial reflex or [[Bainbridge reflex]], associated with varying rates of blood flow to the atria. Increased venous return stretches the walls of the atria where specialized baroreceptors are located. However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase heart rate. The opposite is also true.<ref name="CNX2014"/> Chemoreceptors present in the carotid body or adjacent to the aorta in an aortic body respond to the blood's oxygen, carbon dioxide levels. Low oxygen or high carbon dioxide will stimulate firing of the receptors.{{sfn|Guyton & Hall|2011|p=212}}

<!--Other-->Exercise and fitness levels, age, body temperature, [[basal metabolic rate]], and even a person's emotional state can all affect the heart rate. High levels of the hormones [[epinephrine]], norepinephrine, and [[thyroid hormone]]s can increase the heart rate. The levels of electrolytes including calcium, potassium, and sodium can also influence the speed and regularity of the heart rate; [[hypoxemia|low blood oxygen]], low [[blood pressure]] and [[dehydration]] may increase it.<ref name="CNX2014"/>