Editing Heart (section)

====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"/>