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I know that when we do tasks which require extra physical effort (like running etc.), our heart beats faster.

I also know that the reason behind this is that these jobs require more energy than usual and, hence, require more oxygen for breaking down food. And, since, oxygen is transported throughout body by blood (by forming compound with Haemoglobin (present in RBCs), called Oxyhaemoglobin), the heart needs to beat faster to pump more blood.

However, I am not aware of how does the heart automatically start beating faster.

  • Is there any involvement of the Nervous System?
  • Does the heart have some sort of special mechanism that activates when the body performs a strenuous job?
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  • $\begingroup$ What research have you done before asking it here? $\endgroup$ – another 'Homo sapien' May 31 '17 at 5:47
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I am sure that there are many ways that your body can regulate heart rate, but I am going to cover this topic briefly from the perspective of $\ce{CO2}$/$\ce{O2}$ concentrations, and pH.

The medulla oblongata

The medulla oblongata is part of the central nervous system, and makes up the lower portion of the brain stem. It is responsible for performing autonomic functions, including the regulation of heart rate through chemoreceptors. The medulla oblongata receives its blood supply from the vertebral artery and makes both afferent and efferent connections with the vagus nerve.

Chemoreceptors

There are two main types of chemoreceptors in your cells that are responsible for regulating heart rate.

  1. Central chemoreceptors

These are located in the medulla oblongata, and are responsible for measuring the concentration of $\ce{H+}$ ions (ie. pH). When the pH is low, neural cells with these receptors will signal the release of norepinephrine and epinephrine.

  1. Peripheral chemoreceptors

These are located in the carotid and aortic bodies and measure the concentration of $\ce{O2}$. The receptor cells of these structures make connection with afferent nerves, to send a signal to the central nervous system in times of hypoxemia.

$\ce{CO2}$/$\ce{O2}$ Exchange

Blood is the tissue responsible for distributing chemicals around your body. It is therefore also involved in the $\ce{CO2}$/$\ce{O2}$ exchange between tissues. Blood cells use hemoglobin as as a carrier molecule for this exchange.

When $\ce{O2}$ is delivered to tissues, it will be processed by the mitochondria of the cells to synthesise ATP, and $\ce{CO2}$ as a biproduct. ATP is also responsible for muscle contractions, and so, with exercise comes an increased utilisation of ATP. With ATP stores running low, your cells will begin to use more $\ce{O2}$, and therefore, they will produce more $\ce{CO2}$.

$\ce{CO2}$ and Acidity

So, I kinda lied when I said hemoglobin is the carrier of $\ce{CO2}$. This is mostly true, but $\ce{CO2}/$$\ce{O2}$ is also somewhat soluble in blood, and this accounts for changes in blood-pH.

$\ce{CO2 + H2O}$ $\ce{<=>}$ $\ce{H2CO3}$ $\ce{<=>}$ $\ce{HCO3- + H+}$

The above equilibrium is catalysed by carbonic anhydrase, and accounts for a decrease in pH levels.

Signalling in response to pH

So your exercising, your body is using lots of $\ce{O2}$ to produce ATP. Your $\ce{CO2}$ levels have increased, and your internal pH has dropped. Now what?

Well, as I'm sure you have guessed, your nervous system takes over.

The central chemoreceptors of the medulla oblongata send an impulse to the adrenal glands in response to the decreased pH. This signals the release of the hormones epinephrine and norepinephrine in to the bloodstream. The release of epinephrine and norepinephrine is regulated by the sympathetic nervous system. This accelerates the heart rate by binding β-receptors in your heart tissue. Efferent nerve connections from the vagus nerve also participate in this process (I think).

For mechanisms on heart contration, see here; everything following "excitation contraction coupling" is probably the most relevant.

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  • $\begingroup$ I think this answer is good and very extensive. However, I didn't immediately see the (important) aspect that all these measures are transfered to the heart via the sympathetic nervous system. Since the OP specifically asked for involvement of the NS I suggest adding a disclaimer stating this involvement to the top of your answer. $\endgroup$ – Nicolai May 31 '17 at 4:49
  • $\begingroup$ I am guessing from neural cells and @Nicolai 's comment that the involvement of the Nervous System is there. Am I right? $\endgroup$ – Arjun May 31 '17 at 6:10
  • $\begingroup$ @Nicolai Thank you for your suggestion. I have edited my question. Does this seem better? And yes Arjun, of course! But, I think it is more interesting to know what it is exactly that elicits the nervous system's involvement in a function. $\endgroup$ – Bob May 31 '17 at 6:39
  • $\begingroup$ @Arjun Yes, I think this way it's much clearer where the different receptors are and how the signal is transferred to the heart $\endgroup$ – Nicolai May 31 '17 at 7:33
  • $\begingroup$ Whenever we think about our body changes, we have to think what come first, what is cause and what is effect. How the heart automatically start beating faster? $\endgroup$ – MYO10293 Jun 10 '17 at 5:33
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In short, whenever there is role of controlling mechanism, neural and humoral used to get involve. Paraventricular nucleus of hypothalamus controlling the medulla autonomic nervous system. While resting vagal tone is inhibited, sympathetic predominates leading to positive chronotropic and inotropic effect. Emotional and other factor also involve. Even anticipatory effort can have the effect. Humoral is through hypothalamo-hypophisial-adrenal medullary axis.

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  • $\begingroup$ Thank you for your answer and welcome to Biology.SE. Could you back up your answer with references? Thank you! $\endgroup$ – AlexDeLarge May 30 '17 at 15:54
  • $\begingroup$ Whenever we think about our body changes, we have to think what come first, what is cause and what is effect. How the heart automatically start beating faster? For this question, blood gas changes come later which is the effect of physical activity that will regulate heart rate further. All the senses visual, auditory, touch etc even the act of thinking physical activity will increase heart rate. Our body has extensive feedback system, in preparation for body need. So many references are there. I used to refer Ganong physiology review. $\endgroup$ – MYO10293 Jun 10 '17 at 5:46

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