There are just 2 scenarios involved here. Either the heart rate must be increased or decreased.
1. The heart rate must go up because the blood pressure is low, there’s a shortage of oxygen, an excess of CO2 or the pH is too low – some of these go together e.g. O2, CO2 and pH interaction during exercise, stress, etc.
The hormone that increases heart rate is noradrenaline. This can be released by the brain’s medulla via the sympathetic neurons when aortic and carotid baroreceptors which sense low blood pressure or chemoreceptors which sense excess or a lack of certain chemicals send signals via sensory neurons. The aorta is the principal vessel carrying oxygenated blood from the heart to the rest of the body, while the carotid arteries carry it on to the brain via the neck.
Noradrenaline then binds to the heart’s sino-atrial node which results in increased contractions.
2. The heart rate must go down because the blood pressure is high, there’s excess O2, low CO2 or the pH is too high
The sensing process is identical – baroreceptors and chemoreceptors do their jobs. The only difference? Of course, the released product must be different – the medulla orders acetylcholine instead of noradrenaline, and this signal follows the parasympathetic pathway.
Acetylcholine makes the SA node slow contractions.
The autonomic nervous system is hence central to the increase of heart rate as part of the sympathetic stress or flight/fight response. Upon perception of any stimulus that causes this response, the system directs the release of adrenaline to increase heart rate, alongside other outcomes such as increased blood pressure, inhibition of digestion, etc.
wow just checked out your website and think it’s pretty cool.
Eager bug The Student Room
Just a huge thank you for spending your time helping others. I love your site and I'm seriously very grateful. No word of a lie
Neuron13 The Student Room
Good topic notes and cool videos. I'll definitely recommend it to my students.
Seema Sehgal AQA Examiner and biology teacher on LinkedIn
Hi! I have recently finished my first year doing biology at uni and I subscribed to you back when I was doing AQA A levels (glad I don’t live in Wales after watching this vid lol) and you and your website helped me so much. Just wanted to say it’s so nice to see you
Mr Peanutbutter YouTube
I got A* in A-level biology (Cambridge), thanks! I love your videos
Sherif Negm Facebook