Cardiac Output (Heart Rate)

Unlike stroke volume, the heart rate is influenced by ‘electrical events’ and hence heavily regulated by chronotropic (rhythm) and dromotropic (electric) factors. Hence, stimuli of the autonomic nervous system and reflex control have a significant impact on patients’ heart rate.

The parasympathetic nervous system (PNS) decreases the heart rate by releasing the hormone acetylcholine to slow the heart rate. It also increases the atrioventricular node delay. In contrast, the sympathetic nervous system increases the heart rate in response to fight or flight situations such as stress and danger by releasing hormones such as catecholamines.

The ‘Bainbridge reflex’ is an example of reflex control which regulates the heart rate. Responsible for 40-60% of the increase in HR, the Bainbridge reflex increases the heart rate increases in response to a rise in atrial pressure.

Sources:

https://www.ncbi.nlm.nih.gov/books/NBK470455/

https://www.mayoclinic.org/healthy-lifestyle/fitness/expert-answers/heart-rate/faq-20057979#:~:text=A%20normal%20resting%20heart%20rate%20for%20adults%20ranges%20from%2060,to%2040%20beats%20per%20minute.

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