Back to Basics: Homeostasis Explained

We understand that heart rate variability is a complex topic which requires an understanding of multiple processes to fully grasp.  However, we can make it simple with a sound understanding of the basic principles behind HRV. 

In this article, we’ll revisit one of the fundamental concepts behind HRV and ultimately survival: homeostasis.

What is Homeostasis? 

We can derive the basic concept of homeostasis simply by breaking down the word into its two components:

homeo-, from the Greek word homoio, means similar

-stasis, from the Greek word stásis, means a state of inactivity or equilibrium

So we can simplify our understanding of homeostasis as: a process that keeps conditions similar or in a state of equilibrium

Applying this concept to the body, homeostasis is:

“The maintenance of the body’s interval environment within the required physiological ranges in the face of varying internal and external stressors.”

Why are we so concerned about keeping the body’s internal environment stable?

As the physiological definition suggests, there are certain ranges that variables such as blood pressure and oxygen content must remain within for the body to survive and function properly.  

What is Really Being Regulated?

You understand now that homeostasis is about maintaining the body’s internal environment, but what physiological variables are actually regulated?

Temperature

pH

Blood pressure

Blood oxygen content

Blood carbon dioxide concentration

Blood glucose concentration, just to name a few

In the case of regulating these variables and others, there are always at least 3 components at play:

1.) The receptor: senses the change in the body’s internal environment (the stimulus) and sends this information to the control center

2.) The control center: interprets the appropriate response to bring the internal environment back to the required physiological range and signals the effector(s)

3.) The effector: usually muscles of glands which receive the signal from the control center and enact the necessary change, returning the internal environment back to the required physiological range

To give you an example of how the three components of homeostasis work, we’ll look at the process that keeps body temperature within the necessary range during exercise (Dabrowski):

Stimulus: body temperature rises as cellular metabolism increases to provide muscles with sufficient energy (ATP).   

Receptor: sensory receptors of the skin and other organs (thermoreceptors) detect the increase in temperature and convey the signal to the hypothalamus.

Control center: the hypothalamus interprets the increase in temperature relative to the set point of 37°C (98.6°F) and signals effectors to carry out temperature-reducing changes.

Effectors/Response: activity of the sympathetic nervous system (fight or flight) is inhibited, causing blood vessels beneath the skin to dilate and metabolic rate to be reduced.  The increased surface area of the blood vessels allows more heat to be lost from the skin, and the reduction of metabolic rate decreases the amount of heat produced in the body core.  Sweat glands are signaled to secrete sweat which evaporates from the skin to further cool the body. 

The net result is that the body temperature is returned to normal and the body’s internal environment remains stable.  Normal physiological processes are able to continue uninterrupted.

So the process of homeostasis allows the body to provide muscles with additional energy during exercise and subsequently cool the body as the temperature increases due to heightened metabolic activity.  

Homeostasis and Heart Rate Variability

As we saw in the thermoregulation example, the process of homeostasis is often carried out by changing sympathetic and/or parasympathetic activity. 

Why is that?

The two branches of the autonomic nervous system regulate a variety of variables, including: 

Given that homeostasis is the way the body maintains its internal environment in the face of stress, we can begin to understand how it relates to HRV…

HRV provides a measure of how well the body is responding to stress, be that training stress, work stress, diet stress, or otherwise. 

In other words, HRV measures the effectiveness of the autonomic nervous system at maintaining homeostasis. 

While these are overly simplified explanations, they provide a general illustration of what we’re talking about:

When HRV raises in an attempt to rest and recover following a stressor, the parasympathetic nervous system is actively trying to replenish energy/resource stores that were depleted by the stressful event. 

Following recovery, the parasympathetic input will decrease, the sympathetic component will increase, or both in order to return the body’s internal environment to the normal physiological range (back to homeostasis). 

Similarly, when HRV dips to provide the energy needed for an acute stress response, the sympathetic nervous system is working to mobilize resources that will keep the body alive in the face of increased energy demands. 

Following the acute response in a healthy individual, the sympathetic input will decrease, the parasympathetic component will increase, or both in order to return the body’s internal environment back to homeostatic norms. 

So while there are homeostatic mechanisms outside of autonomic control (such as behavioral changes in response to stimuli), the concept of HRV and homeostasis are inextricably linked.  

Works Referenced

California Institute of Technology. "Variability in heart beat keeps the body in balance." ScienceDaily. ScienceDaily, 22 September 2014. <www.sciencedaily.com/releases/2014/09/140922130749.htm>.

Dabrowski, D. (n.d.). Neurophysiology of Temperature Regulation. Retrieved October 13, 2014. <http://dwb.unl.edu/teacher/nsf/c01/c01links/www.science.mcmaster.ca/biology/4s03/thermoregulation.html>