How stress affects our metabolism: heart rate & headaches (pt1)

Our heart rate and breathing rate are controlled by two different parts of the brain. The medulla (as part of the brain stem) and the hypothalamus both exert control over our heart rate and breathing rate. We can also consciously and deliberately control our breathing rate. We can deliberately move our skeletal muscles, but we cannot deliberately control our heart rate.

Our medulla changes our heart rate and breathing rate in response to “pressure” and “CO2/O2 levels” that our brain gets from the neurons associated with the smooth muscle of our blood vessels. If we start moving fast, our muscles will use more O2 and produce more CO2. This causes the CO2/O2 ratio to rise, the increased CO2/O2 ratio sensory information is processed by our medulla, and our medulla sends out signals that make our hearts beat faster and us breathe faster. This helps more O2 get to the moving muscles faster.

If we did not increase our heart rates and breathing rates in response to increased CO2, our blood would become too acidic and our bodies would start shutting down. It is important that our hearts can beat faster and that we can breathe faster during times that we’re trying to move.

Our hypothalamus changes our heart rate and breathing rate in response to “emotional” stimuli. This can be things like “I’m late for class” (you may feel like this is a fact and not an emotion, but the processing is similar) or “I saw something out of the corner of my eye that I need to run from” (this is part of a reflex response that also involves the superior colliculi of the brain stem, but has an emotional component) or “I’m stressed out” (a more obvious emotional component).

Our bodies are built to run away from our problems (literally). When we get “stressful” stimuli, our hypothalamus releases signals to prepare us to run. While our medulla will respond to signals like increased CO2 once we’ve started running and increase our heart rate, our hypothalamus will respond to signals that say we should start running and increase our heart rate.

What happens when our hypothalamus sends signals to increase our heart rate in anticipation that we’ll start moving, but we don’t actually start moving? Our hypothalamus has sent signals to increase our heart rate and breathing rate to prepare us to move. This allows our heart rate and breathing rate to deliver more O2 to our muslces before the CO2 levels rise.

So what happens if we don’t start moving? If we aren’t moving, our muscles won’t be using extra O2 to produce energy, so they won’t have CO2 to put into the blood. Now, our CO2/O2 ratio is going to drop – a signal that we have too much O2 in our blood.

Our medullas will sense that there’s not enough CO2 and will try to even our bodies back out so that there’s the right amount of CO2 in our blood. Just like we can’t let our blood get too acidic with too much CO2, we also can’t let our blood get too basic with not enough CO2.

Can our medullas get our hearts to slow down if our hypothalamus is telling our hearts to speed up? No. Our ability to try to predict what we need to do next is so strong that as long as we think we need to move fast away from something, our hypothalamus will win and keep our heart rates up. It will also try to keep our breathing rate up. But then how does our body deal with the CO2/O2 problem?

Our medulla can’t change our heart rate if the hypothalamus is increasing it, but it can change our blood pressure. Our medulla will sense that we need to get our CO2/O2 levels back to a normal range. The options are to slow down our heart rate (which our hypothalamus won’t let happen), start moving (which we have decided to not do), or to drop our blood pressure. The medulla will send signals to our blood vessels to open up and get wider (vasodilate) so that it takes more pressure to push blood around. This slows down how fast the blood comes back to the lungs and gives the blood more time to give up O2 to the muscles and pick up CO2.

Rapid changes in blood pressure can make us lightheaded. If you have ever been really stressed out and gotten dizzy, it’s because your blood pressure dropped rapidly in response to your medulla trying to even out your CO2/O2 levels while your hypothalamus is telling your heart to beat faster. This is also why some people end up hyperventilating when they’re stressed – their hypothalamus is telling them to breathe faster but there’s nowhere for the air to go because, if they aren’t moving, there’s not much CO2 to exhale.

Can we do anything about this mismatch between the hypothalamus, the medulla, and our movement? Yes.

Our systems will reset themselves if:

1.     the stressful stimulus is taken away,

2.     we bring our heart rate down,

3.     we restore our CO2/O2 levels back to normal.

We don’t have conscious or deliberate control over the stimulus or our heart rate, but we can do things that reduce how stressful we perceive things to be (hard and takes a long time – but is worth is) and we can do things to restore our CO2/O2 levels to normal (easy and takes a short time – and helps with the first thing).

Questions:

1. Name two things you think you can physically do to bring your CO2/O2 levels back to normal when you’re stressed out? These should be physical activities that you have deliberate control over and that you can do immediately.

2. How do you think that consciously taking deep breathes can make us feel “calmer”?

3. Find your pulse and take your current heart rate. Then find a comfortable seat somewhere and put one hand on your chest and one hand on your belly. Take 5 – 10 “box breathes”. Take your pulse again. Did it change?

References:

Arthur, W. and GC Kaye. (2000) The pathophysiology of common causes of syncope. Postgraduate Medical Journal. Vol 76.

Whitworth, JA. et al. (2005) Cardiovascular consequences of cortisol excess. Vascular Health Risk Management. Vol 1.