Stuff I Learned – Getting out of Bed

In the first week of my Exercise Physiology course we studied how the circulatory system responds to changes in position, particularly in the transition from supine to standing. If you’re anything like me, you may consider studying getting out of bed about as interesting as sodden cardboard. Interestingly, it turns out that this seemingly simple transition involves numerous adaptations that are unique to the human organism.

One of the problems with our human-ness involves rearing up on our hind legs. OK – so being a biped isn’t necessarily a problem, though the vertical orientation creates some serious design challenges. Our brains are at the top of our vertical bodies, which means that gravity is constantly pulling blood away from our brains. Brains are very hungry consumers of blood (much like zombies are hungry consumers of brains) and if the brain doesn’t get a sufficient blood supply, you will quickly pass out.

Buddy Cat has 70% of his blood above his heart. Like any quadruped, this means that getting blood to the brain isn’t all that difficult – the blood is already above the heart and is pretty close to the brain. Humans, on the other hand, have 70% of their blood below their hearts. This means that our hearts have to work hard to pump the blood uphill into the brain. 

When you move from supine to standing, you’re at risk of game-over. What keeps us (mostly) from passing out when standing up? It’s actually a series of adaptations that facilitates this seemingly simple transition.

  • Your rest and digest (parasympathetic) nervous system lowers your heart rate while you’re resting. Upon standing, the parasympathetic nervous system stops lowering your heart rate, and your heart rate increases quite a bit in what’s called parasympathetic withdrawal. For a few moments, your faster heart beat will help pump blood uphill to your brain.
  • Unfortunately, the heart can only pump what it receives, and when you first stand up most of the blood remains stuck in the floppy veins of the legs. Thankfully, your body has the capacity to “tighten” the veins (vasoconstriction) to help facilitate the flow of blood upward into the brain. Upon standing, vasoconstriction helps move the blood out of your legs and up into your torso.
  • In moving from supine to standing, you use muscles in your legs to accomplish that transition. The contracting muscles squeeze the veins, which further helps to push the blood upstream and into your brain.
  • The muscle pump is accompanied by the respiratory pump. (I wrote about the respiratory pump in an earlier blog posting.) When you inhale, the pressure in your thoracic cavity is reduced, which tends to pull blood uphill and into your heart. When you exhale, the pressure in your thoracic cavity increases, which tends to push the blood out of your heart. The respiratory pump also helps to improve circulation to the brain.
  • In your abdominal cavity, organs also help in this process of moving blood. For example, your liver is capable of storing a significant amount of blood volume. Related to the respiratory pump, when you breathe, blood that’s stored in the liver is lifted upward into active circulation.
These are just a few of the adaptations that are unique to the human animal. I’m just getting back into the rhythm of the new school year, so I’ll keep this first blog posting brief and to-the-point. I look forward to learning more and sharing more about this wondrous vehicle that we each inhabit.

‘Til next week!

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