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.
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.
‘Til next week!