Blood vessels. They’re more than just pipes. They’re really cool pipes that can be selectively adjusted to deliver more or less blood to particular areas. A prepper who knows how these adjustments occur can work the system to keep themselves out of harm’s way.
This article is part two of an ongoing series that looks at various physiological systems from a prepping point of view. You can find the first of the series, on the gut, here. I (Spice) am not a physician. I am a physiologist, and want to help you understand how you work and use that when trouble heads your way.
We made a companion podcast and video (the podcast is the soundtrack of the video) to further demonstrate more information on this topic. You can listen to and watch them here:
https://www.youtube.com/watch?v=rfZnSystIRo&feature=youtu.be
Blood vessel basic physiology
The diagram below will help you get an overview of how blood vessels are put together to accomplish circulation.
Freshly oxygenated blood emerges from the heart into the arteries. These subdivide, like a superhighway branching off into multiple state highways. Different branches keep heading off to different regions. By the time you get to the ‘two-lane blacktop’ level, the vessels start getting wrapped in smooth muscle and are called arterioles.
The arterioles branch into the tiny little capillaries that actually trade materials with the body cells, giving them oxygen and nutrients and picking put their wastes. Afterwards the blood collects into small venules. Venules join together like river tributaries to make veins. Eventually the biggest vein dumps the blood back into the heart.
Of all the blood vessels, only the capillaries actually deliver and collect material from the cells. Thanks for the image to David Nascari and Alan Sved, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Controlling blood vessel size
The arterioles, venules, and veins are all wrapped with smooth muscle. As the little spring images on the diagram are trying to show, the smooth muscle is wrapped around the tube at right angles to blood flow. Contracting the muscle makes the diameter of the vessels smaller.
Reducing the diameter of a blood vessel makes it harder for blood to move through. Less blood flow moves through narrower vessels. It’s like road construction: Fewer traffic lanes means fewer cars can pass along the road per minute.
And what else happens when traffic lanes are closed to construction? Traffic jams. Traffic stacks up behind the narrowed part. Similarly, when blood vessels constrict, blood pressure goes up.
Why we adjust blood vessel diameter
Blood flow through a tissue is vital. The more active the tissue, the more blood flow it needs. To fill this need, we’ve got some nifty systems to detect when a tissue isn’t getting enough blood flow for its activity level. Then the smooth muscle is relaxed so the vessels serving the tissue get wider. Voila, more flow! Unless something else is monkeying with the system.
Blood flow is also important to body temperature control. When we start to get too hot, we’ll relax the muscles on blood vessels feeding the skin. More flow gets near the surface, increasing heat loss. When we don’t want that cooling effect, we’ll constrict vessels to the skin to reduce how much nice warm blood gets near the nasty cold outside world. This conserves body heat.
Blood pressure is a consideration too. When we feel a need to raise our blood pressure, we’ll constrict a lot of blood vessels. This is usually because the pressure’s lower than average, or we’re stressed. During times of stress our bodies Try to keep the pressure higher.
Using blood vessel adjustments to your advantage
The first one is well known, but often overlooked: Because active tissues need more flow, you’ve got mechanisms where active tissues release chemical signals that make their blood vessels dilate and give them more flow. When you “warm up” muscles before serious work, you’re doing it to get the blood flow to increase. That makes the tissues work better and be less prone to injury.
If you find yourself needing to do heavy work, get over any notion that warming up is something only gym rats do. If starting easy isn’t an option, start wearing plenty of insulation. But use layers so you can start stripping them off so you don’t get sweaty. Wet and cold are a bad combo, and the heat from exercise doesn’t last much longer than the work.
On a related note: Keeping moving is more effective than thick socks for repelling frostbite. Activity produces body heat, so you open up vessels more. Limbs get more nice warm blood. When I bike ride in the winter, I know my hands and feet will be cold for the first two miles. After that, if I’ve chosen the right clothing, I’m good until I stop. But I do have to bring snacks — I find that if I run low on fuel, I’ll chill even though I’m still working.
Knowing how your blood vessels work tells you not to do:
Don’t let yourself get dehydrated. It will make it harder for you to deal with temperature extremes. Dehydration lowers blood pressure. You won’t be able to send as much blood to the skin to release heat if you need to, and you’ll shut down flow more than usual if cold. Remember to drink when working in the cold; you’re still losing a fair bit but don’t notice it like you do in summer.
Don’t drink alcohol to warm yourself. Alcohol reduces constriction signals to skin vessels. That makes you *feel* warmer because the skin is warmer, but you’re losing body heat much faster.
Don’t put a hypothermic person’s hands and feet in warm water to try to warm them. Warming the limbs first can actually increase the risk in the hypothermic person. Warm skin makes the vessels in the region dilate. If you warm the skin of hands and feet, their vessels relax. More blood goes out to the limb.
But a lot of the limb is still cold. The blood making the trip is chilled. When it goes back to the heart, the body core is further chilled. That might even stop the heart. Heat the core instead. Warm packs in armpits, chest, back, and groin are better ideas. (Click here for more on treating hypothermia.)
In short: Activity and heat cause blood vessels to enlarge. Cold and stress make them constrict. If you want to conserve body heat or raise blood pressure you want smaller vessels. If you want to feed a tissue better you want it to have larger vessels.