Vascular Hemodynamics for Dummies

Vascular Hemodynamics for Dummies Astronauts Chest Swelling

Read on to see why astronauts end up looking like this – and why it matters for everyone else.

Despite the title, we don’t think you’re a dummy. Even among the “big players” frequently condemning sitting and extolling the virtues of standing desks, this never gets mentioned. And it’s a big deal.

In fact, we think it’s the biggest single contributor to the health issues of sitting.

And although those first two words in the title look pretty sciency, we’re going to make this quite simple.

Introduction to Your Circulatory System

We’ll keep this short, but we need to review a few vocabulary words to get started.  As you may already know, with every beat your heart expels fresh oxygenated blood that just arrived from your lungs.  An elegant system of one-way valves ensures the blood flows only one direction.  A contraction of your heart muscle generates a pressure that pushes blood outward to the rest of your body via your arteries, which fan out almost like hollow branches in a tree.

At their smallest point, arteries are called capillaries, which are like a spider web that is woven through every living tissue in your body.  Capillaries have such a  thin skin (only one cell thick in fact), that oxygen and nutrients can pass out of them, and carbon dioxide and waste products can pass into them.   After the nutrient/waste exchange, the branching pattern reverses.  The web of capillaries recombines into veins, and these veins provide a path for spent blood to return to your heart and lungs to repeat the process.

Quitting Sitting Arteries Veins Straws Balloons Calf Pump Vascular Hemodynamics

Your arteries are like straws, and your veins are like balloons.

Here are a few things to remember.  Arteries are like straws that direct pressurized blood from your heart to the rest of your body.  Doctors are interested in the pressure in your arteries for various reasons.  Veins form the return system, they operate at lower pressure and they are more like balloons than straws.  This balloon-like behavior allows them to store blood — both by design to adapt to changing activity levels and positions, and through mis-use.  Today’s post is about veins.

The Little Known Peripheral Heart

Did you know that astronauts experience torso swelling while in zero gravity? It turns out, our big strong hearts are pretty dependent on gravity to get blood all the way out to our feet. So those floating space-station-dwellers end up with less blood in their legs, and more in their torsos, puffing them up like the genie from Aladdin (pictured).

Here on Earth, we always have Newton’s trusty friend helping us out. Gravity does a great job of getting blood to our feet when we’re sitting or standing. But our hearts aren’t designed to pump that blood all the way up. Our veins are soft and somewhat balloon-like, so even the heart of a lion couldn’t push the blood back up without first inflating our veins to the max. Which is why we have calves (ok, it’s not the only reason).

Since our hearts don’t efficiently pump blood back up from our lower legs, we depend on our calf muscles to do the pumping for us. Contracting our calves squeezes our the veins just like when you step on a ketchup packet.  Veins also have one-way valves that prevent blood flowing backwards, so squeezing them forces blood against gravity, and back up to the heart! Here’s how it’s supposed to work:

Quitting Sitting Calf Pump Vascular Hemodynamics for Dummies

Pretty simple, right? It works great while we’re walking and pumping those calves aplenty. But once we sit (or stand) still, the blood starts to pool in our lower legs. Laying down doesn’t pose a problem, because when we’re horizontal, gravity isn’t pushing our blood to our feet.

Our abdomen and chest also participate in this pumping process.  Each deep breath we take puts pressure on the veins in our chest and helps to push blood back up to our hearts.

Quantified Calves – Pumping for Science

When we were learning about this stuff, we thought, “Ok, sure, blood pools. But probably not much, and we’ve got plenty.” Au contraire, past us! We’re actually talking about a significant amount of blood – up to 800mL. That’s over 15% off our blood just stuck down there, doing nothing good.

That pooling causes bad stuff in our legs (we’ll get to that in a second), and it also reduces the amount of blood available to the rest of our bodies. So much so, that it can reduce our cardiac output by 2L/min, resulting in a reduction from average healthy cardiac output of more than 35%.

Where Blood Pooling Gets Really Nasty

You’ve no doubt heard that sitting reduces circulation while raising blood pressure. Now you know that lack of calf pump is a huge contributor.  We’ve also made a pretty compelling case that lazy calves cause higher blood sugar and a reduction in fat burning.

Blood pooling causes more local problems too. Having all that extra blood in your calves effectively removes it from circulation, so your heart has to work harder. And the pressure that builds up in your thin veins actually forces water out of your blood and into the surrounding tissue. So your blood gets more concentrated and your tissues get swollen. Stagnant, slow moving, blood can also cause disorders like deep vein thrombosis, undesired clotting, and even result in a fatal pulmonary embolism. The one-way valves in your veins can even become damaged over time they have to support a column of blood because the veins are never squeezed out and de-pressurized by the calf muscle – allowing the the veins to remain constantly swollen and balloon-like. 

Bottom line: blood pooling in your lower legs due to sitting (or stationary standing) causes problems for three reasons: not enough blood in your torso, damage to your precious venous valves, and too much blood pressure in your lower legs.

Calf Pumping is an Easy Solution

All of this stuff sounds pretty grim, but fear not – there’s a pretty simple solution. Just like nature intended, the way to get that blood flowing correctly is by flexing your calves. Researchers have conducted some pretty cool science to measure the effectiveness of your calves:

Quitting Sitting Calf Pump Venous Volume Vascular Hemodynamics for Dummies

Calf blood volume over course of activity sequence.

That graph is simpler than it looks. The experiment started with the subject lying down with his leg elevated, so gravity pushed the blood out of his leg (hence the low readout in mL of calf blood volume at the far left). He then stood still, which resulted in blood pooling in his legs, increasing his calf volume (takes place over about 90 seconds).

The really interesting part starts at (d), when he performs calf raises. You can see he performed 8-10 reps, but his blood volume dropped to healthy levels after only the first 2-3 calf raises!

So the good news is that maintaining healthy blood flow while standing doesn’t require much calf activation. It only takes a little bit of movement every few minutes to keep things going smoothly. Even better, your calves are pretty big (relative to the portion of your heart that pushes blood into your legs), so it takes several heart beats to refill your calf veins after they’ve been emptied.

What You Should Do Right Now

Whether you’re sitting or standing, do three firm calf flexes right this instant. Can’t you just feel your blood rushing up to your heart? This is fun!

Now do those calf flexes/raises every few minutes the whole time you’re stationary. I agree, it’s starting to sound like a bother. Especially if you’re sitting. When you’re standing, making this happen is much more natural. In fact, one of the most positive aspects of standing is that it provides for greater range of motion than sitting. So use it!

Don’t stay in one place or posture too long. In addition to your normal movement, do three calf raises every time you complete a task or send an email. Hell, have a private dance party every time there’s a break in your workflow. Who said healthy isn’t fun?

Did you learn something new today? Or did you already know about calf blood volume for some reason? Let us know in the comments.

Note: Once you hit that enviable flow state and lose focus on anything but your work, remembering to switch up your posture is basically impossible. Fortunately, ErgoDriven is working on a very nifty solution for that (stay tuned!)



  1. […] desk is configured correctly). But it’s important to stand right, because when it comes to blood flow, impaired metabolism, and disease – standing still is just as bad as sitting. That’s […]

    September 30, 2014
  2. Hunter said:

    I know that blood clotting in your legs is a problem with long flights (people tell you to move around and stretch and stuff) but I never really understood why…

    This makes perfect sense. Would just flexing your legs on a plane prevent that?

    October 1, 2014
    • Kit Perkins said:

      Hey Hunter,

      I think that graph of the blood volume through various exercises shows that calf pumping would be a very effective way to avoid that issue. I would guess that seated calf pumps don’t work quite as well as standing (since there’s less load on the muscle), but 5 or 6 firm ones should work well to clear that pooled blood.

      October 1, 2014
      • Hunter said:

        That makes sense. I fly a lot, so I wear compression socks on long flights (I got these ones), but I’ll try calf flexing too and let you know how it goes.

        October 11, 2014
  3. Justin said:

    I didn’t really get that graph with the little guys standing over it. What does that mean?

    October 1, 2014
    • Kit Perkins said:

      Hey Justin,

      That graph is showing how much blood is in one man’s calves over the course of several actions (so that’s actually one guy shown in several different positions across the top). The gist of it is that laying down with your leg up pretty much keeps the blood out of your calves, whereas standing up causes blood to pool down there.

      The really cool part is that just a few calf pumps (starting at (d)) caused all that blood to leave the calves!

      Does that make sense?

      October 1, 2014
      • Justin said:

        Yea I got it, I totally thought they were different people before.

        October 11, 2014
  4. Ouida M Barker said:

    What is the easiest way to do the most effective calf flex while standing and working.

    May 4, 2017

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