Your circulatory system is composed of two primary sub-systems: the Venous system and the Arterial System. The Arterial system pumps oxygenated blood that is received by the heart from the lungs to the rest of the body. The Venous system returns blood (now de-oxygenated and full of CO2) to the heart which then pumps it through the lungs back to the heart and the Arterial system.
Now consider how close the heart is your lungs and your head. Not very far, The top of your head is about 1 to 1.5 feet above your heart. Everything else in your body is pretty much at the same level or below your heart. So the hardest part of pumping blood in the Arterial system is pumping blood UP to your head, your lungs, and your arms. On the other hand pumping blood down to your abdomen, legs, feet etc. means pumping a distance of about 4-5 feet, depending on how tall you are. Pumping the blood down doesn't require quite as much effort, because gravity is working with your heart, not against it.
What most people don't realize is that the Venous system, which returns blood to the heart, is assisted by your muscular system, primarily your legs. In fact, your legs might actually be a stronger pumping system than your heart (more about this later). Your veins have little valves in them that prevent the back flow of blood when the arterial portion of the heart temporarily stops. What? Yes, that's what your pulse is all about. The heart pumps by contracting - which it does rhythmically. Pump - pause - pump - pause - pump - pause. During that pause, if you didn't have those little valves in your veins, the blood in your veins would start to flow back down in your body because gravity still works. When the muscles contract, they actually help pump blood back to your heart.
Why is this important? Well, if you're sitting at your desk, your legs aren't pumping blood back to the heart! When you're sitting down, the distance from your feet to your heart is about 3 feet. That's actually a VERY long distance to pump fluid against gravity! And it puts a lot of strain on your heart - all day.
In fact, a good demonstration of how much we depend upon our muscular system to pump blood that 3 or 4 feet back to the heart is to simply stand on your head or hang by your feet for about 30 seconds. Generally, you'll find that your face will get flushed, your sinuses will start to feel congested, and you might even start to get a head-ache because blood is starting to pool in your cranium. Your heart is having a hard time pumping blood uphill against gravity to your feet, abdomen, etc. All the things that are normally BELOW your heart. It pools because your muscular system can't help the heart get blood out of the Venous system.
The human body evolved to depend on the muscles in our legs to help out the heart. We used to walk, squat, get up and down all day long before we got cars and desk jobs.
So what's a poor desk jockey to do? Every couple of minutes stand up. Do some calf raises and a few squats. Or just tense your leg muscles with isometric exercises.
Give your heart a break....