So how does blood pool in your body?
When your heart pumps, it pushes blood through your arteries and keeps all the factors mixed up. But by the time blood gets through the capillaries and into the veins, it doesn't move as fast. Gravity makes it even HARDER for blood to flow back up to the heart from your lower body.
Because of this, your body has other 'pumps,' which help move blood back to the heart.
The most important pump is called the 'muscular pump.' When you're moving around, the muscles in your legs contract and relax. When they do this, they squeeze the veins in your legs. Your veins have one-way valves that only let blood flow forward. So if the veins are squeezed, this forces blood to move in one direction--toward your heart. But in order for this pump to work, you need to be using the muscles in your legs.
For people who don't exercise or can't move around very well, the muscular pump won't be as active, and therefore can't cycle blood back to the heart as well. This increases the risk of blood pooling in the legs.
As we mentioned earlier, when blood pools in areas of your body, it makes it more likely for unwanted clots to form.