Here’s a graphic showing the health risks of prolonged sitting, which I’ve blogged about before:

Besides the reasons shown here and described in the NY Times article link, here are a couple of more reasons why prolonged sitting creates dis-ease and why movement is good for you:

1. The lymphatic system aids the immune system in destroying pathogens and filtering waste, and it delivers nutrients, oxygen, and hormones to the cells. It has no central pump, like the circulatory system. Instead, the lymphatic system depends on muscular movement, breathing, and gravity to move lymph throughout the body. Frequent movement is critical to move lymph. 
2. Walking moves the sacrum, which acts as a pump for cerebro-spinal fluid, the fluid that surrounds the brain and spinal cord. Cerebro-spinal fluid nourishes, removes toxins, and cushions the brain and spinal cord. 

Next, tips if you have a job that requires sitting.