"a person who sits for hours on a long airplane flight will have problems will blood pooling below the knees. why? "

The long flight can make the person feel weak and unhealthy. The long flight can cause dehydration, as well as infection. Sometimes, the individual needs emergency medical services

The deep vein thrombosis can occur in the lower leg after a long flight. This might occur because the blood is not able to return to the heart properly and the skeletal muscle pump below the knees. This problem is more common in over weight individual.