how do lymph form and return to blood stream

Blood vessels tend to seep fluid into surrounding tissue. The lymphatic system drains off any extra fluid to stop the tissues from puffing up. The feet in particular are prone to puffiness due to the fact that the lymphatic drainage system is working against gravity and the legs are mostly down

The entire lymphatic system flows toward the bloodstream, returning fluid from body tissues to the blood. If there were no way for excess fluid to return to the blood, our body tissues would become swollen. For example, when a body part swells, it may be because there is too much fluid in the tissues in that area. The lymph vessels collect that excess fluid and carry it to the veins through the lymphatic system.

The answers above aren't doing a good job answering how the lymph and blood system are connected.

Lymph is an important part of the circulatory system. It helps maintain the fluid balance in blood by carrying fluid from extracellular environments into the blood stream (such as water, proteins, and other substances). Lymph also plays a primary role in one's immunity because it is rich in white blood cells (lymph nodes are also the sight for lymphocyte production, which remove microorganisms and other foreign materials by utilization of phagocytosis). The two main ducts that drain lymph into the bloodstream include:

1) Right lymphatic duct- drains lymph from upper right portion of body into the right subclavian vein.

2) Thoracic duct- drains lymph from the rest of the body to the left subclavian vein (just before it becomes the superior vena cava).