Compare the two flow charts and answer the following.

From your observation it is clear that in fig -11(a) blood flows through heart only once to complete one circulation.

If blood flows through heart only once for completing one circulation it is called single circulation.

If the blood flows through the heart twice for completing one circulation it is called double circulation

Have you ever observed what happened to your feet after overnight journey, in sitting position without moving? We feel that our foot wear is little tight. In elders it will be clear that the lower part of the legs will be swollen. This stage is called edema

As blood flows through tissues and through blood capillaries some amount of fluids and certain solid materials are constantly flowing out of them at different junctions. Such materials are to be collected and sent back into blood circulation.

We know that blood circulates in the blood vessels, pushed by the heart. From the heart it flows into the arteries and finally into the capillaries. To supply nutrients to the cells (tissues), the liquid portion of the blood with nutrients flows out of the capillaries. This is called tissue fluid.

The tissue fluid which is present in the tissues should be transported into the blood vessels again. Some portion of the tissue fluid enters into the venules, which in turn form the veins, which carry blood to the heart. What about the remaining tissue fluid? To transport the tissue fluid in to the main blood stream, a separate system is present. That is called lymphatic system. In latin lymph means water.

Lymph is the vital link between blood and tissues by which essential substances pass from blood to cells and excretory products from cells to blood. The lymphatic system is a parallel system to venous system which collects tissue fluid from tissues and transports it to the venous system.