Each of the billions of cells which go to make up the body need a regular supply of oxygen, gasses, nutrients and vitamins which are dissolved in the plasma - the liquid component of blood. As the blood is pumped round the body the blood vessels gradually get smaller and smaller until they form tiny little capillaries. The walls of the capillaries are so thin that some of the fluid can seep out into the area around the cells.
This fluid carries nutrients and dissolved oxygen with it and has several jobs to do. Carbon dioxide, any dead cells, waste products, bacteria and other debris are removed in the fluid plasma which acts like a refuse collector. The fluid is then reabsorbed into the blood through the walls of the capillaries and makes its way back to the heart. A common form of poor circulation is cold hands and feet.