When we eat, the food must get to the cells, because the energy and building blocks contained in the food will be used in the cells (not in the stomach).  The mouth, stomach and intestines use mechanical digestion and chemical digestion to break down food into particles small enough to enter the cells.  After the molecules of food have been broken down in the stomach and intestines, these molecules diffuse through the walls of the villi lining the small intestine into the capillaries which surround the villi.  At this point, the digestive system connects with the circulatory system.  The capillaries that have absorbed food molecules lead to larger blood vessels called veins which lead back to the heart.  From there, this food-rich blood is pumped through arteries to cells in all parts of the body.  The arteries branch into smaller and smaller vessels until they become microscopic capillaries.  From the capillaries, the food molecules diffuse into the cells that they touch to act as fuel and building blocks in the life of the cell.