oxygen goes to the lungs when we breath, from there it spreads to the bloodstream. by use of red blood cells, the blood then circulates to the various body parts.
when we eat glucose and other food nutrients are digested into small molecules, these molecules are then absorbed through the small intestine into the blood stream.
when we eat proteins, they are broken down into separate amino acids, through a process called biosynthesis, using the same process they are also converted back to protein.