Why does the heart pump blood to the lungs before it pumps blood to the rest of the body?

The heart is really two pumps in one. It receives blood into its right side. This blood has been all around the body giving up its oxygen to all the various cells and collecting carbon dioxide. [This is the by-product of the cells using oxygen]. The heart then pumps this blood to the lungs where it exchanges the carbon dioxide that it has collected and takes on more oxygen. The blood is the returned to the heart but this time to the leftside. It is then pumped all around the body again. This freshly oxygenated blood contains 21% oxygen. When it returns to the right side of the heart having been all around the body, it still contains 17% of its oxygen which is why we are able to give mouth to mouth and successfully revive a person that has stopped breathing. Hope this helps.