I love GOOOOooOOOOLLLD.

Imagine if our blood had gold, or silver instead of iron as the primary metallic element.

That'd make oxygen-consuming life pretty much impossible.

First of all, hemoglobin is the ordered agglomeration of four hemes, four proteins that each contain 1 iron atom. That makes for 4 iron atoms per hemoglobin. The role of the iron atom is to make a bond with oxygen (to be oxydized) to transport it, and release it in a volume containing low oxygen.

In fact, our very DNA contains traces of events collectively known as "the war for iron", which are evolutions and mutations of the DNA to make different proteins to protect against different evolutions of other organisms that would use their proteins to steal the iron of their victims. Friendly reminder that such evolution can take place over hundreds of thousands of years, predating the people of Ancient Egypt.

Now here's a big problem: gold doesn't bond with oxygen very well, rendering it useless as a substitute for iron in hemoglobin. That's why you can find shining gold in nature (look up Brasilian gold extractors in French Guyana, I'm sure you'll find pictures of a pan or two with gold flakes inside), unlike iron, usually found oxidized, like in rust and in ochres, and needing either chemical or thermic treatment to be returned to its non-oxidized state (see: high-furnaces).

Silver binds to oxygen, however not 1 but 2 silvers are needed to bind 1 oxygen, and 5 silvers can bind 4 oxygens in a crystalline form. This combination doesn't release the oxygen easily to its surrounding, making it ill-suited as a stand-in for iron in hemoglobin.