How lead is chemically similar to calcium? They are on opposite...
Then you go on to explain how lead is like calcium in its bioavailability, so in relation to the operations of a human body, lead is chemically similar to calcium (only dangerous).
"Ability to replace calcium in certain biochemical processes" is not nearly the same as "chemically similar to calcium". It's like saying if I stole your login for facebook I would look like you and nobody would be able to distinguish between us without DNA test. Completely different things. Biochemical processes are known for accepting a lot of substitutes, that's how a lot of drugs work. That does not mean all those are chemically similar in general - they just play similar roles in certain processes.
Then you go on to explain how lead is like calcium in its bioavailability, so in relation to the operations of a human body, lead is chemically similar to calcium (only dangerous).
http://brain.oxfordjournals.org/content/126/1/5.full
For example, lead’s ability to pass through the blood–brain barrier (BBB) is due in large part to its ability to substitute for calcium ions (Ca2+)