What is a Block header?

Other nodes on the network can validate the node's hash solution and determine whether the proposed block warrants the further checking required to secure its place as the top-most link in the longest chain of valid proof of work.

The body of the block contains the transactions. These are hashed only indirectly through the Merkle root. Because transactions aren't hashed directly, the proof of work needed to mine a block with 1 transaction takes exactly the same amount of effort as for a block with 10,000, 10,000,000 or 10,000,000,000 transactions.