It rises in fits and starts probably depending on how far back different journals have decided to back-register. If you look at the second graph there does seem to be a flattening of the curve in the sixties and onward indicating that the growth may be linear after all. Sadly, for blood pressure that means 18 000 articles per year as of 2014, and the rate increases with another thousand per year every three years. Working in several fields means trying to keep up with each of them, and makes for a grand total that does not bear thinking about.
Papers to read more thoroughly. In addition, I regularly scan Pubmed for relevant articles, as you do when writing papers, grants, and lectures.