When you walk through one of the red pools (Blood Springs), your avatar takes on a red tint, and an animation appears above its head. Similar happens to your followers if they go through a red pool.
What's the effect of it? Does it affect your health (it doesn't appear to unless I have some mitigating effect of a piece of equipment), or is it another "zero effect but looks cool" thing like the tar pits in act 2, which the Enchantress endearingly tells you not to walk through because your boots will get dirty?