[deadlyshaolin]

hey redrum, if you really want to know...

The sensation of nausea and subsequent retching/vomiting is controlled by the vomiting centers in the reticular formations of the medulla (in your brainstem). It receives signals from four sources:
1. the chemoreceptor trigger zone (see below)
2. visceral nerves from the GI tract (ate something bad, stomach flu, food poisoning, etc)
3. visceral nerves from outside the GI tract (e.g. gall bladder stone causing you to vomit, heart attack causing intense nausea, etc)
4. nerves from the extramedullary centers in the brain (e.g. bad odors, fear, motion sickness, brain injury)

What I want to focus on is #1, the chemoreceptor trigger zone. This "zone" is located in your brainstem and gets activated whenever an abnormal systemic, whole-body event is happening, like hypoxia, diabetic ketoacidosis, uremia, vomit-inducing drugs, AND IN THIS CASE, lactic acidosis.

The decrease in pH caused by lactic acid buildup gets sensed by your chemoreceptor trigger zone, which tells the vomiting center in your reticular formation that should be nauseous and possibly vomit.