Lactic acid in the human body is merely a by-product (and an unwanted one at that) of anaerobic respiration*. When we indulge in strenuous or protracted exercise, our bodies cannot produce enough oxygen quickly enough to react with glucose to produce energy to keep the muscles pumping. Because of this, our bodies produce energy by breaking the glucose down chemically, and instead of producing CO2 -as it would were aerobic respiration** carried out- which can be exhaled through the lungs, unlike lactic acid. Because of this our bodies have to get rid of the lactic acid that hangs around like a tramp outside a South London bookie's shop. This is done by taking in extra oxygen to react with the lactic acid. This is why we pant and puff for some time even after we have finished exercising - our bodies are clearing the "Oxygen Debt" brought on by anaerobic respiration. If we don't do this -that is, if we let the lactic acid remain in our system, it can lead to muscle cramps (which is why "warm downs" are carried out by athletes after training). Don't think that we are the only ones to make use of anaerobic respiration, however. Certain bacteria respire exclusively the anaerobic way, and hippopotami can sustain the reactions assosciated with it to such an extent thet they can stay underwater for over ten minutes! *Anaerobic respiration: Producing energy without using oxygen from the air. **Aerobic respiration: Producing energy using oxygen from the air. PS: I daresay the chemists have a myriad different uses for lactic acid.

Before you know functions, you need to know some background on lactic acid... Your body takes in sugar (glucose) for energy. This sugar than goes into a biological pathway (series of reactions) that break it down into compounds we can use for energy, and an extra substance called pyruvic acid. If there's oxygen, it goes into aerobic respiration to produce more of these energy providing compounds, but with no oxygen, it goes into anaerobic respiration. In anaerobic respiration, no more energy-providing compounds are produced, but the pyruvic acid that was created gets conveted into another substance. It can get converted to ethyl alcohol, which we drink. However, it can also get converted into lactic acid. Lactic acid is a 3-carbon molecule that's made with an ENZYME + PYRUVIC ACID (from sugar breaking down). Lactic acid, as said by the last guy's answer, accumulates in muscle cells during strenuous exercise when you may not be breathing enough (Remember: with oxygen=energy, without oxygen=acid). How it works: Without oxygen, lactic acid accumulates; the acid makes it hard for cells to contract; muscle fatigue, pain, cramps. However, lactic acid has another function besides hurting your muscles. LACITC ACID creation in microorganisms (bacteria) HELPS MAKE YOGURT AND CHEESE.

lactic acid sucks!!!!!!!!!!!!!!!