Why is lack of oxygen during cellular respiration result in formation of lactic acid?

Answer 1

Basically, oxygen is used in aerobic respiration, which combines it with glucose to form adenosine triphosphate (ATP), which cells use for energy. It also produces the waste product carbon dioxide, which is removed from the body when you breathe out.

When not enough oxygen is available to produce the amount of ATP, the body can also use anaerobic respiration, which uses glucose to produce ATP. However, it is less efficient than aerobic respiration, and produces lactic acid as a waste product.

Your muscles hurt when you exercise too much largely because when you work them too hard, they can't get enough oxygen, and they have to use anaerobic respiration as well as aerobic respiration. This causes lactic acid to build up inside them, which hurts.

Wikipedia's page on cellular respiration has more details if you need them.

Answer 2

Lactic acid moves the pyruvate plus continuing glycolysis.