In the absence of oxygen, the enzyme lactate dehydrogenase converts pyruvate to lactate.
During the reaction, NADH is oxidized to NAD+.
Glycolysis (the process of converting glucose to pyruvate) results in a net gain of ATP and the reduction of the coenzyme NAD to NADH.
So, in order for glycolysis to occur, a constant supply of NAD is required.
In anaerobic respiration (without the use of oxygen), pyruvate is converted into lactic acid by NADH, which donates H+ to pyruvate to produce lactic acid and NAD.
This is then reused in glycolysis to keep the cycle going and the person experiences fatigue.
Accumulation of lactic acid:
Lack of oxygen or anaerobic respiration leads to the formation of lactic acid.
This mainly occurs in the myocytes during muscle spasms when insufficient oxygen is available.
As a result, lactic acid forms and accumulates in the muscle cells causing hypercontraction, pain, and swelling.
