Does lactic acid fermentation in humans happen only in the muscle? I have read in this Wikipedia page that human muscle cells sometimes respire anaerobically to produce ATP and lactic acid. Does it happen anywhere else in our body?
Cells which lack mitochondria or do not have enough supply of oxygen perform lactic acid fermentation for the energy.
Aerobic and anaerobic respiration:
The NADH gets converted to NAD+ during the process of fermentation. Aerobic respiration and anaerobic respiration follow through glycolysis, the Krebs cycle and finally the Electron Transport Chain (ETC). The difference between aerobic and anaerobic respiration is that, in anaerobic respiration another molecule , other than oxygen, is used as the electron acceptor in the ETC.
But during fermentation, there is no ETC ( or Krebs cycle). Because of the lack of oxygen, the cells performing fermentation have glycolysis followed by some extra reaction ( to get back NAD+) for their energy requirement in the absence of oxygen. During glycolysis NAD+ gets reduced into NADH. Then the NADH oxidises in the ETC to form NAD+. This can again be used in glycolysis. But there is no ETC in fermentation. So, the pyruvate (or a derivative of it) after glycolysis acts as the electron acceptor and oxidises the NADH back to NAD+ in fermentation. Based on the electron acceptor during fermentation, it can be classified as lactic acid fermentation or alcoholic fermentation.
The two main type of cells that perform fermentation are the erythrocytes (RBCs) and SKELETAL muscle cells. As mature RBCs lack mitochondria, which is essential for aerobic respiration, they only performs lactic acid fermentation. Skeletal muscle cells on the other hand perform lactic acid fermentation when the energy requirement is higher than the rate at which the muscles cells can get oxygen. This is usually during strenuous exercises.
I would like to know if any other cell in the human body might be doing this. So the answer might be a cell which lacks mitochondria or that experiences hypoxia and needs energy.