Answer:
Oxygen
Explanation:
We as humans are typical eukaryotic organisms and like other eukaryotic organisms, our cells undergo a metabolic process in order to release energy, called RESPIRATION. Cellular respiration can either be aerobic (presence of oxygen) or anaerobic (absence of oxygen).
Our cells undergo aerobic respiration, which involves the breaking down of organic food molecules (glucose) in the presence of oxygen to produce CO2, water and energy (ATP). Aerobic cellular respiration consists of three steps; Glycolysis, Kreb's cycle and Oxidative phosphorylation. Glycolysis occurs without oxygen but the other two processes require oxygen as the terminal electron acceptor.
However, when the cells are in short supply of oxygen (O2), the cells use an alternative method to produce energy. This alternative method is called LACTIC ACID FERMENTATION, which involves the conversion of pyruvate (from glycolysis), in a series of reactions, to lactic acid without oxygen.
In a prolonged oxygen deficiency, the lactic acid builds up or accumulates in the muscle cells to cause MUSCLE PULL.