Conditions without oxygen are referred to as anaerobic. In this case, the pyruvate will be converted to lactate in the cytoplasm of the cell as shown below.
Figure 6.281 Pyruvate fork in the road, what happens depends on whether it is aerobic or anaerobic respiration1
What happens if oxygen isn't available to serve as the final electron acceptor? As shown in the following video, the ETC becomes backed up with electrons and can't accept them from NADH and FADH2.
Video: What happens when you run out of oxygen? (0:37)
This leads to a problem in glycolysis because NAD is needed to accept electrons, as shown below. Without the electron transport chain functioning, all NAD has been reduced to NADH and glycolysis cannot continue to produce ATP from glucose.
Figure 6.282 Why NAD needs to be regenerated under anaerobic conditions2
Thus, there is a workaround to regenerate NAD by converting pyruvate (pyruvic acid) to lactate (lactic acid) as shown below.
Figure 6.283. The conversion of pyruvic acid to lactic acid regenerates NAD3,4
However, anaerobic respiration only produces 2 ATP per molecule of glucose, compared to 32 ATP for aerobic respiration. The biggest producer of lactate is the muscle. Through what is known as the Cori cycle, lactate produced in the muscle can be sent to the liver. In the liver, through a process known as gluconeogenesis, glucose can be regenerated and sent back to the muscle to be used again for anaerobic respiration forming a cycle as shown below.
Figure 6.284 The Cori cycle5
It is worth noting that the Cori cycle also functions during times of limited glucose (like fasting) to spare glucose by not completely oxidizing it.
References & Links
What happens when your run out of oxygen? - http://www.youtube.com/watch?v=StXlo1W3Gvg