Final answer:
In animals, exertion without enough oxygen leads to the production of lactic acid via anaerobic respiration or fermentation, allowing for a rapid generation of energy and the recycling of NADH to NAD+ to continue glycolysis.
Step-by-step explanation:
In animals, exertion without sufficient oxygen leads to the production of lactic acid. During strenuous physical activities, if oxygen is limited or not available, cells will undergo anaerobic respiration or fermentation to meet the immediate energy demands. This process involves the conversion of pyruvate, the end product of glycolysis, into lactic acid.
The formation of lactic acid allows for the recycling of NADH to NAD+, which is crucial for the continuation of glycolysis, and although it does not produce ATP, it enables cells to generate energy rapidly for short periods of time. Accumulation of lactic acid in muscles can lead to sensations of fatigue.